build: move package sources from src/pkg to src

Preparation was in CL 134570043.
This CL contains only the effect of 'hg mv src/pkg/* src'.
For more about the move, see golang.org/s/go14nopkg.

125 files changed, 32623 insertions, 0 deletions

diff --git a/src/go/ast/ast.go b/src/go/ast/ast.go
new file mode 100644
index 000000000..312e3d1b9
--- /dev/null
+++ b/src/go/ast/ast.go
@@ -0,0 +1,995 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package ast declares the types used to represent syntax trees for Go
+// packages.
+//
+package ast
+
+import (
+	"go/token"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+// ----------------------------------------------------------------------------
+// Interfaces
+//
+// There are 3 main classes of nodes: Expressions and type nodes,
+// statement nodes, and declaration nodes. The node names usually
+// match the corresponding Go spec production names to which they
+// correspond. The node fields correspond to the individual parts
+// of the respective productions.
+//
+// All nodes contain position information marking the beginning of
+// the corresponding source text segment; it is accessible via the
+// Pos accessor method. Nodes may contain additional position info
+// for language constructs where comments may be found between parts
+// of the construct (typically any larger, parenthesized subpart).
+// That position information is needed to properly position comments
+// when printing the construct.
+
+// All node types implement the Node interface.
+type Node interface {
+	Pos() token.Pos // position of first character belonging to the node
+	End() token.Pos // position of first character immediately after the node
+}
+
+// All expression nodes implement the Expr interface.
+type Expr interface {
+	Node
+	exprNode()
+}
+
+// All statement nodes implement the Stmt interface.
+type Stmt interface {
+	Node
+	stmtNode()
+}
+
+// All declaration nodes implement the Decl interface.
+type Decl interface {
+	Node
+	declNode()
+}
+
+// ----------------------------------------------------------------------------
+// Comments
+
+// A Comment node represents a single //-style or /*-style comment.
+type Comment struct {
+	Slash token.Pos // position of "/" starting the comment
+	Text  string    // comment text (excluding '\n' for //-style comments)
+}
+
+func (c *Comment) Pos() token.Pos { return c.Slash }
+func (c *Comment) End() token.Pos { return token.Pos(int(c.Slash) + len(c.Text)) }
+
+// A CommentGroup represents a sequence of comments
+// with no other tokens and no empty lines between.
+//
+type CommentGroup struct {
+	List []*Comment // len(List) > 0
+}
+
+func (g *CommentGroup) Pos() token.Pos { return g.List[0].Pos() }
+func (g *CommentGroup) End() token.Pos { return g.List[len(g.List)-1].End() }
+
+func isWhitespace(ch byte) bool { return ch == ' ' || ch == '\t' || ch == '\n' || ch == '\r' }
+
+func stripTrailingWhitespace(s string) string {
+	i := len(s)
+	for i > 0 && isWhitespace(s[i-1]) {
+		i--
+	}
+	return s[0:i]
+}
+
+// Text returns the text of the comment.
+// Comment markers (//, /*, and */), the first space of a line comment, and
+// leading and trailing empty lines are removed. Multiple empty lines are
+// reduced to one, and trailing space on lines is trimmed. Unless the result
+// is empty, it is newline-terminated.
+//
+func (g *CommentGroup) Text() string {
+	if g == nil {
+		return ""
+	}
+	comments := make([]string, len(g.List))
+	for i, c := range g.List {
+		comments[i] = string(c.Text)
+	}
+
+	lines := make([]string, 0, 10) // most comments are less than 10 lines
+	for _, c := range comments {
+		// Remove comment markers.
+		// The parser has given us exactly the comment text.
+		switch c[1] {
+		case '/':
+			//-style comment (no newline at the end)
+			c = c[2:]
+			// strip first space - required for Example tests
+			if len(c) > 0 && c[0] == ' ' {
+				c = c[1:]
+			}
+		case '*':
+			/*-style comment */
+			c = c[2 : len(c)-2]
+		}
+
+		// Split on newlines.
+		cl := strings.Split(c, "\n")
+
+		// Walk lines, stripping trailing white space and adding to list.
+		for _, l := range cl {
+			lines = append(lines, stripTrailingWhitespace(l))
+		}
+	}
+
+	// Remove leading blank lines; convert runs of
+	// interior blank lines to a single blank line.
+	n := 0
+	for _, line := range lines {
+		if line != "" || n > 0 && lines[n-1] != "" {
+			lines[n] = line
+			n++
+		}
+	}
+	lines = lines[0:n]
+
+	// Add final "" entry to get trailing newline from Join.
+	if n > 0 && lines[n-1] != "" {
+		lines = append(lines, "")
+	}
+
+	return strings.Join(lines, "\n")
+}
+
+// ----------------------------------------------------------------------------
+// Expressions and types
+
+// A Field represents a Field declaration list in a struct type,
+// a method list in an interface type, or a parameter/result declaration
+// in a signature.
+//
+type Field struct {
+	Doc     *CommentGroup // associated documentation; or nil
+	Names   []*Ident      // field/method/parameter names; or nil if anonymous field
+	Type    Expr          // field/method/parameter type
+	Tag     *BasicLit     // field tag; or nil
+	Comment *CommentGroup // line comments; or nil
+}
+
+func (f *Field) Pos() token.Pos {
+	if len(f.Names) > 0 {
+		return f.Names[0].Pos()
+	}
+	return f.Type.Pos()
+}
+
+func (f *Field) End() token.Pos {
+	if f.Tag != nil {
+		return f.Tag.End()
+	}
+	return f.Type.End()
+}
+
+// A FieldList represents a list of Fields, enclosed by parentheses or braces.
+type FieldList struct {
+	Opening token.Pos // position of opening parenthesis/brace, if any
+	List    []*Field  // field list; or nil
+	Closing token.Pos // position of closing parenthesis/brace, if any
+}
+
+func (f *FieldList) Pos() token.Pos {
+	if f.Opening.IsValid() {
+		return f.Opening
+	}
+	// the list should not be empty in this case;
+	// be conservative and guard against bad ASTs
+	if len(f.List) > 0 {
+		return f.List[0].Pos()
+	}
+	return token.NoPos
+}
+
+func (f *FieldList) End() token.Pos {
+	if f.Closing.IsValid() {
+		return f.Closing + 1
+	}
+	// the list should not be empty in this case;
+	// be conservative and guard against bad ASTs
+	if n := len(f.List); n > 0 {
+		return f.List[n-1].End()
+	}
+	return token.NoPos
+}
+
+// NumFields returns the number of (named and anonymous fields) in a FieldList.
+func (f *FieldList) NumFields() int {
+	n := 0
+	if f != nil {
+		for _, g := range f.List {
+			m := len(g.Names)
+			if m == 0 {
+				m = 1 // anonymous field
+			}
+			n += m
+		}
+	}
+	return n
+}
+
+// An expression is represented by a tree consisting of one
+// or more of the following concrete expression nodes.
+//
+type (
+	// A BadExpr node is a placeholder for expressions containing
+	// syntax errors for which no correct expression nodes can be
+	// created.
+	//
+	BadExpr struct {
+		From, To token.Pos // position range of bad expression
+	}
+
+	// An Ident node represents an identifier.
+	Ident struct {
+		NamePos token.Pos // identifier position
+		Name    string    // identifier name
+		Obj     *Object   // denoted object; or nil
+	}
+
+	// An Ellipsis node stands for the "..." type in a
+	// parameter list or the "..." length in an array type.
+	//
+	Ellipsis struct {
+		Ellipsis token.Pos // position of "..."
+		Elt      Expr      // ellipsis element type (parameter lists only); or nil
+	}
+
+	// A BasicLit node represents a literal of basic type.
+	BasicLit struct {
+		ValuePos token.Pos   // literal position
+		Kind     token.Token // token.INT, token.FLOAT, token.IMAG, token.CHAR, or token.STRING
+		Value    string      // literal string; e.g. 42, 0x7f, 3.14, 1e-9, 2.4i, 'a', '\x7f', "foo" or `\m\n\o`
+	}
+
+	// A FuncLit node represents a function literal.
+	FuncLit struct {
+		Type *FuncType  // function type
+		Body *BlockStmt // function body
+	}
+
+	// A CompositeLit node represents a composite literal.
+	CompositeLit struct {
+		Type   Expr      // literal type; or nil
+		Lbrace token.Pos // position of "{"
+		Elts   []Expr    // list of composite elements; or nil
+		Rbrace token.Pos // position of "}"
+	}
+
+	// A ParenExpr node represents a parenthesized expression.
+	ParenExpr struct {
+		Lparen token.Pos // position of "("
+		X      Expr      // parenthesized expression
+		Rparen token.Pos // position of ")"
+	}
+
+	// A SelectorExpr node represents an expression followed by a selector.
+	SelectorExpr struct {
+		X   Expr   // expression
+		Sel *Ident // field selector
+	}
+
+	// An IndexExpr node represents an expression followed by an index.
+	IndexExpr struct {
+		X      Expr      // expression
+		Lbrack token.Pos // position of "["
+		Index  Expr      // index expression
+		Rbrack token.Pos // position of "]"
+	}
+
+	// An SliceExpr node represents an expression followed by slice indices.
+	SliceExpr struct {
+		X      Expr      // expression
+		Lbrack token.Pos // position of "["
+		Low    Expr      // begin of slice range; or nil
+		High   Expr      // end of slice range; or nil
+		Max    Expr      // maximum capacity of slice; or nil
+		Slice3 bool      // true if 3-index slice (2 colons present)
+		Rbrack token.Pos // position of "]"
+	}
+
+	// A TypeAssertExpr node represents an expression followed by a
+	// type assertion.
+	//
+	TypeAssertExpr struct {
+		X      Expr      // expression
+		Lparen token.Pos // position of "("
+		Type   Expr      // asserted type; nil means type switch X.(type)
+		Rparen token.Pos // position of ")"
+	}
+
+	// A CallExpr node represents an expression followed by an argument list.
+	CallExpr struct {
+		Fun      Expr      // function expression
+		Lparen   token.Pos // position of "("
+		Args     []Expr    // function arguments; or nil
+		Ellipsis token.Pos // position of "...", if any
+		Rparen   token.Pos // position of ")"
+	}
+
+	// A StarExpr node represents an expression of the form "*" Expression.
+	// Semantically it could be a unary "*" expression, or a pointer type.
+	//
+	StarExpr struct {
+		Star token.Pos // position of "*"
+		X    Expr      // operand
+	}
+
+	// A UnaryExpr node represents a unary expression.
+	// Unary "*" expressions are represented via StarExpr nodes.
+	//
+	UnaryExpr struct {
+		OpPos token.Pos   // position of Op
+		Op    token.Token // operator
+		X     Expr        // operand
+	}
+
+	// A BinaryExpr node represents a binary expression.
+	BinaryExpr struct {
+		X     Expr        // left operand
+		OpPos token.Pos   // position of Op
+		Op    token.Token // operator
+		Y     Expr        // right operand
+	}
+
+	// A KeyValueExpr node represents (key : value) pairs
+	// in composite literals.
+	//
+	KeyValueExpr struct {
+		Key   Expr
+		Colon token.Pos // position of ":"
+		Value Expr
+	}
+)
+
+// The direction of a channel type is indicated by one
+// of the following constants.
+//
+type ChanDir int
+
+const (
+	SEND ChanDir = 1 << iota
+	RECV
+)
+
+// A type is represented by a tree consisting of one
+// or more of the following type-specific expression
+// nodes.
+//
+type (
+	// An ArrayType node represents an array or slice type.
+	ArrayType struct {
+		Lbrack token.Pos // position of "["
+		Len    Expr      // Ellipsis node for [...]T array types, nil for slice types
+		Elt    Expr      // element type
+	}
+
+	// A StructType node represents a struct type.
+	StructType struct {
+		Struct     token.Pos  // position of "struct" keyword
+		Fields     *FieldList // list of field declarations
+		Incomplete bool       // true if (source) fields are missing in the Fields list
+	}
+
+	// Pointer types are represented via StarExpr nodes.
+
+	// A FuncType node represents a function type.
+	FuncType struct {
+		Func    token.Pos  // position of "func" keyword (token.NoPos if there is no "func")
+		Params  *FieldList // (incoming) parameters; non-nil
+		Results *FieldList // (outgoing) results; or nil
+	}
+
+	// An InterfaceType node represents an interface type.
+	InterfaceType struct {
+		Interface  token.Pos  // position of "interface" keyword
+		Methods    *FieldList // list of methods
+		Incomplete bool       // true if (source) methods are missing in the Methods list
+	}
+
+	// A MapType node represents a map type.
+	MapType struct {
+		Map   token.Pos // position of "map" keyword
+		Key   Expr
+		Value Expr
+	}
+
+	// A ChanType node represents a channel type.
+	ChanType struct {
+		Begin token.Pos // position of "chan" keyword or "<-" (whichever comes first)
+		Arrow token.Pos // position of "<-" (token.NoPos if there is no "<-")
+		Dir   ChanDir   // channel direction
+		Value Expr      // value type
+	}
+)
+
+// Pos and End implementations for expression/type nodes.
+//
+func (x *BadExpr) Pos() token.Pos  { return x.From }
+func (x *Ident) Pos() token.Pos    { return x.NamePos }
+func (x *Ellipsis) Pos() token.Pos { return x.Ellipsis }
+func (x *BasicLit) Pos() token.Pos { return x.ValuePos }
+func (x *FuncLit) Pos() token.Pos  { return x.Type.Pos() }
+func (x *CompositeLit) Pos() token.Pos {
+	if x.Type != nil {
+		return x.Type.Pos()
+	}
+	return x.Lbrace
+}
+func (x *ParenExpr) Pos() token.Pos      { return x.Lparen }
+func (x *SelectorExpr) Pos() token.Pos   { return x.X.Pos() }
+func (x *IndexExpr) Pos() token.Pos      { return x.X.Pos() }
+func (x *SliceExpr) Pos() token.Pos      { return x.X.Pos() }
+func (x *TypeAssertExpr) Pos() token.Pos { return x.X.Pos() }
+func (x *CallExpr) Pos() token.Pos       { return x.Fun.Pos() }
+func (x *StarExpr) Pos() token.Pos       { return x.Star }
+func (x *UnaryExpr) Pos() token.Pos      { return x.OpPos }
+func (x *BinaryExpr) Pos() token.Pos     { return x.X.Pos() }
+func (x *KeyValueExpr) Pos() token.Pos   { return x.Key.Pos() }
+func (x *ArrayType) Pos() token.Pos      { return x.Lbrack }
+func (x *StructType) Pos() token.Pos     { return x.Struct }
+func (x *FuncType) Pos() token.Pos {
+	if x.Func.IsValid() || x.Params == nil { // see issue 3870
+		return x.Func
+	}
+	return x.Params.Pos() // interface method declarations have no "func" keyword
+}
+func (x *InterfaceType) Pos() token.Pos { return x.Interface }
+func (x *MapType) Pos() token.Pos       { return x.Map }
+func (x *ChanType) Pos() token.Pos      { return x.Begin }
+
+func (x *BadExpr) End() token.Pos { return x.To }
+func (x *Ident) End() token.Pos   { return token.Pos(int(x.NamePos) + len(x.Name)) }
+func (x *Ellipsis) End() token.Pos {
+	if x.Elt != nil {
+		return x.Elt.End()
+	}
+	return x.Ellipsis + 3 // len("...")
+}
+func (x *BasicLit) End() token.Pos       { return token.Pos(int(x.ValuePos) + len(x.Value)) }
+func (x *FuncLit) End() token.Pos        { return x.Body.End() }
+func (x *CompositeLit) End() token.Pos   { return x.Rbrace + 1 }
+func (x *ParenExpr) End() token.Pos      { return x.Rparen + 1 }
+func (x *SelectorExpr) End() token.Pos   { return x.Sel.End() }
+func (x *IndexExpr) End() token.Pos      { return x.Rbrack + 1 }
+func (x *SliceExpr) End() token.Pos      { return x.Rbrack + 1 }
+func (x *TypeAssertExpr) End() token.Pos { return x.Rparen + 1 }
+func (x *CallExpr) End() token.Pos       { return x.Rparen + 1 }
+func (x *StarExpr) End() token.Pos       { return x.X.End() }
+func (x *UnaryExpr) End() token.Pos      { return x.X.End() }
+func (x *BinaryExpr) End() token.Pos     { return x.Y.End() }
+func (x *KeyValueExpr) End() token.Pos   { return x.Value.End() }
+func (x *ArrayType) End() token.Pos      { return x.Elt.End() }
+func (x *StructType) End() token.Pos     { return x.Fields.End() }
+func (x *FuncType) End() token.Pos {
+	if x.Results != nil {
+		return x.Results.End()
+	}
+	return x.Params.End()
+}
+func (x *InterfaceType) End() token.Pos { return x.Methods.End() }
+func (x *MapType) End() token.Pos       { return x.Value.End() }
+func (x *ChanType) End() token.Pos      { return x.Value.End() }
+
+// exprNode() ensures that only expression/type nodes can be
+// assigned to an ExprNode.
+//
+func (*BadExpr) exprNode()        {}
+func (*Ident) exprNode()          {}
+func (*Ellipsis) exprNode()       {}
+func (*BasicLit) exprNode()       {}
+func (*FuncLit) exprNode()        {}
+func (*CompositeLit) exprNode()   {}
+func (*ParenExpr) exprNode()      {}
+func (*SelectorExpr) exprNode()   {}
+func (*IndexExpr) exprNode()      {}
+func (*SliceExpr) exprNode()      {}
+func (*TypeAssertExpr) exprNode() {}
+func (*CallExpr) exprNode()       {}
+func (*StarExpr) exprNode()       {}
+func (*UnaryExpr) exprNode()      {}
+func (*BinaryExpr) exprNode()     {}
+func (*KeyValueExpr) exprNode()   {}
+
+func (*ArrayType) exprNode()     {}
+func (*StructType) exprNode()    {}
+func (*FuncType) exprNode()      {}
+func (*InterfaceType) exprNode() {}
+func (*MapType) exprNode()       {}
+func (*ChanType) exprNode()      {}
+
+// ----------------------------------------------------------------------------
+// Convenience functions for Idents
+
+// NewIdent creates a new Ident without position.
+// Useful for ASTs generated by code other than the Go parser.
+//
+func NewIdent(name string) *Ident { return &Ident{token.NoPos, name, nil} }
+
+// IsExported reports whether name is an exported Go symbol
+// (that is, whether it begins with an upper-case letter).
+//
+func IsExported(name string) bool {
+	ch, _ := utf8.DecodeRuneInString(name)
+	return unicode.IsUpper(ch)
+}
+
+// IsExported reports whether id is an exported Go symbol
+// (that is, whether it begins with an uppercase letter).
+//
+func (id *Ident) IsExported() bool { return IsExported(id.Name) }
+
+func (id *Ident) String() string {
+	if id != nil {
+		return id.Name
+	}
+	return "<nil>"
+}
+
+// ----------------------------------------------------------------------------
+// Statements
+
+// A statement is represented by a tree consisting of one
+// or more of the following concrete statement nodes.
+//
+type (
+	// A BadStmt node is a placeholder for statements containing
+	// syntax errors for which no correct statement nodes can be
+	// created.
+	//
+	BadStmt struct {
+		From, To token.Pos // position range of bad statement
+	}
+
+	// A DeclStmt node represents a declaration in a statement list.
+	DeclStmt struct {
+		Decl Decl // *GenDecl with CONST, TYPE, or VAR token
+	}
+
+	// An EmptyStmt node represents an empty statement.
+	// The "position" of the empty statement is the position
+	// of the immediately preceding semicolon.
+	//
+	EmptyStmt struct {
+		Semicolon token.Pos // position of preceding ";"
+	}
+
+	// A LabeledStmt node represents a labeled statement.
+	LabeledStmt struct {
+		Label *Ident
+		Colon token.Pos // position of ":"
+		Stmt  Stmt
+	}
+
+	// An ExprStmt node represents a (stand-alone) expression
+	// in a statement list.
+	//
+	ExprStmt struct {
+		X Expr // expression
+	}
+
+	// A SendStmt node represents a send statement.
+	SendStmt struct {
+		Chan  Expr
+		Arrow token.Pos // position of "<-"
+		Value Expr
+	}
+
+	// An IncDecStmt node represents an increment or decrement statement.
+	IncDecStmt struct {
+		X      Expr
+		TokPos token.Pos   // position of Tok
+		Tok    token.Token // INC or DEC
+	}
+
+	// An AssignStmt node represents an assignment or
+	// a short variable declaration.
+	//
+	AssignStmt struct {
+		Lhs    []Expr
+		TokPos token.Pos   // position of Tok
+		Tok    token.Token // assignment token, DEFINE
+		Rhs    []Expr
+	}
+
+	// A GoStmt node represents a go statement.
+	GoStmt struct {
+		Go   token.Pos // position of "go" keyword
+		Call *CallExpr
+	}
+
+	// A DeferStmt node represents a defer statement.
+	DeferStmt struct {
+		Defer token.Pos // position of "defer" keyword
+		Call  *CallExpr
+	}
+
+	// A ReturnStmt node represents a return statement.
+	ReturnStmt struct {
+		Return  token.Pos // position of "return" keyword
+		Results []Expr    // result expressions; or nil
+	}
+
+	// A BranchStmt node represents a break, continue, goto,
+	// or fallthrough statement.
+	//
+	BranchStmt struct {
+		TokPos token.Pos   // position of Tok
+		Tok    token.Token // keyword token (BREAK, CONTINUE, GOTO, FALLTHROUGH)
+		Label  *Ident      // label name; or nil
+	}
+
+	// A BlockStmt node represents a braced statement list.
+	BlockStmt struct {
+		Lbrace token.Pos // position of "{"
+		List   []Stmt
+		Rbrace token.Pos // position of "}"
+	}
+
+	// An IfStmt node represents an if statement.
+	IfStmt struct {
+		If   token.Pos // position of "if" keyword
+		Init Stmt      // initialization statement; or nil
+		Cond Expr      // condition
+		Body *BlockStmt
+		Else Stmt // else branch; or nil
+	}
+
+	// A CaseClause represents a case of an expression or type switch statement.
+	CaseClause struct {
+		Case  token.Pos // position of "case" or "default" keyword
+		List  []Expr    // list of expressions or types; nil means default case
+		Colon token.Pos // position of ":"
+		Body  []Stmt    // statement list; or nil
+	}
+
+	// A SwitchStmt node represents an expression switch statement.
+	SwitchStmt struct {
+		Switch token.Pos  // position of "switch" keyword
+		Init   Stmt       // initialization statement; or nil
+		Tag    Expr       // tag expression; or nil
+		Body   *BlockStmt // CaseClauses only
+	}
+
+	// An TypeSwitchStmt node represents a type switch statement.
+	TypeSwitchStmt struct {
+		Switch token.Pos  // position of "switch" keyword
+		Init   Stmt       // initialization statement; or nil
+		Assign Stmt       // x := y.(type) or y.(type)
+		Body   *BlockStmt // CaseClauses only
+	}
+
+	// A CommClause node represents a case of a select statement.
+	CommClause struct {
+		Case  token.Pos // position of "case" or "default" keyword
+		Comm  Stmt      // send or receive statement; nil means default case
+		Colon token.Pos // position of ":"
+		Body  []Stmt    // statement list; or nil
+	}
+
+	// An SelectStmt node represents a select statement.
+	SelectStmt struct {
+		Select token.Pos  // position of "select" keyword
+		Body   *BlockStmt // CommClauses only
+	}
+
+	// A ForStmt represents a for statement.
+	ForStmt struct {
+		For  token.Pos // position of "for" keyword
+		Init Stmt      // initialization statement; or nil
+		Cond Expr      // condition; or nil
+		Post Stmt      // post iteration statement; or nil
+		Body *BlockStmt
+	}
+
+	// A RangeStmt represents a for statement with a range clause.
+	RangeStmt struct {
+		For        token.Pos   // position of "for" keyword
+		Key, Value Expr        // Key, Value may be nil
+		TokPos     token.Pos   // position of Tok; invalid if Key == nil
+		Tok        token.Token // ILLEGAL if Key == nil, ASSIGN, DEFINE
+		X          Expr        // value to range over
+		Body       *BlockStmt
+	}
+)
+
+// Pos and End implementations for statement nodes.
+//
+func (s *BadStmt) Pos() token.Pos        { return s.From }
+func (s *DeclStmt) Pos() token.Pos       { return s.Decl.Pos() }
+func (s *EmptyStmt) Pos() token.Pos      { return s.Semicolon }
+func (s *LabeledStmt) Pos() token.Pos    { return s.Label.Pos() }
+func (s *ExprStmt) Pos() token.Pos       { return s.X.Pos() }
+func (s *SendStmt) Pos() token.Pos       { return s.Chan.Pos() }
+func (s *IncDecStmt) Pos() token.Pos     { return s.X.Pos() }
+func (s *AssignStmt) Pos() token.Pos     { return s.Lhs[0].Pos() }
+func (s *GoStmt) Pos() token.Pos         { return s.Go }
+func (s *DeferStmt) Pos() token.Pos      { return s.Defer }
+func (s *ReturnStmt) Pos() token.Pos     { return s.Return }
+func (s *BranchStmt) Pos() token.Pos     { return s.TokPos }
+func (s *BlockStmt) Pos() token.Pos      { return s.Lbrace }
+func (s *IfStmt) Pos() token.Pos         { return s.If }
+func (s *CaseClause) Pos() token.Pos     { return s.Case }
+func (s *SwitchStmt) Pos() token.Pos     { return s.Switch }
+func (s *TypeSwitchStmt) Pos() token.Pos { return s.Switch }
+func (s *CommClause) Pos() token.Pos     { return s.Case }
+func (s *SelectStmt) Pos() token.Pos     { return s.Select }
+func (s *ForStmt) Pos() token.Pos        { return s.For }
+func (s *RangeStmt) Pos() token.Pos      { return s.For }
+
+func (s *BadStmt) End() token.Pos  { return s.To }
+func (s *DeclStmt) End() token.Pos { return s.Decl.End() }
+func (s *EmptyStmt) End() token.Pos {
+	return s.Semicolon + 1 /* len(";") */
+}
+func (s *LabeledStmt) End() token.Pos { return s.Stmt.End() }
+func (s *ExprStmt) End() token.Pos    { return s.X.End() }
+func (s *SendStmt) End() token.Pos    { return s.Value.End() }
+func (s *IncDecStmt) End() token.Pos {
+	return s.TokPos + 2 /* len("++") */
+}
+func (s *AssignStmt) End() token.Pos { return s.Rhs[len(s.Rhs)-1].End() }
+func (s *GoStmt) End() token.Pos     { return s.Call.End() }
+func (s *DeferStmt) End() token.Pos  { return s.Call.End() }
+func (s *ReturnStmt) End() token.Pos {
+	if n := len(s.Results); n > 0 {
+		return s.Results[n-1].End()
+	}
+	return s.Return + 6 // len("return")
+}
+func (s *BranchStmt) End() token.Pos {
+	if s.Label != nil {
+		return s.Label.End()
+	}
+	return token.Pos(int(s.TokPos) + len(s.Tok.String()))
+}
+func (s *BlockStmt) End() token.Pos { return s.Rbrace + 1 }
+func (s *IfStmt) End() token.Pos {
+	if s.Else != nil {
+		return s.Else.End()
+	}
+	return s.Body.End()
+}
+func (s *CaseClause) End() token.Pos {
+	if n := len(s.Body); n > 0 {
+		return s.Body[n-1].End()
+	}
+	return s.Colon + 1
+}
+func (s *SwitchStmt) End() token.Pos     { return s.Body.End() }
+func (s *TypeSwitchStmt) End() token.Pos { return s.Body.End() }
+func (s *CommClause) End() token.Pos {
+	if n := len(s.Body); n > 0 {
+		return s.Body[n-1].End()
+	}
+	return s.Colon + 1
+}
+func (s *SelectStmt) End() token.Pos { return s.Body.End() }
+func (s *ForStmt) End() token.Pos    { return s.Body.End() }
+func (s *RangeStmt) End() token.Pos  { return s.Body.End() }
+
+// stmtNode() ensures that only statement nodes can be
+// assigned to a StmtNode.
+//
+func (*BadStmt) stmtNode()        {}
+func (*DeclStmt) stmtNode()       {}
+func (*EmptyStmt) stmtNode()      {}
+func (*LabeledStmt) stmtNode()    {}
+func (*ExprStmt) stmtNode()       {}
+func (*SendStmt) stmtNode()       {}
+func (*IncDecStmt) stmtNode()     {}
+func (*AssignStmt) stmtNode()     {}
+func (*GoStmt) stmtNode()         {}
+func (*DeferStmt) stmtNode()      {}
+func (*ReturnStmt) stmtNode()     {}
+func (*BranchStmt) stmtNode()     {}
+func (*BlockStmt) stmtNode()      {}
+func (*IfStmt) stmtNode()         {}
+func (*CaseClause) stmtNode()     {}
+func (*SwitchStmt) stmtNode()     {}
+func (*TypeSwitchStmt) stmtNode() {}
+func (*CommClause) stmtNode()     {}
+func (*SelectStmt) stmtNode()     {}
+func (*ForStmt) stmtNode()        {}
+func (*RangeStmt) stmtNode()      {}
+
+// ----------------------------------------------------------------------------
+// Declarations
+
+// A Spec node represents a single (non-parenthesized) import,
+// constant, type, or variable declaration.
+//
+type (
+	// The Spec type stands for any of *ImportSpec, *ValueSpec, and *TypeSpec.
+	Spec interface {
+		Node
+		specNode()
+	}
+
+	// An ImportSpec node represents a single package import.
+	ImportSpec struct {
+		Doc     *CommentGroup // associated documentation; or nil
+		Name    *Ident        // local package name (including "."); or nil
+		Path    *BasicLit     // import path
+		Comment *CommentGroup // line comments; or nil
+		EndPos  token.Pos     // end of spec (overrides Path.Pos if nonzero)
+	}
+
+	// A ValueSpec node represents a constant or variable declaration
+	// (ConstSpec or VarSpec production).
+	//
+	ValueSpec struct {
+		Doc     *CommentGroup // associated documentation; or nil
+		Names   []*Ident      // value names (len(Names) > 0)
+		Type    Expr          // value type; or nil
+		Values  []Expr        // initial values; or nil
+		Comment *CommentGroup // line comments; or nil
+	}
+
+	// A TypeSpec node represents a type declaration (TypeSpec production).
+	TypeSpec struct {
+		Doc     *CommentGroup // associated documentation; or nil
+		Name    *Ident        // type name
+		Type    Expr          // *Ident, *ParenExpr, *SelectorExpr, *StarExpr, or any of the *XxxTypes
+		Comment *CommentGroup // line comments; or nil
+	}
+)
+
+// Pos and End implementations for spec nodes.
+//
+func (s *ImportSpec) Pos() token.Pos {
+	if s.Name != nil {
+		return s.Name.Pos()
+	}
+	return s.Path.Pos()
+}
+func (s *ValueSpec) Pos() token.Pos { return s.Names[0].Pos() }
+func (s *TypeSpec) Pos() token.Pos  { return s.Name.Pos() }
+
+func (s *ImportSpec) End() token.Pos {
+	if s.EndPos != 0 {
+		return s.EndPos
+	}
+	return s.Path.End()
+}
+
+func (s *ValueSpec) End() token.Pos {
+	if n := len(s.Values); n > 0 {
+		return s.Values[n-1].End()
+	}
+	if s.Type != nil {
+		return s.Type.End()
+	}
+	return s.Names[len(s.Names)-1].End()
+}
+func (s *TypeSpec) End() token.Pos { return s.Type.End() }
+
+// specNode() ensures that only spec nodes can be
+// assigned to a Spec.
+//
+func (*ImportSpec) specNode() {}
+func (*ValueSpec) specNode()  {}
+func (*TypeSpec) specNode()   {}
+
+// A declaration is represented by one of the following declaration nodes.
+//
+type (
+	// A BadDecl node is a placeholder for declarations containing
+	// syntax errors for which no correct declaration nodes can be
+	// created.
+	//
+	BadDecl struct {
+		From, To token.Pos // position range of bad declaration
+	}
+
+	// A GenDecl node (generic declaration node) represents an import,
+	// constant, type or variable declaration. A valid Lparen position
+	// (Lparen.Line > 0) indicates a parenthesized declaration.
+	//
+	// Relationship between Tok value and Specs element type:
+	//
+	//	token.IMPORT  *ImportSpec
+	//	token.CONST   *ValueSpec
+	//	token.TYPE    *TypeSpec
+	//	token.VAR     *ValueSpec
+	//
+	GenDecl struct {
+		Doc    *CommentGroup // associated documentation; or nil
+		TokPos token.Pos     // position of Tok
+		Tok    token.Token   // IMPORT, CONST, TYPE, VAR
+		Lparen token.Pos     // position of '(', if any
+		Specs  []Spec
+		Rparen token.Pos // position of ')', if any
+	}
+
+	// A FuncDecl node represents a function declaration.
+	FuncDecl struct {
+		Doc  *CommentGroup // associated documentation; or nil
+		Recv *FieldList    // receiver (methods); or nil (functions)
+		Name *Ident        // function/method name
+		Type *FuncType     // function signature: parameters, results, and position of "func" keyword
+		Body *BlockStmt    // function body; or nil (forward declaration)
+	}
+)
+
+// Pos and End implementations for declaration nodes.
+//
+func (d *BadDecl) Pos() token.Pos  { return d.From }
+func (d *GenDecl) Pos() token.Pos  { return d.TokPos }
+func (d *FuncDecl) Pos() token.Pos { return d.Type.Pos() }
+
+func (d *BadDecl) End() token.Pos { return d.To }
+func (d *GenDecl) End() token.Pos {
+	if d.Rparen.IsValid() {
+		return d.Rparen + 1
+	}
+	return d.Specs[0].End()
+}
+func (d *FuncDecl) End() token.Pos {
+	if d.Body != nil {
+		return d.Body.End()
+	}
+	return d.Type.End()
+}
+
+// declNode() ensures that only declaration nodes can be
+// assigned to a DeclNode.
+//
+func (*BadDecl) declNode()  {}
+func (*GenDecl) declNode()  {}
+func (*FuncDecl) declNode() {}
+
+// ----------------------------------------------------------------------------
+// Files and packages
+
+// A File node represents a Go source file.
+//
+// The Comments list contains all comments in the source file in order of
+// appearance, including the comments that are pointed to from other nodes
+// via Doc and Comment fields.
+//
+type File struct {
+	Doc        *CommentGroup   // associated documentation; or nil
+	Package    token.Pos       // position of "package" keyword
+	Name       *Ident          // package name
+	Decls      []Decl          // top-level declarations; or nil
+	Scope      *Scope          // package scope (this file only)
+	Imports    []*ImportSpec   // imports in this file
+	Unresolved []*Ident        // unresolved identifiers in this file
+	Comments   []*CommentGroup // list of all comments in the source file
+}
+
+func (f *File) Pos() token.Pos { return f.Package }
+func (f *File) End() token.Pos {
+	if n := len(f.Decls); n > 0 {
+		return f.Decls[n-1].End()
+	}
+	return f.Name.End()
+}
+
+// A Package node represents a set of source files
+// collectively building a Go package.
+//
+type Package struct {
+	Name    string             // package name
+	Scope   *Scope             // package scope across all files
+	Imports map[string]*Object // map of package id -> package object
+	Files   map[string]*File   // Go source files by filename
+}
+
+func (p *Package) Pos() token.Pos { return token.NoPos }
+func (p *Package) End() token.Pos { return token.NoPos }
diff --git a/src/go/ast/ast_test.go b/src/go/ast/ast_test.go
new file mode 100644
index 000000000..1a6a283f2
--- /dev/null
+++ b/src/go/ast/ast_test.go
@@ -0,0 +1,50 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast
+
+import (
+	"testing"
+)
+
+var comments = []struct {
+	list []string
+	text string
+}{
+	{[]string{"//"}, ""},
+	{[]string{"//   "}, ""},
+	{[]string{"//", "//", "//   "}, ""},
+	{[]string{"// foo   "}, "foo\n"},
+	{[]string{"//", "//", "// foo"}, "foo\n"},
+	{[]string{"// foo  bar  "}, "foo  bar\n"},
+	{[]string{"// foo", "// bar"}, "foo\nbar\n"},
+	{[]string{"// foo", "//", "//", "//", "// bar"}, "foo\n\nbar\n"},
+	{[]string{"// foo", "/* bar */"}, "foo\n bar\n"},
+	{[]string{"//", "//", "//", "// foo", "//", "//", "//"}, "foo\n"},
+
+	{[]string{"/**/"}, ""},
+	{[]string{"/*   */"}, ""},
+	{[]string{"/**/", "/**/", "/*   */"}, ""},
+	{[]string{"/* Foo   */"}, " Foo\n"},
+	{[]string{"/* Foo  Bar  */"}, " Foo  Bar\n"},
+	{[]string{"/* Foo*/", "/* Bar*/"}, " Foo\n Bar\n"},
+	{[]string{"/* Foo*/", "/**/", "/**/", "/**/", "// Bar"}, " Foo\n\nBar\n"},
+	{[]string{"/* Foo*/", "/*\n*/", "//", "/*\n*/", "// Bar"}, " Foo\n\nBar\n"},
+	{[]string{"/* Foo*/", "// Bar"}, " Foo\nBar\n"},
+	{[]string{"/* Foo\n Bar*/"}, " Foo\n Bar\n"},
+}
+
+func TestCommentText(t *testing.T) {
+	for i, c := range comments {
+		list := make([]*Comment, len(c.list))
+		for i, s := range c.list {
+			list[i] = &Comment{Text: s}
+		}
+
+		text := (&CommentGroup{list}).Text()
+		if text != c.text {
+			t.Errorf("case %d: got %q; expected %q", i, text, c.text)
+		}
+	}
+}
diff --git a/src/go/ast/commentmap.go b/src/go/ast/commentmap.go
new file mode 100644
index 000000000..ac999d627
--- /dev/null
+++ b/src/go/ast/commentmap.go
@@ -0,0 +1,332 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast
+
+import (
+	"bytes"
+	"fmt"
+	"go/token"
+	"sort"
+)
+
+type byPos []*CommentGroup
+
+func (a byPos) Len() int           { return len(a) }
+func (a byPos) Less(i, j int) bool { return a[i].Pos() < a[j].Pos() }
+func (a byPos) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+
+// sortComments sorts the list of comment groups in source order.
+//
+func sortComments(list []*CommentGroup) {
+	// TODO(gri): Does it make sense to check for sorted-ness
+	//            first (because we know that sorted-ness is
+	//            very likely)?
+	if orderedList := byPos(list); !sort.IsSorted(orderedList) {
+		sort.Sort(orderedList)
+	}
+}
+
+// A CommentMap maps an AST node to a list of comment groups
+// associated with it. See NewCommentMap for a description of
+// the association.
+//
+type CommentMap map[Node][]*CommentGroup
+
+func (cmap CommentMap) addComment(n Node, c *CommentGroup) {
+	list := cmap[n]
+	if len(list) == 0 {
+		list = []*CommentGroup{c}
+	} else {
+		list = append(list, c)
+	}
+	cmap[n] = list
+}
+
+type byInterval []Node
+
+func (a byInterval) Len() int { return len(a) }
+func (a byInterval) Less(i, j int) bool {
+	pi, pj := a[i].Pos(), a[j].Pos()
+	return pi < pj || pi == pj && a[i].End() > a[j].End()
+}
+func (a byInterval) Swap(i, j int) { a[i], a[j] = a[j], a[i] }
+
+// nodeList returns the list of nodes of the AST n in source order.
+//
+func nodeList(n Node) []Node {
+	var list []Node
+	Inspect(n, func(n Node) bool {
+		// don't collect comments
+		switch n.(type) {
+		case nil, *CommentGroup, *Comment:
+			return false
+		}
+		list = append(list, n)
+		return true
+	})
+	// Note: The current implementation assumes that Inspect traverses the
+	//       AST in depth-first and thus _source_ order. If AST traversal
+	//       does not follow source order, the sorting call below will be
+	//       required.
+	// sort.Sort(byInterval(list))
+	return list
+}
+
+// A commentListReader helps iterating through a list of comment groups.
+//
+type commentListReader struct {
+	fset     *token.FileSet
+	list     []*CommentGroup
+	index    int
+	comment  *CommentGroup  // comment group at current index
+	pos, end token.Position // source interval of comment group at current index
+}
+
+func (r *commentListReader) eol() bool {
+	return r.index >= len(r.list)
+}
+
+func (r *commentListReader) next() {
+	if !r.eol() {
+		r.comment = r.list[r.index]
+		r.pos = r.fset.Position(r.comment.Pos())
+		r.end = r.fset.Position(r.comment.End())
+		r.index++
+	}
+}
+
+// A nodeStack keeps track of nested nodes.
+// A node lower on the stack lexically contains the nodes higher on the stack.
+//
+type nodeStack []Node
+
+// push pops all nodes that appear lexically before n
+// and then pushes n on the stack.
+//
+func (s *nodeStack) push(n Node) {
+	s.pop(n.Pos())
+	*s = append((*s), n)
+}
+
+// pop pops all nodes that appear lexically before pos
+// (i.e., whose lexical extent has ended before or at pos).
+// It returns the last node popped.
+//
+func (s *nodeStack) pop(pos token.Pos) (top Node) {
+	i := len(*s)
+	for i > 0 && (*s)[i-1].End() <= pos {
+		top = (*s)[i-1]
+		i--
+	}
+	*s = (*s)[0:i]
+	return top
+}
+
+// NewCommentMap creates a new comment map by associating comment groups
+// of the comments list with the nodes of the AST specified by node.
+//
+// A comment group g is associated with a node n if:
+//
+//   - g starts on the same line as n ends
+//   - g starts on the line immediately following n, and there is
+//     at least one empty line after g and before the next node
+//   - g starts before n and is not associated to the node before n
+//     via the previous rules
+//
+// NewCommentMap tries to associate a comment group to the "largest"
+// node possible: For instance, if the comment is a line comment
+// trailing an assignment, the comment is associated with the entire
+// assignment rather than just the last operand in the assignment.
+//
+func NewCommentMap(fset *token.FileSet, node Node, comments []*CommentGroup) CommentMap {
+	if len(comments) == 0 {
+		return nil // no comments to map
+	}
+
+	cmap := make(CommentMap)
+
+	// set up comment reader r
+	tmp := make([]*CommentGroup, len(comments))
+	copy(tmp, comments) // don't change incoming comments
+	sortComments(tmp)
+	r := commentListReader{fset: fset, list: tmp} // !r.eol() because len(comments) > 0
+	r.next()
+
+	// create node list in lexical order
+	nodes := nodeList(node)
+	nodes = append(nodes, nil) // append sentinel
+
+	// set up iteration variables
+	var (
+		p     Node           // previous node
+		pend  token.Position // end of p
+		pg    Node           // previous node group (enclosing nodes of "importance")
+		pgend token.Position // end of pg
+		stack nodeStack      // stack of node groups
+	)
+
+	for _, q := range nodes {
+		var qpos token.Position
+		if q != nil {
+			qpos = fset.Position(q.Pos()) // current node position
+		} else {
+			// set fake sentinel position to infinity so that
+			// all comments get processed before the sentinel
+			const infinity = 1 << 30
+			qpos.Offset = infinity
+			qpos.Line = infinity
+		}
+
+		// process comments before current node
+		for r.end.Offset <= qpos.Offset {
+			// determine recent node group
+			if top := stack.pop(r.comment.Pos()); top != nil {
+				pg = top
+				pgend = fset.Position(pg.End())
+			}
+			// Try to associate a comment first with a node group
+			// (i.e., a node of "importance" such as a declaration);
+			// if that fails, try to associate it with the most recent
+			// node.
+			// TODO(gri) try to simplify the logic below
+			var assoc Node
+			switch {
+			case pg != nil &&
+				(pgend.Line == r.pos.Line ||
+					pgend.Line+1 == r.pos.Line && r.end.Line+1 < qpos.Line):
+				// 1) comment starts on same line as previous node group ends, or
+				// 2) comment starts on the line immediately after the
+				//    previous node group and there is an empty line before
+				//    the current node
+				// => associate comment with previous node group
+				assoc = pg
+			case p != nil &&
+				(pend.Line == r.pos.Line ||
+					pend.Line+1 == r.pos.Line && r.end.Line+1 < qpos.Line ||
+					q == nil):
+				// same rules apply as above for p rather than pg,
+				// but also associate with p if we are at the end (q == nil)
+				assoc = p
+			default:
+				// otherwise, associate comment with current node
+				if q == nil {
+					// we can only reach here if there was no p
+					// which would imply that there were no nodes
+					panic("internal error: no comments should be associated with sentinel")
+				}
+				assoc = q
+			}
+			cmap.addComment(assoc, r.comment)
+			if r.eol() {
+				return cmap
+			}
+			r.next()
+		}
+
+		// update previous node
+		p = q
+		pend = fset.Position(p.End())
+
+		// update previous node group if we see an "important" node
+		switch q.(type) {
+		case *File, *Field, Decl, Spec, Stmt:
+			stack.push(q)
+		}
+	}
+
+	return cmap
+}
+
+// Update replaces an old node in the comment map with the new node
+// and returns the new node. Comments that were associated with the
+// old node are associated with the new node.
+//
+func (cmap CommentMap) Update(old, new Node) Node {
+	if list := cmap[old]; len(list) > 0 {
+		delete(cmap, old)
+		cmap[new] = append(cmap[new], list...)
+	}
+	return new
+}
+
+// Filter returns a new comment map consisting of only those
+// entries of cmap for which a corresponding node exists in
+// the AST specified by node.
+//
+func (cmap CommentMap) Filter(node Node) CommentMap {
+	umap := make(CommentMap)
+	Inspect(node, func(n Node) bool {
+		if g := cmap[n]; len(g) > 0 {
+			umap[n] = g
+		}
+		return true
+	})
+	return umap
+}
+
+// Comments returns the list of comment groups in the comment map.
+// The result is sorted is source order.
+//
+func (cmap CommentMap) Comments() []*CommentGroup {
+	list := make([]*CommentGroup, 0, len(cmap))
+	for _, e := range cmap {
+		list = append(list, e...)
+	}
+	sortComments(list)
+	return list
+}
+
+func summary(list []*CommentGroup) string {
+	const maxLen = 40
+	var buf bytes.Buffer
+
+	// collect comments text
+loop:
+	for _, group := range list {
+		// Note: CommentGroup.Text() does too much work for what we
+		//       need and would only replace this innermost loop.
+		//       Just do it explicitly.
+		for _, comment := range group.List {
+			if buf.Len() >= maxLen {
+				break loop
+			}
+			buf.WriteString(comment.Text)
+		}
+	}
+
+	// truncate if too long
+	if buf.Len() > maxLen {
+		buf.Truncate(maxLen - 3)
+		buf.WriteString("...")
+	}
+
+	// replace any invisibles with blanks
+	bytes := buf.Bytes()
+	for i, b := range bytes {
+		switch b {
+		case '\t', '\n', '\r':
+			bytes[i] = ' '
+		}
+	}
+
+	return string(bytes)
+}
+
+func (cmap CommentMap) String() string {
+	var buf bytes.Buffer
+	fmt.Fprintln(&buf, "CommentMap {")
+	for node, comment := range cmap {
+		// print name of identifiers; print node type for other nodes
+		var s string
+		if ident, ok := node.(*Ident); ok {
+			s = ident.Name
+		} else {
+			s = fmt.Sprintf("%T", node)
+		}
+		fmt.Fprintf(&buf, "\t%p  %20s:  %s\n", node, s, summary(comment))
+	}
+	fmt.Fprintln(&buf, "}")
+	return buf.String()
+}
diff --git a/src/go/ast/commentmap_test.go b/src/go/ast/commentmap_test.go
new file mode 100644
index 000000000..e372eab74
--- /dev/null
+++ b/src/go/ast/commentmap_test.go
@@ -0,0 +1,143 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// To avoid a cyclic dependency with go/parser, this file is in a separate package.
+
+package ast_test
+
+import (
+	"bytes"
+	"fmt"
+	. "go/ast"
+	"go/parser"
+	"go/token"
+	"sort"
+	"testing"
+)
+
+const src = `
+// the very first comment
+
+// package p
+package p /* the name is p */
+
+// imports
+import (
+	"bytes"     // bytes
+	"fmt"       // fmt
+	"go/ast"
+	"go/parser"
+)
+
+// T
+type T struct {
+	a, b, c int // associated with a, b, c
+	// associated with x, y
+	x, y float64    // float values
+	z    complex128 // complex value
+}
+// also associated with T
+
+// x
+var x = 0 // x = 0
+// also associated with x
+
+// f1
+func f1() {
+	/* associated with s1 */
+	s1()
+	// also associated with s1
+	
+	// associated with s2
+	
+	// also associated with s2
+	s2() // line comment for s2
+}
+// associated with f1
+// also associated with f1
+
+// associated with f2
+
+// f2
+func f2() {
+}
+
+func f3() {
+	i := 1 /* 1 */ + 2 // addition
+	_ = i
+}
+
+// the very last comment
+`
+
+// res maps a key of the form "line number: node type"
+// to the associated comments' text.
+//
+var res = map[string]string{
+	" 5: *ast.File":       "the very first comment\npackage p\n",
+	" 5: *ast.Ident":      " the name is p\n",
+	" 8: *ast.GenDecl":    "imports\n",
+	" 9: *ast.ImportSpec": "bytes\n",
+	"10: *ast.ImportSpec": "fmt\n",
+	"16: *ast.GenDecl":    "T\nalso associated with T\n",
+	"17: *ast.Field":      "associated with a, b, c\n",
+	"19: *ast.Field":      "associated with x, y\nfloat values\n",
+	"20: *ast.Field":      "complex value\n",
+	"25: *ast.GenDecl":    "x\nx = 0\nalso associated with x\n",
+	"29: *ast.FuncDecl":   "f1\nassociated with f1\nalso associated with f1\n",
+	"31: *ast.ExprStmt":   " associated with s1\nalso associated with s1\n",
+	"37: *ast.ExprStmt":   "associated with s2\nalso associated with s2\nline comment for s2\n",
+	"45: *ast.FuncDecl":   "associated with f2\nf2\n",
+	"49: *ast.AssignStmt": "addition\n",
+	"49: *ast.BasicLit":   " 1\n",
+	"50: *ast.Ident":      "the very last comment\n",
+}
+
+func ctext(list []*CommentGroup) string {
+	var buf bytes.Buffer
+	for _, g := range list {
+		buf.WriteString(g.Text())
+	}
+	return buf.String()
+}
+
+func TestCommentMap(t *testing.T) {
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	cmap := NewCommentMap(fset, f, f.Comments)
+
+	// very correct association of comments
+	for n, list := range cmap {
+		key := fmt.Sprintf("%2d: %T", fset.Position(n.Pos()).Line, n)
+		got := ctext(list)
+		want := res[key]
+		if got != want {
+			t.Errorf("%s: got %q; want %q", key, got, want)
+		}
+	}
+
+	// verify that no comments got lost
+	if n := len(cmap.Comments()); n != len(f.Comments) {
+		t.Errorf("got %d comment groups in map; want %d", n, len(f.Comments))
+	}
+
+	// support code to update test:
+	// set genMap to true to generate res map
+	const genMap = false
+	if genMap {
+		out := make([]string, 0, len(cmap))
+		for n, list := range cmap {
+			out = append(out, fmt.Sprintf("\t\"%2d: %T\":\t%q,", fset.Position(n.Pos()).Line, n, ctext(list)))
+		}
+		sort.Strings(out)
+		for _, s := range out {
+			fmt.Println(s)
+		}
+	}
+}
+
+// TODO(gri): add tests for Filter.
diff --git a/src/go/ast/example_test.go b/src/go/ast/example_test.go
new file mode 100644
index 000000000..d2e734f2c
--- /dev/null
+++ b/src/go/ast/example_test.go
@@ -0,0 +1,210 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast_test
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/format"
+	"go/parser"
+	"go/token"
+)
+
+// This example demonstrates how to inspect the AST of a Go program.
+func ExampleInspect() {
+	// src is the input for which we want to inspect the AST.
+	src := `
+package p
+const c = 1.0
+var X = f(3.14)*2 + c
+`
+
+	// Create the AST by parsing src.
+	fset := token.NewFileSet() // positions are relative to fset
+	f, err := parser.ParseFile(fset, "src.go", src, 0)
+	if err != nil {
+		panic(err)
+	}
+
+	// Inspect the AST and print all identifiers and literals.
+	ast.Inspect(f, func(n ast.Node) bool {
+		var s string
+		switch x := n.(type) {
+		case *ast.BasicLit:
+			s = x.Value
+		case *ast.Ident:
+			s = x.Name
+		}
+		if s != "" {
+			fmt.Printf("%s:\t%s\n", fset.Position(n.Pos()), s)
+		}
+		return true
+	})
+
+	// output:
+	// src.go:2:9:	p
+	// src.go:3:7:	c
+	// src.go:3:11:	1.0
+	// src.go:4:5:	X
+	// src.go:4:9:	f
+	// src.go:4:11:	3.14
+	// src.go:4:17:	2
+	// src.go:4:21:	c
+}
+
+// This example shows what an AST looks like when printed for debugging.
+func ExamplePrint() {
+	// src is the input for which we want to print the AST.
+	src := `
+package main
+func main() {
+	println("Hello, World!")
+}
+`
+
+	// Create the AST by parsing src.
+	fset := token.NewFileSet() // positions are relative to fset
+	f, err := parser.ParseFile(fset, "", src, 0)
+	if err != nil {
+		panic(err)
+	}
+
+	// Print the AST.
+	ast.Print(fset, f)
+
+	// output:
+	//      0  *ast.File {
+	//      1  .  Package: 2:1
+	//      2  .  Name: *ast.Ident {
+	//      3  .  .  NamePos: 2:9
+	//      4  .  .  Name: "main"
+	//      5  .  }
+	//      6  .  Decls: []ast.Decl (len = 1) {
+	//      7  .  .  0: *ast.FuncDecl {
+	//      8  .  .  .  Name: *ast.Ident {
+	//      9  .  .  .  .  NamePos: 3:6
+	//     10  .  .  .  .  Name: "main"
+	//     11  .  .  .  .  Obj: *ast.Object {
+	//     12  .  .  .  .  .  Kind: func
+	//     13  .  .  .  .  .  Name: "main"
+	//     14  .  .  .  .  .  Decl: *(obj @ 7)
+	//     15  .  .  .  .  }
+	//     16  .  .  .  }
+	//     17  .  .  .  Type: *ast.FuncType {
+	//     18  .  .  .  .  Func: 3:1
+	//     19  .  .  .  .  Params: *ast.FieldList {
+	//     20  .  .  .  .  .  Opening: 3:10
+	//     21  .  .  .  .  .  Closing: 3:11
+	//     22  .  .  .  .  }
+	//     23  .  .  .  }
+	//     24  .  .  .  Body: *ast.BlockStmt {
+	//     25  .  .  .  .  Lbrace: 3:13
+	//     26  .  .  .  .  List: []ast.Stmt (len = 1) {
+	//     27  .  .  .  .  .  0: *ast.ExprStmt {
+	//     28  .  .  .  .  .  .  X: *ast.CallExpr {
+	//     29  .  .  .  .  .  .  .  Fun: *ast.Ident {
+	//     30  .  .  .  .  .  .  .  .  NamePos: 4:2
+	//     31  .  .  .  .  .  .  .  .  Name: "println"
+	//     32  .  .  .  .  .  .  .  }
+	//     33  .  .  .  .  .  .  .  Lparen: 4:9
+	//     34  .  .  .  .  .  .  .  Args: []ast.Expr (len = 1) {
+	//     35  .  .  .  .  .  .  .  .  0: *ast.BasicLit {
+	//     36  .  .  .  .  .  .  .  .  .  ValuePos: 4:10
+	//     37  .  .  .  .  .  .  .  .  .  Kind: STRING
+	//     38  .  .  .  .  .  .  .  .  .  Value: "\"Hello, World!\""
+	//     39  .  .  .  .  .  .  .  .  }
+	//     40  .  .  .  .  .  .  .  }
+	//     41  .  .  .  .  .  .  .  Ellipsis: -
+	//     42  .  .  .  .  .  .  .  Rparen: 4:25
+	//     43  .  .  .  .  .  .  }
+	//     44  .  .  .  .  .  }
+	//     45  .  .  .  .  }
+	//     46  .  .  .  .  Rbrace: 5:1
+	//     47  .  .  .  }
+	//     48  .  .  }
+	//     49  .  }
+	//     50  .  Scope: *ast.Scope {
+	//     51  .  .  Objects: map[string]*ast.Object (len = 1) {
+	//     52  .  .  .  "main": *(obj @ 11)
+	//     53  .  .  }
+	//     54  .  }
+	//     55  .  Unresolved: []*ast.Ident (len = 1) {
+	//     56  .  .  0: *(obj @ 29)
+	//     57  .  }
+	//     58  }
+}
+
+// This example illustrates how to remove a variable declaration
+// in a Go program while maintaining correct comment association
+// using an ast.CommentMap.
+func ExampleCommentMap() {
+	// src is the input for which we create the AST that we
+	// are going to manipulate.
+	src := `
+// This is the package comment.
+package main
+
+// This comment is associated with the hello constant.
+const hello = "Hello, World!" // line comment 1
+
+// This comment is associated with the foo variable.
+var foo = hello // line comment 2 
+
+// This comment is associated with the main function.
+func main() {
+	fmt.Println(hello) // line comment 3
+}
+`
+
+	// Create the AST by parsing src.
+	fset := token.NewFileSet() // positions are relative to fset
+	f, err := parser.ParseFile(fset, "src.go", src, parser.ParseComments)
+	if err != nil {
+		panic(err)
+	}
+
+	// Create an ast.CommentMap from the ast.File's comments.
+	// This helps keeping the association between comments
+	// and AST nodes.
+	cmap := ast.NewCommentMap(fset, f, f.Comments)
+
+	// Remove the first variable declaration from the list of declarations.
+	f.Decls = removeFirstVarDecl(f.Decls)
+
+	// Use the comment map to filter comments that don't belong anymore
+	// (the comments associated with the variable declaration), and create
+	// the new comments list.
+	f.Comments = cmap.Filter(f).Comments()
+
+	// Print the modified AST.
+	var buf bytes.Buffer
+	if err := format.Node(&buf, fset, f); err != nil {
+		panic(err)
+	}
+	fmt.Printf("%s", buf.Bytes())
+
+	// output:
+	// // This is the package comment.
+	// package main
+	//
+	// // This comment is associated with the hello constant.
+	// const hello = "Hello, World!" // line comment 1
+	//
+	// // This comment is associated with the main function.
+	// func main() {
+	// 	fmt.Println(hello) // line comment 3
+	// }
+}
+
+func removeFirstVarDecl(list []ast.Decl) []ast.Decl {
+	for i, decl := range list {
+		if gen, ok := decl.(*ast.GenDecl); ok && gen.Tok == token.VAR {
+			copy(list[i:], list[i+1:])
+			return list[:len(list)-1]
+		}
+	}
+	panic("variable declaration not found")
+}
diff --git a/src/go/ast/filter.go b/src/go/ast/filter.go
new file mode 100644
index 000000000..fc3eeb4a1
--- /dev/null
+++ b/src/go/ast/filter.go
@@ -0,0 +1,466 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast
+
+import (
+	"go/token"
+	"sort"
+)
+
+// ----------------------------------------------------------------------------
+// Export filtering
+
+// exportFilter is a special filter function to extract exported nodes.
+func exportFilter(name string) bool {
+	return IsExported(name)
+}
+
+// FileExports trims the AST for a Go source file in place such that
+// only exported nodes remain: all top-level identifiers which are not exported
+// and their associated information (such as type, initial value, or function
+// body) are removed. Non-exported fields and methods of exported types are
+// stripped. The File.Comments list is not changed.
+//
+// FileExports returns true if there are exported declarations;
+// it returns false otherwise.
+//
+func FileExports(src *File) bool {
+	return filterFile(src, exportFilter, true)
+}
+
+// PackageExports trims the AST for a Go package in place such that
+// only exported nodes remain. The pkg.Files list is not changed, so that
+// file names and top-level package comments don't get lost.
+//
+// PackageExports returns true if there are exported declarations;
+// it returns false otherwise.
+//
+func PackageExports(pkg *Package) bool {
+	return filterPackage(pkg, exportFilter, true)
+}
+
+// ----------------------------------------------------------------------------
+// General filtering
+
+type Filter func(string) bool
+
+func filterIdentList(list []*Ident, f Filter) []*Ident {
+	j := 0
+	for _, x := range list {
+		if f(x.Name) {
+			list[j] = x
+			j++
+		}
+	}
+	return list[0:j]
+}
+
+// fieldName assumes that x is the type of an anonymous field and
+// returns the corresponding field name. If x is not an acceptable
+// anonymous field, the result is nil.
+//
+func fieldName(x Expr) *Ident {
+	switch t := x.(type) {
+	case *Ident:
+		return t
+	case *SelectorExpr:
+		if _, ok := t.X.(*Ident); ok {
+			return t.Sel
+		}
+	case *StarExpr:
+		return fieldName(t.X)
+	}
+	return nil
+}
+
+func filterFieldList(fields *FieldList, filter Filter, export bool) (removedFields bool) {
+	if fields == nil {
+		return false
+	}
+	list := fields.List
+	j := 0
+	for _, f := range list {
+		keepField := false
+		if len(f.Names) == 0 {
+			// anonymous field
+			name := fieldName(f.Type)
+			keepField = name != nil && filter(name.Name)
+		} else {
+			n := len(f.Names)
+			f.Names = filterIdentList(f.Names, filter)
+			if len(f.Names) < n {
+				removedFields = true
+			}
+			keepField = len(f.Names) > 0
+		}
+		if keepField {
+			if export {
+				filterType(f.Type, filter, export)
+			}
+			list[j] = f
+			j++
+		}
+	}
+	if j < len(list) {
+		removedFields = true
+	}
+	fields.List = list[0:j]
+	return
+}
+
+func filterParamList(fields *FieldList, filter Filter, export bool) bool {
+	if fields == nil {
+		return false
+	}
+	var b bool
+	for _, f := range fields.List {
+		if filterType(f.Type, filter, export) {
+			b = true
+		}
+	}
+	return b
+}
+
+func filterType(typ Expr, f Filter, export bool) bool {
+	switch t := typ.(type) {
+	case *Ident:
+		return f(t.Name)
+	case *ParenExpr:
+		return filterType(t.X, f, export)
+	case *ArrayType:
+		return filterType(t.Elt, f, export)
+	case *StructType:
+		if filterFieldList(t.Fields, f, export) {
+			t.Incomplete = true
+		}
+		return len(t.Fields.List) > 0
+	case *FuncType:
+		b1 := filterParamList(t.Params, f, export)
+		b2 := filterParamList(t.Results, f, export)
+		return b1 || b2
+	case *InterfaceType:
+		if filterFieldList(t.Methods, f, export) {
+			t.Incomplete = true
+		}
+		return len(t.Methods.List) > 0
+	case *MapType:
+		b1 := filterType(t.Key, f, export)
+		b2 := filterType(t.Value, f, export)
+		return b1 || b2
+	case *ChanType:
+		return filterType(t.Value, f, export)
+	}
+	return false
+}
+
+func filterSpec(spec Spec, f Filter, export bool) bool {
+	switch s := spec.(type) {
+	case *ValueSpec:
+		s.Names = filterIdentList(s.Names, f)
+		if len(s.Names) > 0 {
+			if export {
+				filterType(s.Type, f, export)
+			}
+			return true
+		}
+	case *TypeSpec:
+		if f(s.Name.Name) {
+			if export {
+				filterType(s.Type, f, export)
+			}
+			return true
+		}
+		if !export {
+			// For general filtering (not just exports),
+			// filter type even if name is not filtered
+			// out.
+			// If the type contains filtered elements,
+			// keep the declaration.
+			return filterType(s.Type, f, export)
+		}
+	}
+	return false
+}
+
+func filterSpecList(list []Spec, f Filter, export bool) []Spec {
+	j := 0
+	for _, s := range list {
+		if filterSpec(s, f, export) {
+			list[j] = s
+			j++
+		}
+	}
+	return list[0:j]
+}
+
+// FilterDecl trims the AST for a Go declaration in place by removing
+// all names (including struct field and interface method names, but
+// not from parameter lists) that don't pass through the filter f.
+//
+// FilterDecl returns true if there are any declared names left after
+// filtering; it returns false otherwise.
+//
+func FilterDecl(decl Decl, f Filter) bool {
+	return filterDecl(decl, f, false)
+}
+
+func filterDecl(decl Decl, f Filter, export bool) bool {
+	switch d := decl.(type) {
+	case *GenDecl:
+		d.Specs = filterSpecList(d.Specs, f, export)
+		return len(d.Specs) > 0
+	case *FuncDecl:
+		return f(d.Name.Name)
+	}
+	return false
+}
+
+// FilterFile trims the AST for a Go file in place by removing all
+// names from top-level declarations (including struct field and
+// interface method names, but not from parameter lists) that don't
+// pass through the filter f. If the declaration is empty afterwards,
+// the declaration is removed from the AST. The File.Comments list
+// is not changed.
+//
+// FilterFile returns true if there are any top-level declarations
+// left after filtering; it returns false otherwise.
+//
+func FilterFile(src *File, f Filter) bool {
+	return filterFile(src, f, false)
+}
+
+func filterFile(src *File, f Filter, export bool) bool {
+	j := 0
+	for _, d := range src.Decls {
+		if filterDecl(d, f, export) {
+			src.Decls[j] = d
+			j++
+		}
+	}
+	src.Decls = src.Decls[0:j]
+	return j > 0
+}
+
+// FilterPackage trims the AST for a Go package in place by removing
+// all names from top-level declarations (including struct field and
+// interface method names, but not from parameter lists) that don't
+// pass through the filter f. If the declaration is empty afterwards,
+// the declaration is removed from the AST. The pkg.Files list is not
+// changed, so that file names and top-level package comments don't get
+// lost.
+//
+// FilterPackage returns true if there are any top-level declarations
+// left after filtering; it returns false otherwise.
+//
+func FilterPackage(pkg *Package, f Filter) bool {
+	return filterPackage(pkg, f, false)
+}
+
+func filterPackage(pkg *Package, f Filter, export bool) bool {
+	hasDecls := false
+	for _, src := range pkg.Files {
+		if filterFile(src, f, export) {
+			hasDecls = true
+		}
+	}
+	return hasDecls
+}
+
+// ----------------------------------------------------------------------------
+// Merging of package files
+
+// The MergeMode flags control the behavior of MergePackageFiles.
+type MergeMode uint
+
+const (
+	// If set, duplicate function declarations are excluded.
+	FilterFuncDuplicates MergeMode = 1 << iota
+	// If set, comments that are not associated with a specific
+	// AST node (as Doc or Comment) are excluded.
+	FilterUnassociatedComments
+	// If set, duplicate import declarations are excluded.
+	FilterImportDuplicates
+)
+
+// nameOf returns the function (foo) or method name (foo.bar) for
+// the given function declaration. If the AST is incorrect for the
+// receiver, it assumes a function instead.
+//
+func nameOf(f *FuncDecl) string {
+	if r := f.Recv; r != nil && len(r.List) == 1 {
+		// looks like a correct receiver declaration
+		t := r.List[0].Type
+		// dereference pointer receiver types
+		if p, _ := t.(*StarExpr); p != nil {
+			t = p.X
+		}
+		// the receiver type must be a type name
+		if p, _ := t.(*Ident); p != nil {
+			return p.Name + "." + f.Name.Name
+		}
+		// otherwise assume a function instead
+	}
+	return f.Name.Name
+}
+
+// separator is an empty //-style comment that is interspersed between
+// different comment groups when they are concatenated into a single group
+//
+var separator = &Comment{token.NoPos, "//"}
+
+// MergePackageFiles creates a file AST by merging the ASTs of the
+// files belonging to a package. The mode flags control merging behavior.
+//
+func MergePackageFiles(pkg *Package, mode MergeMode) *File {
+	// Count the number of package docs, comments and declarations across
+	// all package files. Also, compute sorted list of filenames, so that
+	// subsequent iterations can always iterate in the same order.
+	ndocs := 0
+	ncomments := 0
+	ndecls := 0
+	filenames := make([]string, len(pkg.Files))
+	i := 0
+	for filename, f := range pkg.Files {
+		filenames[i] = filename
+		i++
+		if f.Doc != nil {
+			ndocs += len(f.Doc.List) + 1 // +1 for separator
+		}
+		ncomments += len(f.Comments)
+		ndecls += len(f.Decls)
+	}
+	sort.Strings(filenames)
+
+	// Collect package comments from all package files into a single
+	// CommentGroup - the collected package documentation. In general
+	// there should be only one file with a package comment; but it's
+	// better to collect extra comments than drop them on the floor.
+	var doc *CommentGroup
+	var pos token.Pos
+	if ndocs > 0 {
+		list := make([]*Comment, ndocs-1) // -1: no separator before first group
+		i := 0
+		for _, filename := range filenames {
+			f := pkg.Files[filename]
+			if f.Doc != nil {
+				if i > 0 {
+					// not the first group - add separator
+					list[i] = separator
+					i++
+				}
+				for _, c := range f.Doc.List {
+					list[i] = c
+					i++
+				}
+				if f.Package > pos {
+					// Keep the maximum package clause position as
+					// position for the package clause of the merged
+					// files.
+					pos = f.Package
+				}
+			}
+		}
+		doc = &CommentGroup{list}
+	}
+
+	// Collect declarations from all package files.
+	var decls []Decl
+	if ndecls > 0 {
+		decls = make([]Decl, ndecls)
+		funcs := make(map[string]int) // map of func name -> decls index
+		i := 0                        // current index
+		n := 0                        // number of filtered entries
+		for _, filename := range filenames {
+			f := pkg.Files[filename]
+			for _, d := range f.Decls {
+				if mode&FilterFuncDuplicates != 0 {
+					// A language entity may be declared multiple
+					// times in different package files; only at
+					// build time declarations must be unique.
+					// For now, exclude multiple declarations of
+					// functions - keep the one with documentation.
+					//
+					// TODO(gri): Expand this filtering to other
+					//            entities (const, type, vars) if
+					//            multiple declarations are common.
+					if f, isFun := d.(*FuncDecl); isFun {
+						name := nameOf(f)
+						if j, exists := funcs[name]; exists {
+							// function declared already
+							if decls[j] != nil && decls[j].(*FuncDecl).Doc == nil {
+								// existing declaration has no documentation;
+								// ignore the existing declaration
+								decls[j] = nil
+							} else {
+								// ignore the new declaration
+								d = nil
+							}
+							n++ // filtered an entry
+						} else {
+							funcs[name] = i
+						}
+					}
+				}
+				decls[i] = d
+				i++
+			}
+		}
+
+		// Eliminate nil entries from the decls list if entries were
+		// filtered. We do this using a 2nd pass in order to not disturb
+		// the original declaration order in the source (otherwise, this
+		// would also invalidate the monotonically increasing position
+		// info within a single file).
+		if n > 0 {
+			i = 0
+			for _, d := range decls {
+				if d != nil {
+					decls[i] = d
+					i++
+				}
+			}
+			decls = decls[0:i]
+		}
+	}
+
+	// Collect import specs from all package files.
+	var imports []*ImportSpec
+	if mode&FilterImportDuplicates != 0 {
+		seen := make(map[string]bool)
+		for _, filename := range filenames {
+			f := pkg.Files[filename]
+			for _, imp := range f.Imports {
+				if path := imp.Path.Value; !seen[path] {
+					// TODO: consider handling cases where:
+					// - 2 imports exist with the same import path but
+					//   have different local names (one should probably
+					//   keep both of them)
+					// - 2 imports exist but only one has a comment
+					// - 2 imports exist and they both have (possibly
+					//   different) comments
+					imports = append(imports, imp)
+					seen[path] = true
+				}
+			}
+		}
+	} else {
+		for _, f := range pkg.Files {
+			imports = append(imports, f.Imports...)
+		}
+	}
+
+	// Collect comments from all package files.
+	var comments []*CommentGroup
+	if mode&FilterUnassociatedComments == 0 {
+		comments = make([]*CommentGroup, ncomments)
+		i := 0
+		for _, f := range pkg.Files {
+			i += copy(comments[i:], f.Comments)
+		}
+	}
+
+	// TODO(gri) need to compute unresolved identifiers!
+	return &File{doc, pos, NewIdent(pkg.Name), decls, pkg.Scope, imports, nil, comments}
+}
diff --git a/src/go/ast/filter_test.go b/src/go/ast/filter_test.go
new file mode 100644
index 000000000..9fd86cb46
--- /dev/null
+++ b/src/go/ast/filter_test.go
@@ -0,0 +1,86 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// To avoid a cyclic dependency with go/parser, this file is in a separate package.
+
+package ast_test
+
+import (
+	"bytes"
+	"go/ast"
+	"go/format"
+	"go/parser"
+	"go/token"
+	"testing"
+)
+
+const input = `package p
+
+type t1 struct{}
+type t2 struct{}
+
+func f1() {}
+func f1() {}
+func f2() {}
+
+func (*t1) f1() {}
+func (t1) f1() {}
+func (t1) f2() {}
+
+func (t2) f1() {}
+func (t2) f2() {}
+func (x *t2) f2() {}
+`
+
+// Calling ast.MergePackageFiles with ast.FilterFuncDuplicates
+// keeps a duplicate entry with attached documentation in favor
+// of one without, and it favors duplicate entries appearing
+// later in the source over ones appearing earlier. This is why
+// (*t2).f2 is kept and t2.f2 is eliminated in this test case.
+//
+const golden = `package p
+
+type t1 struct{}
+type t2 struct{}
+
+func f1() {}
+func f2() {}
+
+func (t1) f1() {}
+func (t1) f2() {}
+
+func (t2) f1() {}
+
+func (x *t2) f2() {}
+`
+
+func TestFilterDuplicates(t *testing.T) {
+	// parse input
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, "", input, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// create package
+	files := map[string]*ast.File{"": file}
+	pkg, err := ast.NewPackage(fset, files, nil, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// filter
+	merged := ast.MergePackageFiles(pkg, ast.FilterFuncDuplicates)
+
+	// pretty-print
+	var buf bytes.Buffer
+	if err := format.Node(&buf, fset, merged); err != nil {
+		t.Fatal(err)
+	}
+	output := buf.String()
+
+	if output != golden {
+		t.Errorf("incorrect output:\n%s", output)
+	}
+}
diff --git a/src/go/ast/import.go b/src/go/ast/import.go
new file mode 100644
index 000000000..d2770d16c
--- /dev/null
+++ b/src/go/ast/import.go
@@ -0,0 +1,196 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast
+
+import (
+	"go/token"
+	"sort"
+	"strconv"
+)
+
+// SortImports sorts runs of consecutive import lines in import blocks in f.
+// It also removes duplicate imports when it is possible to do so without data loss.
+func SortImports(fset *token.FileSet, f *File) {
+	for _, d := range f.Decls {
+		d, ok := d.(*GenDecl)
+		if !ok || d.Tok != token.IMPORT {
+			// Not an import declaration, so we're done.
+			// Imports are always first.
+			break
+		}
+
+		if !d.Lparen.IsValid() {
+			// Not a block: sorted by default.
+			continue
+		}
+
+		// Identify and sort runs of specs on successive lines.
+		i := 0
+		specs := d.Specs[:0]
+		for j, s := range d.Specs {
+			if j > i && fset.Position(s.Pos()).Line > 1+fset.Position(d.Specs[j-1].End()).Line {
+				// j begins a new run.  End this one.
+				specs = append(specs, sortSpecs(fset, f, d.Specs[i:j])...)
+				i = j
+			}
+		}
+		specs = append(specs, sortSpecs(fset, f, d.Specs[i:])...)
+		d.Specs = specs
+
+		// Deduping can leave a blank line before the rparen; clean that up.
+		if len(d.Specs) > 0 {
+			lastSpec := d.Specs[len(d.Specs)-1]
+			lastLine := fset.Position(lastSpec.Pos()).Line
+			if rParenLine := fset.Position(d.Rparen).Line; rParenLine > lastLine+1 {
+				fset.File(d.Rparen).MergeLine(rParenLine - 1)
+			}
+		}
+	}
+}
+
+func importPath(s Spec) string {
+	t, err := strconv.Unquote(s.(*ImportSpec).Path.Value)
+	if err == nil {
+		return t
+	}
+	return ""
+}
+
+func importName(s Spec) string {
+	n := s.(*ImportSpec).Name
+	if n == nil {
+		return ""
+	}
+	return n.Name
+}
+
+func importComment(s Spec) string {
+	c := s.(*ImportSpec).Comment
+	if c == nil {
+		return ""
+	}
+	return c.Text()
+}
+
+// collapse indicates whether prev may be removed, leaving only next.
+func collapse(prev, next Spec) bool {
+	if importPath(next) != importPath(prev) || importName(next) != importName(prev) {
+		return false
+	}
+	return prev.(*ImportSpec).Comment == nil
+}
+
+type posSpan struct {
+	Start token.Pos
+	End   token.Pos
+}
+
+func sortSpecs(fset *token.FileSet, f *File, specs []Spec) []Spec {
+	// Can't short-circuit here even if specs are already sorted,
+	// since they might yet need deduplication.
+	// A lone import, however, may be safely ignored.
+	if len(specs) <= 1 {
+		return specs
+	}
+
+	// Record positions for specs.
+	pos := make([]posSpan, len(specs))
+	for i, s := range specs {
+		pos[i] = posSpan{s.Pos(), s.End()}
+	}
+
+	// Identify comments in this range.
+	// Any comment from pos[0].Start to the final line counts.
+	lastLine := fset.Position(pos[len(pos)-1].End).Line
+	cstart := len(f.Comments)
+	cend := len(f.Comments)
+	for i, g := range f.Comments {
+		if g.Pos() < pos[0].Start {
+			continue
+		}
+		if i < cstart {
+			cstart = i
+		}
+		if fset.Position(g.End()).Line > lastLine {
+			cend = i
+			break
+		}
+	}
+	comments := f.Comments[cstart:cend]
+
+	// Assign each comment to the import spec preceding it.
+	importComment := map[*ImportSpec][]*CommentGroup{}
+	specIndex := 0
+	for _, g := range comments {
+		for specIndex+1 < len(specs) && pos[specIndex+1].Start <= g.Pos() {
+			specIndex++
+		}
+		s := specs[specIndex].(*ImportSpec)
+		importComment[s] = append(importComment[s], g)
+	}
+
+	// Sort the import specs by import path.
+	// Remove duplicates, when possible without data loss.
+	// Reassign the import paths to have the same position sequence.
+	// Reassign each comment to abut the end of its spec.
+	// Sort the comments by new position.
+	sort.Sort(byImportSpec(specs))
+
+	// Dedup. Thanks to our sorting, we can just consider
+	// adjacent pairs of imports.
+	deduped := specs[:0]
+	for i, s := range specs {
+		if i == len(specs)-1 || !collapse(s, specs[i+1]) {
+			deduped = append(deduped, s)
+		} else {
+			p := s.Pos()
+			fset.File(p).MergeLine(fset.Position(p).Line)
+		}
+	}
+	specs = deduped
+
+	// Fix up comment positions
+	for i, s := range specs {
+		s := s.(*ImportSpec)
+		if s.Name != nil {
+			s.Name.NamePos = pos[i].Start
+		}
+		s.Path.ValuePos = pos[i].Start
+		s.EndPos = pos[i].End
+		for _, g := range importComment[s] {
+			for _, c := range g.List {
+				c.Slash = pos[i].End
+			}
+		}
+	}
+
+	sort.Sort(byCommentPos(comments))
+
+	return specs
+}
+
+type byImportSpec []Spec // slice of *ImportSpec
+
+func (x byImportSpec) Len() int      { return len(x) }
+func (x byImportSpec) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
+func (x byImportSpec) Less(i, j int) bool {
+	ipath := importPath(x[i])
+	jpath := importPath(x[j])
+	if ipath != jpath {
+		return ipath < jpath
+	}
+	iname := importName(x[i])
+	jname := importName(x[j])
+	if iname != jname {
+		return iname < jname
+	}
+	return importComment(x[i]) < importComment(x[j])
+}
+
+type byCommentPos []*CommentGroup
+
+func (x byCommentPos) Len() int           { return len(x) }
+func (x byCommentPos) Swap(i, j int)      { x[i], x[j] = x[j], x[i] }
+func (x byCommentPos) Less(i, j int) bool { return x[i].Pos() < x[j].Pos() }
diff --git a/src/go/ast/print.go b/src/go/ast/print.go
new file mode 100644
index 000000000..f15dc11dc
--- /dev/null
+++ b/src/go/ast/print.go
@@ -0,0 +1,251 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains printing support for ASTs.
+
+package ast
+
+import (
+	"fmt"
+	"go/token"
+	"io"
+	"os"
+	"reflect"
+)
+
+// A FieldFilter may be provided to Fprint to control the output.
+type FieldFilter func(name string, value reflect.Value) bool
+
+// NotNilFilter returns true for field values that are not nil;
+// it returns false otherwise.
+func NotNilFilter(_ string, v reflect.Value) bool {
+	switch v.Kind() {
+	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice:
+		return !v.IsNil()
+	}
+	return true
+}
+
+// Fprint prints the (sub-)tree starting at AST node x to w.
+// If fset != nil, position information is interpreted relative
+// to that file set. Otherwise positions are printed as integer
+// values (file set specific offsets).
+//
+// A non-nil FieldFilter f may be provided to control the output:
+// struct fields for which f(fieldname, fieldvalue) is true are
+// printed; all others are filtered from the output. Unexported
+// struct fields are never printed.
+//
+func Fprint(w io.Writer, fset *token.FileSet, x interface{}, f FieldFilter) (err error) {
+	// setup printer
+	p := printer{
+		output: w,
+		fset:   fset,
+		filter: f,
+		ptrmap: make(map[interface{}]int),
+		last:   '\n', // force printing of line number on first line
+	}
+
+	// install error handler
+	defer func() {
+		if e := recover(); e != nil {
+			err = e.(localError).err // re-panics if it's not a localError
+		}
+	}()
+
+	// print x
+	if x == nil {
+		p.printf("nil\n")
+		return
+	}
+	p.print(reflect.ValueOf(x))
+	p.printf("\n")
+
+	return
+}
+
+// Print prints x to standard output, skipping nil fields.
+// Print(fset, x) is the same as Fprint(os.Stdout, fset, x, NotNilFilter).
+func Print(fset *token.FileSet, x interface{}) error {
+	return Fprint(os.Stdout, fset, x, NotNilFilter)
+}
+
+type printer struct {
+	output io.Writer
+	fset   *token.FileSet
+	filter FieldFilter
+	ptrmap map[interface{}]int // *T -> line number
+	indent int                 // current indentation level
+	last   byte                // the last byte processed by Write
+	line   int                 // current line number
+}
+
+var indent = []byte(".  ")
+
+func (p *printer) Write(data []byte) (n int, err error) {
+	var m int
+	for i, b := range data {
+		// invariant: data[0:n] has been written
+		if b == '\n' {
+			m, err = p.output.Write(data[n : i+1])
+			n += m
+			if err != nil {
+				return
+			}
+			p.line++
+		} else if p.last == '\n' {
+			_, err = fmt.Fprintf(p.output, "%6d  ", p.line)
+			if err != nil {
+				return
+			}
+			for j := p.indent; j > 0; j-- {
+				_, err = p.output.Write(indent)
+				if err != nil {
+					return
+				}
+			}
+		}
+		p.last = b
+	}
+	if len(data) > n {
+		m, err = p.output.Write(data[n:])
+		n += m
+	}
+	return
+}
+
+// localError wraps locally caught errors so we can distinguish
+// them from genuine panics which we don't want to return as errors.
+type localError struct {
+	err error
+}
+
+// printf is a convenience wrapper that takes care of print errors.
+func (p *printer) printf(format string, args ...interface{}) {
+	if _, err := fmt.Fprintf(p, format, args...); err != nil {
+		panic(localError{err})
+	}
+}
+
+// Implementation note: Print is written for AST nodes but could be
+// used to print arbitrary data structures; such a version should
+// probably be in a different package.
+//
+// Note: This code detects (some) cycles created via pointers but
+// not cycles that are created via slices or maps containing the
+// same slice or map. Code for general data structures probably
+// should catch those as well.
+
+func (p *printer) print(x reflect.Value) {
+	if !NotNilFilter("", x) {
+		p.printf("nil")
+		return
+	}
+
+	switch x.Kind() {
+	case reflect.Interface:
+		p.print(x.Elem())
+
+	case reflect.Map:
+		p.printf("%s (len = %d) {", x.Type(), x.Len())
+		if x.Len() > 0 {
+			p.indent++
+			p.printf("\n")
+			for _, key := range x.MapKeys() {
+				p.print(key)
+				p.printf(": ")
+				p.print(x.MapIndex(key))
+				p.printf("\n")
+			}
+			p.indent--
+		}
+		p.printf("}")
+
+	case reflect.Ptr:
+		p.printf("*")
+		// type-checked ASTs may contain cycles - use ptrmap
+		// to keep track of objects that have been printed
+		// already and print the respective line number instead
+		ptr := x.Interface()
+		if line, exists := p.ptrmap[ptr]; exists {
+			p.printf("(obj @ %d)", line)
+		} else {
+			p.ptrmap[ptr] = p.line
+			p.print(x.Elem())
+		}
+
+	case reflect.Array:
+		p.printf("%s {", x.Type())
+		if x.Len() > 0 {
+			p.indent++
+			p.printf("\n")
+			for i, n := 0, x.Len(); i < n; i++ {
+				p.printf("%d: ", i)
+				p.print(x.Index(i))
+				p.printf("\n")
+			}
+			p.indent--
+		}
+		p.printf("}")
+
+	case reflect.Slice:
+		if s, ok := x.Interface().([]byte); ok {
+			p.printf("%#q", s)
+			return
+		}
+		p.printf("%s (len = %d) {", x.Type(), x.Len())
+		if x.Len() > 0 {
+			p.indent++
+			p.printf("\n")
+			for i, n := 0, x.Len(); i < n; i++ {
+				p.printf("%d: ", i)
+				p.print(x.Index(i))
+				p.printf("\n")
+			}
+			p.indent--
+		}
+		p.printf("}")
+
+	case reflect.Struct:
+		t := x.Type()
+		p.printf("%s {", t)
+		p.indent++
+		first := true
+		for i, n := 0, t.NumField(); i < n; i++ {
+			// exclude non-exported fields because their
+			// values cannot be accessed via reflection
+			if name := t.Field(i).Name; IsExported(name) {
+				value := x.Field(i)
+				if p.filter == nil || p.filter(name, value) {
+					if first {
+						p.printf("\n")
+						first = false
+					}
+					p.printf("%s: ", name)
+					p.print(value)
+					p.printf("\n")
+				}
+			}
+		}
+		p.indent--
+		p.printf("}")
+
+	default:
+		v := x.Interface()
+		switch v := v.(type) {
+		case string:
+			// print strings in quotes
+			p.printf("%q", v)
+			return
+		case token.Pos:
+			// position values can be printed nicely if we have a file set
+			if p.fset != nil {
+				p.printf("%s", p.fset.Position(v))
+				return
+			}
+		}
+		// default
+		p.printf("%v", v)
+	}
+}
diff --git a/src/go/ast/print_test.go b/src/go/ast/print_test.go
new file mode 100644
index 000000000..210f16430
--- /dev/null
+++ b/src/go/ast/print_test.go
@@ -0,0 +1,97 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+var tests = []struct {
+	x interface{} // x is printed as s
+	s string
+}{
+	// basic types
+	{nil, "0  nil"},
+	{true, "0  true"},
+	{42, "0  42"},
+	{3.14, "0  3.14"},
+	{1 + 2.718i, "0  (1+2.718i)"},
+	{"foobar", "0  \"foobar\""},
+
+	// maps
+	{map[Expr]string{}, `0  map[ast.Expr]string (len = 0) {}`},
+	{map[string]int{"a": 1},
+		`0  map[string]int (len = 1) {
+		1  .  "a": 1
+		2  }`},
+
+	// pointers
+	{new(int), "0  *0"},
+
+	// arrays
+	{[0]int{}, `0  [0]int {}`},
+	{[3]int{1, 2, 3},
+		`0  [3]int {
+		1  .  0: 1
+		2  .  1: 2
+		3  .  2: 3
+		4  }`},
+	{[...]int{42},
+		`0  [1]int {
+		1  .  0: 42
+		2  }`},
+
+	// slices
+	{[]int{}, `0  []int (len = 0) {}`},
+	{[]int{1, 2, 3},
+		`0  []int (len = 3) {
+		1  .  0: 1
+		2  .  1: 2
+		3  .  2: 3
+		4  }`},
+
+	// structs
+	{struct{}{}, `0  struct {} {}`},
+	{struct{ x int }{007}, `0  struct { x int } {}`},
+	{struct{ X, y int }{42, 991},
+		`0  struct { X int; y int } {
+		1  .  X: 42
+		2  }`},
+	{struct{ X, Y int }{42, 991},
+		`0  struct { X int; Y int } {
+		1  .  X: 42
+		2  .  Y: 991
+		3  }`},
+}
+
+// Split s into lines, trim whitespace from all lines, and return
+// the concatenated non-empty lines.
+func trim(s string) string {
+	lines := strings.Split(s, "\n")
+	i := 0
+	for _, line := range lines {
+		line = strings.TrimSpace(line)
+		if line != "" {
+			lines[i] = line
+			i++
+		}
+	}
+	return strings.Join(lines[0:i], "\n")
+}
+
+func TestPrint(t *testing.T) {
+	var buf bytes.Buffer
+	for _, test := range tests {
+		buf.Reset()
+		if err := Fprint(&buf, nil, test.x, nil); err != nil {
+			t.Errorf("Fprint failed: %s", err)
+		}
+		if s, ts := trim(buf.String()), trim(test.s); s != ts {
+			t.Errorf("got:\n%s\nexpected:\n%s\n", s, ts)
+		}
+	}
+}
diff --git a/src/go/ast/resolve.go b/src/go/ast/resolve.go
new file mode 100644
index 000000000..0406bfc58
--- /dev/null
+++ b/src/go/ast/resolve.go
@@ -0,0 +1,174 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements NewPackage.
+
+package ast
+
+import (
+	"fmt"
+	"go/scanner"
+	"go/token"
+	"strconv"
+)
+
+type pkgBuilder struct {
+	fset   *token.FileSet
+	errors scanner.ErrorList
+}
+
+func (p *pkgBuilder) error(pos token.Pos, msg string) {
+	p.errors.Add(p.fset.Position(pos), msg)
+}
+
+func (p *pkgBuilder) errorf(pos token.Pos, format string, args ...interface{}) {
+	p.error(pos, fmt.Sprintf(format, args...))
+}
+
+func (p *pkgBuilder) declare(scope, altScope *Scope, obj *Object) {
+	alt := scope.Insert(obj)
+	if alt == nil && altScope != nil {
+		// see if there is a conflicting declaration in altScope
+		alt = altScope.Lookup(obj.Name)
+	}
+	if alt != nil {
+		prevDecl := ""
+		if pos := alt.Pos(); pos.IsValid() {
+			prevDecl = fmt.Sprintf("\n\tprevious declaration at %s", p.fset.Position(pos))
+		}
+		p.error(obj.Pos(), fmt.Sprintf("%s redeclared in this block%s", obj.Name, prevDecl))
+	}
+}
+
+func resolve(scope *Scope, ident *Ident) bool {
+	for ; scope != nil; scope = scope.Outer {
+		if obj := scope.Lookup(ident.Name); obj != nil {
+			ident.Obj = obj
+			return true
+		}
+	}
+	return false
+}
+
+// An Importer resolves import paths to package Objects.
+// The imports map records the packages already imported,
+// indexed by package id (canonical import path).
+// An Importer must determine the canonical import path and
+// check the map to see if it is already present in the imports map.
+// If so, the Importer can return the map entry.  Otherwise, the
+// Importer should load the package data for the given path into
+// a new *Object (pkg), record pkg in the imports map, and then
+// return pkg.
+type Importer func(imports map[string]*Object, path string) (pkg *Object, err error)
+
+// NewPackage creates a new Package node from a set of File nodes. It resolves
+// unresolved identifiers across files and updates each file's Unresolved list
+// accordingly. If a non-nil importer and universe scope are provided, they are
+// used to resolve identifiers not declared in any of the package files. Any
+// remaining unresolved identifiers are reported as undeclared. If the files
+// belong to different packages, one package name is selected and files with
+// different package names are reported and then ignored.
+// The result is a package node and a scanner.ErrorList if there were errors.
+//
+func NewPackage(fset *token.FileSet, files map[string]*File, importer Importer, universe *Scope) (*Package, error) {
+	var p pkgBuilder
+	p.fset = fset
+
+	// complete package scope
+	pkgName := ""
+	pkgScope := NewScope(universe)
+	for _, file := range files {
+		// package names must match
+		switch name := file.Name.Name; {
+		case pkgName == "":
+			pkgName = name
+		case name != pkgName:
+			p.errorf(file.Package, "package %s; expected %s", name, pkgName)
+			continue // ignore this file
+		}
+
+		// collect top-level file objects in package scope
+		for _, obj := range file.Scope.Objects {
+			p.declare(pkgScope, nil, obj)
+		}
+	}
+
+	// package global mapping of imported package ids to package objects
+	imports := make(map[string]*Object)
+
+	// complete file scopes with imports and resolve identifiers
+	for _, file := range files {
+		// ignore file if it belongs to a different package
+		// (error has already been reported)
+		if file.Name.Name != pkgName {
+			continue
+		}
+
+		// build file scope by processing all imports
+		importErrors := false
+		fileScope := NewScope(pkgScope)
+		for _, spec := range file.Imports {
+			if importer == nil {
+				importErrors = true
+				continue
+			}
+			path, _ := strconv.Unquote(spec.Path.Value)
+			pkg, err := importer(imports, path)
+			if err != nil {
+				p.errorf(spec.Path.Pos(), "could not import %s (%s)", path, err)
+				importErrors = true
+				continue
+			}
+			// TODO(gri) If a local package name != "." is provided,
+			// global identifier resolution could proceed even if the
+			// import failed. Consider adjusting the logic here a bit.
+
+			// local name overrides imported package name
+			name := pkg.Name
+			if spec.Name != nil {
+				name = spec.Name.Name
+			}
+
+			// add import to file scope
+			if name == "." {
+				// merge imported scope with file scope
+				for _, obj := range pkg.Data.(*Scope).Objects {
+					p.declare(fileScope, pkgScope, obj)
+				}
+			} else if name != "_" {
+				// declare imported package object in file scope
+				// (do not re-use pkg in the file scope but create
+				// a new object instead; the Decl field is different
+				// for different files)
+				obj := NewObj(Pkg, name)
+				obj.Decl = spec
+				obj.Data = pkg.Data
+				p.declare(fileScope, pkgScope, obj)
+			}
+		}
+
+		// resolve identifiers
+		if importErrors {
+			// don't use the universe scope without correct imports
+			// (objects in the universe may be shadowed by imports;
+			// with missing imports, identifiers might get resolved
+			// incorrectly to universe objects)
+			pkgScope.Outer = nil
+		}
+		i := 0
+		for _, ident := range file.Unresolved {
+			if !resolve(fileScope, ident) {
+				p.errorf(ident.Pos(), "undeclared name: %s", ident.Name)
+				file.Unresolved[i] = ident
+				i++
+			}
+
+		}
+		file.Unresolved = file.Unresolved[0:i]
+		pkgScope.Outer = universe // reset universe scope
+	}
+
+	p.errors.Sort()
+	return &Package{pkgName, pkgScope, imports, files}, p.errors.Err()
+}
diff --git a/src/go/ast/scope.go b/src/go/ast/scope.go
new file mode 100644
index 000000000..df1529d18
--- /dev/null
+++ b/src/go/ast/scope.go
@@ -0,0 +1,162 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements scopes and the objects they contain.
+
+package ast
+
+import (
+	"bytes"
+	"fmt"
+	"go/token"
+)
+
+// A Scope maintains the set of named language entities declared
+// in the scope and a link to the immediately surrounding (outer)
+// scope.
+//
+type Scope struct {
+	Outer   *Scope
+	Objects map[string]*Object
+}
+
+// NewScope creates a new scope nested in the outer scope.
+func NewScope(outer *Scope) *Scope {
+	const n = 4 // initial scope capacity
+	return &Scope{outer, make(map[string]*Object, n)}
+}
+
+// Lookup returns the object with the given name if it is
+// found in scope s, otherwise it returns nil. Outer scopes
+// are ignored.
+//
+func (s *Scope) Lookup(name string) *Object {
+	return s.Objects[name]
+}
+
+// Insert attempts to insert a named object obj into the scope s.
+// If the scope already contains an object alt with the same name,
+// Insert leaves the scope unchanged and returns alt. Otherwise
+// it inserts obj and returns nil."
+//
+func (s *Scope) Insert(obj *Object) (alt *Object) {
+	if alt = s.Objects[obj.Name]; alt == nil {
+		s.Objects[obj.Name] = obj
+	}
+	return
+}
+
+// Debugging support
+func (s *Scope) String() string {
+	var buf bytes.Buffer
+	fmt.Fprintf(&buf, "scope %p {", s)
+	if s != nil && len(s.Objects) > 0 {
+		fmt.Fprintln(&buf)
+		for _, obj := range s.Objects {
+			fmt.Fprintf(&buf, "\t%s %s\n", obj.Kind, obj.Name)
+		}
+	}
+	fmt.Fprintf(&buf, "}\n")
+	return buf.String()
+}
+
+// ----------------------------------------------------------------------------
+// Objects
+
+// An Object describes a named language entity such as a package,
+// constant, type, variable, function (incl. methods), or label.
+//
+// The Data fields contains object-specific data:
+//
+//	Kind    Data type         Data value
+//	Pkg	*types.Package    package scope
+//	Con     int               iota for the respective declaration
+//	Con     != nil            constant value
+//	Typ     *Scope            (used as method scope during type checking - transient)
+//
+type Object struct {
+	Kind ObjKind
+	Name string      // declared name
+	Decl interface{} // corresponding Field, XxxSpec, FuncDecl, LabeledStmt, AssignStmt, Scope; or nil
+	Data interface{} // object-specific data; or nil
+	Type interface{} // placeholder for type information; may be nil
+}
+
+// NewObj creates a new object of a given kind and name.
+func NewObj(kind ObjKind, name string) *Object {
+	return &Object{Kind: kind, Name: name}
+}
+
+// Pos computes the source position of the declaration of an object name.
+// The result may be an invalid position if it cannot be computed
+// (obj.Decl may be nil or not correct).
+func (obj *Object) Pos() token.Pos {
+	name := obj.Name
+	switch d := obj.Decl.(type) {
+	case *Field:
+		for _, n := range d.Names {
+			if n.Name == name {
+				return n.Pos()
+			}
+		}
+	case *ImportSpec:
+		if d.Name != nil && d.Name.Name == name {
+			return d.Name.Pos()
+		}
+		return d.Path.Pos()
+	case *ValueSpec:
+		for _, n := range d.Names {
+			if n.Name == name {
+				return n.Pos()
+			}
+		}
+	case *TypeSpec:
+		if d.Name.Name == name {
+			return d.Name.Pos()
+		}
+	case *FuncDecl:
+		if d.Name.Name == name {
+			return d.Name.Pos()
+		}
+	case *LabeledStmt:
+		if d.Label.Name == name {
+			return d.Label.Pos()
+		}
+	case *AssignStmt:
+		for _, x := range d.Lhs {
+			if ident, isIdent := x.(*Ident); isIdent && ident.Name == name {
+				return ident.Pos()
+			}
+		}
+	case *Scope:
+		// predeclared object - nothing to do for now
+	}
+	return token.NoPos
+}
+
+// ObjKind describes what an object represents.
+type ObjKind int
+
+// The list of possible Object kinds.
+const (
+	Bad ObjKind = iota // for error handling
+	Pkg                // package
+	Con                // constant
+	Typ                // type
+	Var                // variable
+	Fun                // function or method
+	Lbl                // label
+)
+
+var objKindStrings = [...]string{
+	Bad: "bad",
+	Pkg: "package",
+	Con: "const",
+	Typ: "type",
+	Var: "var",
+	Fun: "func",
+	Lbl: "label",
+}
+
+func (kind ObjKind) String() string { return objKindStrings[kind] }
diff --git a/src/go/ast/walk.go b/src/go/ast/walk.go
new file mode 100644
index 000000000..73ac38647
--- /dev/null
+++ b/src/go/ast/walk.go
@@ -0,0 +1,386 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast
+
+import "fmt"
+
+// A Visitor's Visit method is invoked for each node encountered by Walk.
+// If the result visitor w is not nil, Walk visits each of the children
+// of node with the visitor w, followed by a call of w.Visit(nil).
+type Visitor interface {
+	Visit(node Node) (w Visitor)
+}
+
+// Helper functions for common node lists. They may be empty.
+
+func walkIdentList(v Visitor, list []*Ident) {
+	for _, x := range list {
+		Walk(v, x)
+	}
+}
+
+func walkExprList(v Visitor, list []Expr) {
+	for _, x := range list {
+		Walk(v, x)
+	}
+}
+
+func walkStmtList(v Visitor, list []Stmt) {
+	for _, x := range list {
+		Walk(v, x)
+	}
+}
+
+func walkDeclList(v Visitor, list []Decl) {
+	for _, x := range list {
+		Walk(v, x)
+	}
+}
+
+// TODO(gri): Investigate if providing a closure to Walk leads to
+//            simpler use (and may help eliminate Inspect in turn).
+
+// Walk traverses an AST in depth-first order: It starts by calling
+// v.Visit(node); node must not be nil. If the visitor w returned by
+// v.Visit(node) is not nil, Walk is invoked recursively with visitor
+// w for each of the non-nil children of node, followed by a call of
+// w.Visit(nil).
+//
+func Walk(v Visitor, node Node) {
+	if v = v.Visit(node); v == nil {
+		return
+	}
+
+	// walk children
+	// (the order of the cases matches the order
+	// of the corresponding node types in ast.go)
+	switch n := node.(type) {
+	// Comments and fields
+	case *Comment:
+		// nothing to do
+
+	case *CommentGroup:
+		for _, c := range n.List {
+			Walk(v, c)
+		}
+
+	case *Field:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		walkIdentList(v, n.Names)
+		Walk(v, n.Type)
+		if n.Tag != nil {
+			Walk(v, n.Tag)
+		}
+		if n.Comment != nil {
+			Walk(v, n.Comment)
+		}
+
+	case *FieldList:
+		for _, f := range n.List {
+			Walk(v, f)
+		}
+
+	// Expressions
+	case *BadExpr, *Ident, *BasicLit:
+		// nothing to do
+
+	case *Ellipsis:
+		if n.Elt != nil {
+			Walk(v, n.Elt)
+		}
+
+	case *FuncLit:
+		Walk(v, n.Type)
+		Walk(v, n.Body)
+
+	case *CompositeLit:
+		if n.Type != nil {
+			Walk(v, n.Type)
+		}
+		walkExprList(v, n.Elts)
+
+	case *ParenExpr:
+		Walk(v, n.X)
+
+	case *SelectorExpr:
+		Walk(v, n.X)
+		Walk(v, n.Sel)
+
+	case *IndexExpr:
+		Walk(v, n.X)
+		Walk(v, n.Index)
+
+	case *SliceExpr:
+		Walk(v, n.X)
+		if n.Low != nil {
+			Walk(v, n.Low)
+		}
+		if n.High != nil {
+			Walk(v, n.High)
+		}
+		if n.Max != nil {
+			Walk(v, n.Max)
+		}
+
+	case *TypeAssertExpr:
+		Walk(v, n.X)
+		if n.Type != nil {
+			Walk(v, n.Type)
+		}
+
+	case *CallExpr:
+		Walk(v, n.Fun)
+		walkExprList(v, n.Args)
+
+	case *StarExpr:
+		Walk(v, n.X)
+
+	case *UnaryExpr:
+		Walk(v, n.X)
+
+	case *BinaryExpr:
+		Walk(v, n.X)
+		Walk(v, n.Y)
+
+	case *KeyValueExpr:
+		Walk(v, n.Key)
+		Walk(v, n.Value)
+
+	// Types
+	case *ArrayType:
+		if n.Len != nil {
+			Walk(v, n.Len)
+		}
+		Walk(v, n.Elt)
+
+	case *StructType:
+		Walk(v, n.Fields)
+
+	case *FuncType:
+		if n.Params != nil {
+			Walk(v, n.Params)
+		}
+		if n.Results != nil {
+			Walk(v, n.Results)
+		}
+
+	case *InterfaceType:
+		Walk(v, n.Methods)
+
+	case *MapType:
+		Walk(v, n.Key)
+		Walk(v, n.Value)
+
+	case *ChanType:
+		Walk(v, n.Value)
+
+	// Statements
+	case *BadStmt:
+		// nothing to do
+
+	case *DeclStmt:
+		Walk(v, n.Decl)
+
+	case *EmptyStmt:
+		// nothing to do
+
+	case *LabeledStmt:
+		Walk(v, n.Label)
+		Walk(v, n.Stmt)
+
+	case *ExprStmt:
+		Walk(v, n.X)
+
+	case *SendStmt:
+		Walk(v, n.Chan)
+		Walk(v, n.Value)
+
+	case *IncDecStmt:
+		Walk(v, n.X)
+
+	case *AssignStmt:
+		walkExprList(v, n.Lhs)
+		walkExprList(v, n.Rhs)
+
+	case *GoStmt:
+		Walk(v, n.Call)
+
+	case *DeferStmt:
+		Walk(v, n.Call)
+
+	case *ReturnStmt:
+		walkExprList(v, n.Results)
+
+	case *BranchStmt:
+		if n.Label != nil {
+			Walk(v, n.Label)
+		}
+
+	case *BlockStmt:
+		walkStmtList(v, n.List)
+
+	case *IfStmt:
+		if n.Init != nil {
+			Walk(v, n.Init)
+		}
+		Walk(v, n.Cond)
+		Walk(v, n.Body)
+		if n.Else != nil {
+			Walk(v, n.Else)
+		}
+
+	case *CaseClause:
+		walkExprList(v, n.List)
+		walkStmtList(v, n.Body)
+
+	case *SwitchStmt:
+		if n.Init != nil {
+			Walk(v, n.Init)
+		}
+		if n.Tag != nil {
+			Walk(v, n.Tag)
+		}
+		Walk(v, n.Body)
+
+	case *TypeSwitchStmt:
+		if n.Init != nil {
+			Walk(v, n.Init)
+		}
+		Walk(v, n.Assign)
+		Walk(v, n.Body)
+
+	case *CommClause:
+		if n.Comm != nil {
+			Walk(v, n.Comm)
+		}
+		walkStmtList(v, n.Body)
+
+	case *SelectStmt:
+		Walk(v, n.Body)
+
+	case *ForStmt:
+		if n.Init != nil {
+			Walk(v, n.Init)
+		}
+		if n.Cond != nil {
+			Walk(v, n.Cond)
+		}
+		if n.Post != nil {
+			Walk(v, n.Post)
+		}
+		Walk(v, n.Body)
+
+	case *RangeStmt:
+		if n.Key != nil {
+			Walk(v, n.Key)
+		}
+		if n.Value != nil {
+			Walk(v, n.Value)
+		}
+		Walk(v, n.X)
+		Walk(v, n.Body)
+
+	// Declarations
+	case *ImportSpec:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		if n.Name != nil {
+			Walk(v, n.Name)
+		}
+		Walk(v, n.Path)
+		if n.Comment != nil {
+			Walk(v, n.Comment)
+		}
+
+	case *ValueSpec:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		walkIdentList(v, n.Names)
+		if n.Type != nil {
+			Walk(v, n.Type)
+		}
+		walkExprList(v, n.Values)
+		if n.Comment != nil {
+			Walk(v, n.Comment)
+		}
+
+	case *TypeSpec:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		Walk(v, n.Name)
+		Walk(v, n.Type)
+		if n.Comment != nil {
+			Walk(v, n.Comment)
+		}
+
+	case *BadDecl:
+		// nothing to do
+
+	case *GenDecl:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		for _, s := range n.Specs {
+			Walk(v, s)
+		}
+
+	case *FuncDecl:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		if n.Recv != nil {
+			Walk(v, n.Recv)
+		}
+		Walk(v, n.Name)
+		Walk(v, n.Type)
+		if n.Body != nil {
+			Walk(v, n.Body)
+		}
+
+	// Files and packages
+	case *File:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		Walk(v, n.Name)
+		walkDeclList(v, n.Decls)
+		// don't walk n.Comments - they have been
+		// visited already through the individual
+		// nodes
+
+	case *Package:
+		for _, f := range n.Files {
+			Walk(v, f)
+		}
+
+	default:
+		fmt.Printf("ast.Walk: unexpected node type %T", n)
+		panic("ast.Walk")
+	}
+
+	v.Visit(nil)
+}
+
+type inspector func(Node) bool
+
+func (f inspector) Visit(node Node) Visitor {
+	if f(node) {
+		return f
+	}
+	return nil
+}
+
+// Inspect traverses an AST in depth-first order: It starts by calling
+// f(node); node must not be nil. If f returns true, Inspect invokes f
+// for all the non-nil children of node, recursively.
+//
+func Inspect(node Node, f func(Node) bool) {
+	Walk(inspector(f), node)
+}
diff --git a/src/go/build/build.go b/src/go/build/build.go
new file mode 100644
index 000000000..69cb4b2f6
--- /dev/null
+++ b/src/go/build/build.go
@@ -0,0 +1,1364 @@
+// Copyright 2011 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package build
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"go/ast"
+	"go/doc"
+	"go/parser"
+	"go/token"
+	"io"
+	"io/ioutil"
+	"log"
+	"os"
+	pathpkg "path"
+	"path/filepath"
+	"runtime"
+	"sort"
+	"strconv"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+// A Context specifies the supporting context for a build.
+type Context struct {
+	GOARCH      string // target architecture
+	GOOS        string // target operating system
+	GOROOT      string // Go root
+	GOPATH      string // Go path
+	CgoEnabled  bool   // whether cgo can be used
+	UseAllFiles bool   // use files regardless of +build lines, file names
+	Compiler    string // compiler to assume when computing target paths
+
+	// The build and release tags specify build constraints
+	// that should be considered satisfied when processing +build lines.
+	// Clients creating a new context may customize BuildTags, which
+	// defaults to empty, but it is usually an error to customize ReleaseTags,
+	// which defaults to the list of Go releases the current release is compatible with.
+	// In addition to the BuildTags and ReleaseTags, build constraints
+	// consider the values of GOARCH and GOOS as satisfied tags.
+	BuildTags   []string
+	ReleaseTags []string
+
+	// The install suffix specifies a suffix to use in the name of the installation
+	// directory. By default it is empty, but custom builds that need to keep
+	// their outputs separate can set InstallSuffix to do so. For example, when
+	// using the race detector, the go command uses InstallSuffix = "race", so
+	// that on a Linux/386 system, packages are written to a directory named
+	// "linux_386_race" instead of the usual "linux_386".
+	InstallSuffix string
+
+	// By default, Import uses the operating system's file system calls
+	// to read directories and files.  To read from other sources,
+	// callers can set the following functions.  They all have default
+	// behaviors that use the local file system, so clients need only set
+	// the functions whose behaviors they wish to change.
+
+	// JoinPath joins the sequence of path fragments into a single path.
+	// If JoinPath is nil, Import uses filepath.Join.
+	JoinPath func(elem ...string) string
+
+	// SplitPathList splits the path list into a slice of individual paths.
+	// If SplitPathList is nil, Import uses filepath.SplitList.
+	SplitPathList func(list string) []string
+
+	// IsAbsPath reports whether path is an absolute path.
+	// If IsAbsPath is nil, Import uses filepath.IsAbs.
+	IsAbsPath func(path string) bool
+
+	// IsDir reports whether the path names a directory.
+	// If IsDir is nil, Import calls os.Stat and uses the result's IsDir method.
+	IsDir func(path string) bool
+
+	// HasSubdir reports whether dir is a subdirectory of
+	// (perhaps multiple levels below) root.
+	// If so, HasSubdir sets rel to a slash-separated path that
+	// can be joined to root to produce a path equivalent to dir.
+	// If HasSubdir is nil, Import uses an implementation built on
+	// filepath.EvalSymlinks.
+	HasSubdir func(root, dir string) (rel string, ok bool)
+
+	// ReadDir returns a slice of os.FileInfo, sorted by Name,
+	// describing the content of the named directory.
+	// If ReadDir is nil, Import uses ioutil.ReadDir.
+	ReadDir func(dir string) (fi []os.FileInfo, err error)
+
+	// OpenFile opens a file (not a directory) for reading.
+	// If OpenFile is nil, Import uses os.Open.
+	OpenFile func(path string) (r io.ReadCloser, err error)
+}
+
+// joinPath calls ctxt.JoinPath (if not nil) or else filepath.Join.
+func (ctxt *Context) joinPath(elem ...string) string {
+	if f := ctxt.JoinPath; f != nil {
+		return f(elem...)
+	}
+	return filepath.Join(elem...)
+}
+
+// splitPathList calls ctxt.SplitPathList (if not nil) or else filepath.SplitList.
+func (ctxt *Context) splitPathList(s string) []string {
+	if f := ctxt.SplitPathList; f != nil {
+		return f(s)
+	}
+	return filepath.SplitList(s)
+}
+
+// isAbsPath calls ctxt.IsAbsSPath (if not nil) or else filepath.IsAbs.
+func (ctxt *Context) isAbsPath(path string) bool {
+	if f := ctxt.IsAbsPath; f != nil {
+		return f(path)
+	}
+	return filepath.IsAbs(path)
+}
+
+// isDir calls ctxt.IsDir (if not nil) or else uses os.Stat.
+func (ctxt *Context) isDir(path string) bool {
+	if f := ctxt.IsDir; f != nil {
+		return f(path)
+	}
+	fi, err := os.Stat(path)
+	return err == nil && fi.IsDir()
+}
+
+// hasSubdir calls ctxt.HasSubdir (if not nil) or else uses
+// the local file system to answer the question.
+func (ctxt *Context) hasSubdir(root, dir string) (rel string, ok bool) {
+	if f := ctxt.HasSubdir; f != nil {
+		return f(root, dir)
+	}
+
+	// Try using paths we received.
+	if rel, ok = hasSubdir(root, dir); ok {
+		return
+	}
+
+	// Try expanding symlinks and comparing
+	// expanded against unexpanded and
+	// expanded against expanded.
+	rootSym, _ := filepath.EvalSymlinks(root)
+	dirSym, _ := filepath.EvalSymlinks(dir)
+
+	if rel, ok = hasSubdir(rootSym, dir); ok {
+		return
+	}
+	if rel, ok = hasSubdir(root, dirSym); ok {
+		return
+	}
+	return hasSubdir(rootSym, dirSym)
+}
+
+func hasSubdir(root, dir string) (rel string, ok bool) {
+	const sep = string(filepath.Separator)
+	root = filepath.Clean(root)
+	if !strings.HasSuffix(root, sep) {
+		root += sep
+	}
+	dir = filepath.Clean(dir)
+	if !strings.HasPrefix(dir, root) {
+		return "", false
+	}
+	return filepath.ToSlash(dir[len(root):]), true
+}
+
+// readDir calls ctxt.ReadDir (if not nil) or else ioutil.ReadDir.
+func (ctxt *Context) readDir(path string) ([]os.FileInfo, error) {
+	if f := ctxt.ReadDir; f != nil {
+		return f(path)
+	}
+	return ioutil.ReadDir(path)
+}
+
+// openFile calls ctxt.OpenFile (if not nil) or else os.Open.
+func (ctxt *Context) openFile(path string) (io.ReadCloser, error) {
+	if fn := ctxt.OpenFile; fn != nil {
+		return fn(path)
+	}
+
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err // nil interface
+	}
+	return f, nil
+}
+
+// isFile determines whether path is a file by trying to open it.
+// It reuses openFile instead of adding another function to the
+// list in Context.
+func (ctxt *Context) isFile(path string) bool {
+	f, err := ctxt.openFile(path)
+	if err != nil {
+		return false
+	}
+	f.Close()
+	return true
+}
+
+// gopath returns the list of Go path directories.
+func (ctxt *Context) gopath() []string {
+	var all []string
+	for _, p := range ctxt.splitPathList(ctxt.GOPATH) {
+		if p == "" || p == ctxt.GOROOT {
+			// Empty paths are uninteresting.
+			// If the path is the GOROOT, ignore it.
+			// People sometimes set GOPATH=$GOROOT.
+			// Do not get confused by this common mistake.
+			continue
+		}
+		if strings.HasPrefix(p, "~") {
+			// Path segments starting with ~ on Unix are almost always
+			// users who have incorrectly quoted ~ while setting GOPATH,
+			// preventing it from expanding to $HOME.
+			// The situation is made more confusing by the fact that
+			// bash allows quoted ~ in $PATH (most shells do not).
+			// Do not get confused by this, and do not try to use the path.
+			// It does not exist, and printing errors about it confuses
+			// those users even more, because they think "sure ~ exists!".
+			// The go command diagnoses this situation and prints a
+			// useful error.
+			// On Windows, ~ is used in short names, such as c:\progra~1
+			// for c:\program files.
+			continue
+		}
+		all = append(all, p)
+	}
+	return all
+}
+
+// SrcDirs returns a list of package source root directories.
+// It draws from the current Go root and Go path but omits directories
+// that do not exist.
+func (ctxt *Context) SrcDirs() []string {
+	var all []string
+	if ctxt.GOROOT != "" {
+		dir := ctxt.joinPath(ctxt.GOROOT, "src")
+		if ctxt.isDir(dir) {
+			all = append(all, dir)
+		}
+	}
+	for _, p := range ctxt.gopath() {
+		dir := ctxt.joinPath(p, "src")
+		if ctxt.isDir(dir) {
+			all = append(all, dir)
+		}
+	}
+	return all
+}
+
+// Default is the default Context for builds.
+// It uses the GOARCH, GOOS, GOROOT, and GOPATH environment variables
+// if set, or else the compiled code's GOARCH, GOOS, and GOROOT.
+var Default Context = defaultContext()
+
+var cgoEnabled = map[string]bool{
+	"darwin/386":      true,
+	"darwin/amd64":    true,
+	"dragonfly/386":   true,
+	"dragonfly/amd64": true,
+	"freebsd/386":     true,
+	"freebsd/amd64":   true,
+	"freebsd/arm":     true,
+	"linux/386":       true,
+	"linux/amd64":     true,
+	"linux/arm":       true,
+	"android/386":     true,
+	"android/amd64":   true,
+	"android/arm":     true,
+	"netbsd/386":      true,
+	"netbsd/amd64":    true,
+	"netbsd/arm":      true,
+	"openbsd/386":     true,
+	"openbsd/amd64":   true,
+	"windows/386":     true,
+	"windows/amd64":   true,
+}
+
+func defaultContext() Context {
+	var c Context
+
+	c.GOARCH = envOr("GOARCH", runtime.GOARCH)
+	c.GOOS = envOr("GOOS", runtime.GOOS)
+	c.GOROOT = runtime.GOROOT()
+	c.GOPATH = envOr("GOPATH", "")
+	c.Compiler = runtime.Compiler
+
+	// Each major Go release in the Go 1.x series should add a tag here.
+	// Old tags should not be removed. That is, the go1.x tag is present
+	// in all releases >= Go 1.x. Code that requires Go 1.x or later should
+	// say "+build go1.x", and code that should only be built before Go 1.x
+	// (perhaps it is the stub to use in that case) should say "+build !go1.x".
+	//
+	// When we reach Go 1.4 the line will read
+	//	c.ReleaseTags = []string{"go1.1", "go1.2", "go1.3", "go1.4"}
+	// and so on.
+	c.ReleaseTags = []string{"go1.1", "go1.2", "go1.3"}
+
+	switch os.Getenv("CGO_ENABLED") {
+	case "1":
+		c.CgoEnabled = true
+	case "0":
+		c.CgoEnabled = false
+	default:
+		// cgo must be explicitly enabled for cross compilation builds
+		if runtime.GOARCH == c.GOARCH && runtime.GOOS == c.GOOS {
+			c.CgoEnabled = cgoEnabled[c.GOOS+"/"+c.GOARCH]
+			break
+		}
+		c.CgoEnabled = false
+	}
+
+	return c
+}
+
+func envOr(name, def string) string {
+	s := os.Getenv(name)
+	if s == "" {
+		return def
+	}
+	return s
+}
+
+// An ImportMode controls the behavior of the Import method.
+type ImportMode uint
+
+const (
+	// If FindOnly is set, Import stops after locating the directory
+	// that should contain the sources for a package.  It does not
+	// read any files in the directory.
+	FindOnly ImportMode = 1 << iota
+
+	// If AllowBinary is set, Import can be satisfied by a compiled
+	// package object without corresponding sources.
+	AllowBinary
+
+	// If ImportComment is set, parse import comments on package statements.
+	// Import returns an error if it finds a comment it cannot understand
+	// or finds conflicting comments in multiple source files.
+	// See golang.org/s/go14customimport for more information.
+	ImportComment
+)
+
+// A Package describes the Go package found in a directory.
+type Package struct {
+	Dir           string   // directory containing package sources
+	Name          string   // package name
+	ImportComment string   // path in import comment on package statement
+	Doc           string   // documentation synopsis
+	ImportPath    string   // import path of package ("" if unknown)
+	Root          string   // root of Go tree where this package lives
+	SrcRoot       string   // package source root directory ("" if unknown)
+	PkgRoot       string   // package install root directory ("" if unknown)
+	BinDir        string   // command install directory ("" if unknown)
+	Goroot        bool     // package found in Go root
+	PkgObj        string   // installed .a file
+	AllTags       []string // tags that can influence file selection in this directory
+	ConflictDir   string   // this directory shadows Dir in $GOPATH
+
+	// Source files
+	GoFiles        []string // .go source files (excluding CgoFiles, TestGoFiles, XTestGoFiles)
+	CgoFiles       []string // .go source files that import "C"
+	IgnoredGoFiles []string // .go source files ignored for this build
+	CFiles         []string // .c source files
+	CXXFiles       []string // .cc, .cpp and .cxx source files
+	MFiles         []string // .m (Objective-C) source files
+	HFiles         []string // .h, .hh, .hpp and .hxx source files
+	SFiles         []string // .s source files
+	SwigFiles      []string // .swig files
+	SwigCXXFiles   []string // .swigcxx files
+	SysoFiles      []string // .syso system object files to add to archive
+
+	// Cgo directives
+	CgoCFLAGS    []string // Cgo CFLAGS directives
+	CgoCPPFLAGS  []string // Cgo CPPFLAGS directives
+	CgoCXXFLAGS  []string // Cgo CXXFLAGS directives
+	CgoLDFLAGS   []string // Cgo LDFLAGS directives
+	CgoPkgConfig []string // Cgo pkg-config directives
+
+	// Dependency information
+	Imports   []string                    // imports from GoFiles, CgoFiles
+	ImportPos map[string][]token.Position // line information for Imports
+
+	// Test information
+	TestGoFiles    []string                    // _test.go files in package
+	TestImports    []string                    // imports from TestGoFiles
+	TestImportPos  map[string][]token.Position // line information for TestImports
+	XTestGoFiles   []string                    // _test.go files outside package
+	XTestImports   []string                    // imports from XTestGoFiles
+	XTestImportPos map[string][]token.Position // line information for XTestImports
+}
+
+// IsCommand reports whether the package is considered a
+// command to be installed (not just a library).
+// Packages named "main" are treated as commands.
+func (p *Package) IsCommand() bool {
+	return p.Name == "main"
+}
+
+// ImportDir is like Import but processes the Go package found in
+// the named directory.
+func (ctxt *Context) ImportDir(dir string, mode ImportMode) (*Package, error) {
+	return ctxt.Import(".", dir, mode)
+}
+
+// NoGoError is the error used by Import to describe a directory
+// containing no buildable Go source files. (It may still contain
+// test files, files hidden by build tags, and so on.)
+type NoGoError struct {
+	Dir string
+}
+
+func (e *NoGoError) Error() string {
+	return "no buildable Go source files in " + e.Dir
+}
+
+func nameExt(name string) string {
+	i := strings.LastIndex(name, ".")
+	if i < 0 {
+		return ""
+	}
+	return name[i:]
+}
+
+// Import returns details about the Go package named by the import path,
+// interpreting local import paths relative to the srcDir directory.
+// If the path is a local import path naming a package that can be imported
+// using a standard import path, the returned package will set p.ImportPath
+// to that path.
+//
+// In the directory containing the package, .go, .c, .h, and .s files are
+// considered part of the package except for:
+//
+//	- .go files in package documentation
+//	- files starting with _ or . (likely editor temporary files)
+//	- files with build constraints not satisfied by the context
+//
+// If an error occurs, Import returns a non-nil error and a non-nil
+// *Package containing partial information.
+//
+func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Package, error) {
+	p := &Package{
+		ImportPath: path,
+	}
+	if path == "" {
+		return p, fmt.Errorf("import %q: invalid import path", path)
+	}
+
+	var pkga string
+	var pkgerr error
+	switch ctxt.Compiler {
+	case "gccgo":
+		dir, elem := pathpkg.Split(p.ImportPath)
+		pkga = "pkg/gccgo_" + ctxt.GOOS + "_" + ctxt.GOARCH + "/" + dir + "lib" + elem + ".a"
+	case "gc":
+		suffix := ""
+		if ctxt.InstallSuffix != "" {
+			suffix = "_" + ctxt.InstallSuffix
+		}
+		pkga = "pkg/" + ctxt.GOOS + "_" + ctxt.GOARCH + suffix + "/" + p.ImportPath + ".a"
+	default:
+		// Save error for end of function.
+		pkgerr = fmt.Errorf("import %q: unknown compiler %q", path, ctxt.Compiler)
+	}
+
+	binaryOnly := false
+	if IsLocalImport(path) {
+		pkga = "" // local imports have no installed path
+		if srcDir == "" {
+			return p, fmt.Errorf("import %q: import relative to unknown directory", path)
+		}
+		if !ctxt.isAbsPath(path) {
+			p.Dir = ctxt.joinPath(srcDir, path)
+		}
+		// Determine canonical import path, if any.
+		if ctxt.GOROOT != "" {
+			root := ctxt.joinPath(ctxt.GOROOT, "src")
+			if sub, ok := ctxt.hasSubdir(root, p.Dir); ok {
+				p.Goroot = true
+				p.ImportPath = sub
+				p.Root = ctxt.GOROOT
+				goto Found
+			}
+		}
+		all := ctxt.gopath()
+		for i, root := range all {
+			rootsrc := ctxt.joinPath(root, "src")
+			if sub, ok := ctxt.hasSubdir(rootsrc, p.Dir); ok {
+				// We found a potential import path for dir,
+				// but check that using it wouldn't find something
+				// else first.
+				if ctxt.GOROOT != "" {
+					if dir := ctxt.joinPath(ctxt.GOROOT, "src", sub); ctxt.isDir(dir) {
+						p.ConflictDir = dir
+						goto Found
+					}
+				}
+				for _, earlyRoot := range all[:i] {
+					if dir := ctxt.joinPath(earlyRoot, "src", sub); ctxt.isDir(dir) {
+						p.ConflictDir = dir
+						goto Found
+					}
+				}
+
+				// sub would not name some other directory instead of this one.
+				// Record it.
+				p.ImportPath = sub
+				p.Root = root
+				goto Found
+			}
+		}
+		// It's okay that we didn't find a root containing dir.
+		// Keep going with the information we have.
+	} else {
+		if strings.HasPrefix(path, "/") {
+			return p, fmt.Errorf("import %q: cannot import absolute path", path)
+		}
+
+		// tried records the location of unsuccessful package lookups
+		var tried struct {
+			goroot string
+			gopath []string
+		}
+
+		// Determine directory from import path.
+		if ctxt.GOROOT != "" {
+			dir := ctxt.joinPath(ctxt.GOROOT, "src", path)
+			isDir := ctxt.isDir(dir)
+			binaryOnly = !isDir && mode&AllowBinary != 0 && pkga != "" && ctxt.isFile(ctxt.joinPath(ctxt.GOROOT, pkga))
+			if isDir || binaryOnly {
+				p.Dir = dir
+				p.Goroot = true
+				p.Root = ctxt.GOROOT
+				goto Found
+			}
+			tried.goroot = dir
+		}
+		for _, root := range ctxt.gopath() {
+			dir := ctxt.joinPath(root, "src", path)
+			isDir := ctxt.isDir(dir)
+			binaryOnly = !isDir && mode&AllowBinary != 0 && pkga != "" && ctxt.isFile(ctxt.joinPath(root, pkga))
+			if isDir || binaryOnly {
+				p.Dir = dir
+				p.Root = root
+				goto Found
+			}
+			tried.gopath = append(tried.gopath, dir)
+		}
+
+		// package was not found
+		var paths []string
+		if tried.goroot != "" {
+			paths = append(paths, fmt.Sprintf("\t%s (from $GOROOT)", tried.goroot))
+		} else {
+			paths = append(paths, "\t($GOROOT not set)")
+		}
+		var i int
+		var format = "\t%s (from $GOPATH)"
+		for ; i < len(tried.gopath); i++ {
+			if i > 0 {
+				format = "\t%s"
+			}
+			paths = append(paths, fmt.Sprintf(format, tried.gopath[i]))
+		}
+		if i == 0 {
+			paths = append(paths, "\t($GOPATH not set)")
+		}
+		return p, fmt.Errorf("cannot find package %q in any of:\n%s", path, strings.Join(paths, "\n"))
+	}
+
+Found:
+	if p.Root != "" {
+		p.SrcRoot = ctxt.joinPath(p.Root, "src")
+		p.PkgRoot = ctxt.joinPath(p.Root, "pkg")
+		p.BinDir = ctxt.joinPath(p.Root, "bin")
+		if pkga != "" {
+			p.PkgObj = ctxt.joinPath(p.Root, pkga)
+		}
+	}
+
+	if mode&FindOnly != 0 {
+		return p, pkgerr
+	}
+	if binaryOnly && (mode&AllowBinary) != 0 {
+		return p, pkgerr
+	}
+
+	dirs, err := ctxt.readDir(p.Dir)
+	if err != nil {
+		return p, err
+	}
+
+	var Sfiles []string // files with ".S" (capital S)
+	var firstFile, firstCommentFile string
+	imported := make(map[string][]token.Position)
+	testImported := make(map[string][]token.Position)
+	xTestImported := make(map[string][]token.Position)
+	allTags := make(map[string]bool)
+	fset := token.NewFileSet()
+	for _, d := range dirs {
+		if d.IsDir() {
+			continue
+		}
+
+		name := d.Name()
+		ext := nameExt(name)
+
+		match, data, filename, err := ctxt.matchFile(p.Dir, name, true, allTags)
+		if err != nil {
+			return p, err
+		}
+		if !match {
+			if ext == ".go" {
+				p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
+			}
+			continue
+		}
+
+		// Going to save the file.  For non-Go files, can stop here.
+		switch ext {
+		case ".c":
+			p.CFiles = append(p.CFiles, name)
+			continue
+		case ".cc", ".cpp", ".cxx":
+			p.CXXFiles = append(p.CXXFiles, name)
+			continue
+		case ".m":
+			p.MFiles = append(p.MFiles, name)
+			continue
+		case ".h", ".hh", ".hpp", ".hxx":
+			p.HFiles = append(p.HFiles, name)
+			continue
+		case ".s":
+			p.SFiles = append(p.SFiles, name)
+			continue
+		case ".S":
+			Sfiles = append(Sfiles, name)
+			continue
+		case ".swig":
+			p.SwigFiles = append(p.SwigFiles, name)
+			continue
+		case ".swigcxx":
+			p.SwigCXXFiles = append(p.SwigCXXFiles, name)
+			continue
+		case ".syso":
+			// binary objects to add to package archive
+			// Likely of the form foo_windows.syso, but
+			// the name was vetted above with goodOSArchFile.
+			p.SysoFiles = append(p.SysoFiles, name)
+			continue
+		}
+
+		pf, err := parser.ParseFile(fset, filename, data, parser.ImportsOnly|parser.ParseComments)
+		if err != nil {
+			return p, err
+		}
+
+		pkg := pf.Name.Name
+		if pkg == "documentation" {
+			p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
+			continue
+		}
+
+		isTest := strings.HasSuffix(name, "_test.go")
+		isXTest := false
+		if isTest && strings.HasSuffix(pkg, "_test") {
+			isXTest = true
+			pkg = pkg[:len(pkg)-len("_test")]
+		}
+
+		if p.Name == "" {
+			p.Name = pkg
+			firstFile = name
+		} else if pkg != p.Name {
+			return p, fmt.Errorf("found packages %s (%s) and %s (%s) in %s", p.Name, firstFile, pkg, name, p.Dir)
+		}
+		if pf.Doc != nil && p.Doc == "" {
+			p.Doc = doc.Synopsis(pf.Doc.Text())
+		}
+
+		if mode&ImportComment != 0 {
+			qcom, line := findImportComment(data)
+			if line != 0 {
+				com, err := strconv.Unquote(qcom)
+				if err != nil {
+					return p, fmt.Errorf("%s:%d: cannot parse import comment", filename, line)
+				}
+				if p.ImportComment == "" {
+					p.ImportComment = com
+					firstCommentFile = name
+				} else if p.ImportComment != com {
+					return p, fmt.Errorf("found import comments %q (%s) and %q (%s) in %s", p.ImportComment, firstCommentFile, com, name, p.Dir)
+				}
+			}
+		}
+
+		// Record imports and information about cgo.
+		isCgo := false
+		for _, decl := range pf.Decls {
+			d, ok := decl.(*ast.GenDecl)
+			if !ok {
+				continue
+			}
+			for _, dspec := range d.Specs {
+				spec, ok := dspec.(*ast.ImportSpec)
+				if !ok {
+					continue
+				}
+				quoted := spec.Path.Value
+				path, err := strconv.Unquote(quoted)
+				if err != nil {
+					log.Panicf("%s: parser returned invalid quoted string: <%s>", filename, quoted)
+				}
+				if isXTest {
+					xTestImported[path] = append(xTestImported[path], fset.Position(spec.Pos()))
+				} else if isTest {
+					testImported[path] = append(testImported[path], fset.Position(spec.Pos()))
+				} else {
+					imported[path] = append(imported[path], fset.Position(spec.Pos()))
+				}
+				if path == "C" {
+					if isTest {
+						return p, fmt.Errorf("use of cgo in test %s not supported", filename)
+					}
+					cg := spec.Doc
+					if cg == nil && len(d.Specs) == 1 {
+						cg = d.Doc
+					}
+					if cg != nil {
+						if err := ctxt.saveCgo(filename, p, cg); err != nil {
+							return p, err
+						}
+					}
+					isCgo = true
+				}
+			}
+		}
+		if isCgo {
+			allTags["cgo"] = true
+			if ctxt.CgoEnabled {
+				p.CgoFiles = append(p.CgoFiles, name)
+			} else {
+				p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
+			}
+		} else if isXTest {
+			p.XTestGoFiles = append(p.XTestGoFiles, name)
+		} else if isTest {
+			p.TestGoFiles = append(p.TestGoFiles, name)
+		} else {
+			p.GoFiles = append(p.GoFiles, name)
+		}
+	}
+	if len(p.GoFiles)+len(p.CgoFiles)+len(p.TestGoFiles)+len(p.XTestGoFiles) == 0 {
+		return p, &NoGoError{p.Dir}
+	}
+
+	for tag := range allTags {
+		p.AllTags = append(p.AllTags, tag)
+	}
+	sort.Strings(p.AllTags)
+
+	p.Imports, p.ImportPos = cleanImports(imported)
+	p.TestImports, p.TestImportPos = cleanImports(testImported)
+	p.XTestImports, p.XTestImportPos = cleanImports(xTestImported)
+
+	// add the .S files only if we are using cgo
+	// (which means gcc will compile them).
+	// The standard assemblers expect .s files.
+	if len(p.CgoFiles) > 0 {
+		p.SFiles = append(p.SFiles, Sfiles...)
+		sort.Strings(p.SFiles)
+	}
+
+	return p, pkgerr
+}
+
+func findImportComment(data []byte) (s string, line int) {
+	// expect keyword package
+	word, data := parseWord(data)
+	if string(word) != "package" {
+		return "", 0
+	}
+
+	// expect package name
+	_, data = parseWord(data)
+
+	// now ready for import comment, a // or /* */ comment
+	// beginning and ending on the current line.
+	for len(data) > 0 && (data[0] == ' ' || data[0] == '\t' || data[0] == '\r') {
+		data = data[1:]
+	}
+
+	var comment []byte
+	switch {
+	case bytes.HasPrefix(data, slashSlash):
+		i := bytes.Index(data, newline)
+		if i < 0 {
+			i = len(data)
+		}
+		comment = data[2:i]
+	case bytes.HasPrefix(data, slashStar):
+		data = data[2:]
+		i := bytes.Index(data, starSlash)
+		if i < 0 {
+			// malformed comment
+			return "", 0
+		}
+		comment = data[:i]
+		if bytes.Contains(comment, newline) {
+			return "", 0
+		}
+	}
+	comment = bytes.TrimSpace(comment)
+
+	// split comment into `import`, `"pkg"`
+	word, arg := parseWord(comment)
+	if string(word) != "import" {
+		return "", 0
+	}
+
+	line = 1 + bytes.Count(data[:cap(data)-cap(arg)], newline)
+	return strings.TrimSpace(string(arg)), line
+}
+
+var (
+	slashSlash = []byte("//")
+	slashStar  = []byte("/*")
+	starSlash  = []byte("*/")
+	newline    = []byte("\n")
+)
+
+// skipSpaceOrComment returns data with any leading spaces or comments removed.
+func skipSpaceOrComment(data []byte) []byte {
+	for len(data) > 0 {
+		switch data[0] {
+		case ' ', '\t', '\r', '\n':
+			data = data[1:]
+			continue
+		case '/':
+			if bytes.HasPrefix(data, slashSlash) {
+				i := bytes.Index(data, newline)
+				if i < 0 {
+					return nil
+				}
+				data = data[i+1:]
+				continue
+			}
+			if bytes.HasPrefix(data, slashStar) {
+				data = data[2:]
+				i := bytes.Index(data, starSlash)
+				if i < 0 {
+					return nil
+				}
+				data = data[i+2:]
+				continue
+			}
+		}
+		break
+	}
+	return data
+}
+
+// parseWord skips any leading spaces or comments in data
+// and then parses the beginning of data as an identifier or keyword,
+// returning that word and what remains after the word.
+func parseWord(data []byte) (word, rest []byte) {
+	data = skipSpaceOrComment(data)
+
+	// Parse past leading word characters.
+	rest = data
+	for {
+		r, size := utf8.DecodeRune(rest)
+		if unicode.IsLetter(r) || '0' <= r && r <= '9' || r == '_' {
+			rest = rest[size:]
+			continue
+		}
+		break
+	}
+
+	word = data[:len(data)-len(rest)]
+	if len(word) == 0 {
+		return nil, nil
+	}
+
+	return word, rest
+}
+
+// MatchFile reports whether the file with the given name in the given directory
+// matches the context and would be included in a Package created by ImportDir
+// of that directory.
+//
+// MatchFile considers the name of the file and may use ctxt.OpenFile to
+// read some or all of the file's content.
+func (ctxt *Context) MatchFile(dir, name string) (match bool, err error) {
+	match, _, _, err = ctxt.matchFile(dir, name, false, nil)
+	return
+}
+
+// matchFile determines whether the file with the given name in the given directory
+// should be included in the package being constructed.
+// It returns the data read from the file.
+// If returnImports is true and name denotes a Go program, matchFile reads
+// until the end of the imports (and returns that data) even though it only
+// considers text until the first non-comment.
+// If allTags is non-nil, matchFile records any encountered build tag
+// by setting allTags[tag] = true.
+func (ctxt *Context) matchFile(dir, name string, returnImports bool, allTags map[string]bool) (match bool, data []byte, filename string, err error) {
+	if strings.HasPrefix(name, "_") ||
+		strings.HasPrefix(name, ".") {
+		return
+	}
+
+	i := strings.LastIndex(name, ".")
+	if i < 0 {
+		i = len(name)
+	}
+	ext := name[i:]
+
+	if !ctxt.goodOSArchFile(name, allTags) && !ctxt.UseAllFiles {
+		return
+	}
+
+	switch ext {
+	case ".go", ".c", ".cc", ".cxx", ".cpp", ".m", ".s", ".h", ".hh", ".hpp", ".hxx", ".S", ".swig", ".swigcxx":
+		// tentatively okay - read to make sure
+	case ".syso":
+		// binary, no reading
+		match = true
+		return
+	default:
+		// skip
+		return
+	}
+
+	filename = ctxt.joinPath(dir, name)
+	f, err := ctxt.openFile(filename)
+	if err != nil {
+		return
+	}
+
+	if strings.HasSuffix(filename, ".go") {
+		data, err = readImports(f, false)
+	} else {
+		data, err = readComments(f)
+	}
+	f.Close()
+	if err != nil {
+		err = fmt.Errorf("read %s: %v", filename, err)
+		return
+	}
+
+	// Look for +build comments to accept or reject the file.
+	if !ctxt.shouldBuild(data, allTags) && !ctxt.UseAllFiles {
+		return
+	}
+
+	match = true
+	return
+}
+
+func cleanImports(m map[string][]token.Position) ([]string, map[string][]token.Position) {
+	all := make([]string, 0, len(m))
+	for path := range m {
+		all = append(all, path)
+	}
+	sort.Strings(all)
+	return all, m
+}
+
+// Import is shorthand for Default.Import.
+func Import(path, srcDir string, mode ImportMode) (*Package, error) {
+	return Default.Import(path, srcDir, mode)
+}
+
+// ImportDir is shorthand for Default.ImportDir.
+func ImportDir(dir string, mode ImportMode) (*Package, error) {
+	return Default.ImportDir(dir, mode)
+}
+
+var slashslash = []byte("//")
+
+// shouldBuild reports whether it is okay to use this file,
+// The rule is that in the file's leading run of // comments
+// and blank lines, which must be followed by a blank line
+// (to avoid including a Go package clause doc comment),
+// lines beginning with '// +build' are taken as build directives.
+//
+// The file is accepted only if each such line lists something
+// matching the file.  For example:
+//
+//	// +build windows linux
+//
+// marks the file as applicable only on Windows and Linux.
+//
+func (ctxt *Context) shouldBuild(content []byte, allTags map[string]bool) bool {
+	// Pass 1. Identify leading run of // comments and blank lines,
+	// which must be followed by a blank line.
+	end := 0
+	p := content
+	for len(p) > 0 {
+		line := p
+		if i := bytes.IndexByte(line, '\n'); i >= 0 {
+			line, p = line[:i], p[i+1:]
+		} else {
+			p = p[len(p):]
+		}
+		line = bytes.TrimSpace(line)
+		if len(line) == 0 { // Blank line
+			end = len(content) - len(p)
+			continue
+		}
+		if !bytes.HasPrefix(line, slashslash) { // Not comment line
+			break
+		}
+	}
+	content = content[:end]
+
+	// Pass 2.  Process each line in the run.
+	p = content
+	allok := true
+	for len(p) > 0 {
+		line := p
+		if i := bytes.IndexByte(line, '\n'); i >= 0 {
+			line, p = line[:i], p[i+1:]
+		} else {
+			p = p[len(p):]
+		}
+		line = bytes.TrimSpace(line)
+		if bytes.HasPrefix(line, slashslash) {
+			line = bytes.TrimSpace(line[len(slashslash):])
+			if len(line) > 0 && line[0] == '+' {
+				// Looks like a comment +line.
+				f := strings.Fields(string(line))
+				if f[0] == "+build" {
+					ok := false
+					for _, tok := range f[1:] {
+						if ctxt.match(tok, allTags) {
+							ok = true
+						}
+					}
+					if !ok {
+						allok = false
+					}
+				}
+			}
+		}
+	}
+
+	return allok
+}
+
+// saveCgo saves the information from the #cgo lines in the import "C" comment.
+// These lines set CFLAGS, CPPFLAGS, CXXFLAGS and LDFLAGS and pkg-config directives
+// that affect the way cgo's C code is built.
+//
+// TODO(rsc): This duplicates code in cgo.
+// Once the dust settles, remove this code from cgo.
+func (ctxt *Context) saveCgo(filename string, di *Package, cg *ast.CommentGroup) error {
+	text := cg.Text()
+	for _, line := range strings.Split(text, "\n") {
+		orig := line
+
+		// Line is
+		//	#cgo [GOOS/GOARCH...] LDFLAGS: stuff
+		//
+		line = strings.TrimSpace(line)
+		if len(line) < 5 || line[:4] != "#cgo" || (line[4] != ' ' && line[4] != '\t') {
+			continue
+		}
+
+		// Split at colon.
+		line = strings.TrimSpace(line[4:])
+		i := strings.Index(line, ":")
+		if i < 0 {
+			return fmt.Errorf("%s: invalid #cgo line: %s", filename, orig)
+		}
+		line, argstr := line[:i], line[i+1:]
+
+		// Parse GOOS/GOARCH stuff.
+		f := strings.Fields(line)
+		if len(f) < 1 {
+			return fmt.Errorf("%s: invalid #cgo line: %s", filename, orig)
+		}
+
+		cond, verb := f[:len(f)-1], f[len(f)-1]
+		if len(cond) > 0 {
+			ok := false
+			for _, c := range cond {
+				if ctxt.match(c, nil) {
+					ok = true
+					break
+				}
+			}
+			if !ok {
+				continue
+			}
+		}
+
+		args, err := splitQuoted(argstr)
+		if err != nil {
+			return fmt.Errorf("%s: invalid #cgo line: %s", filename, orig)
+		}
+		for _, arg := range args {
+			if !safeCgoName(arg) {
+				return fmt.Errorf("%s: malformed #cgo argument: %s", filename, arg)
+			}
+		}
+
+		switch verb {
+		case "CFLAGS":
+			di.CgoCFLAGS = append(di.CgoCFLAGS, args...)
+		case "CPPFLAGS":
+			di.CgoCPPFLAGS = append(di.CgoCPPFLAGS, args...)
+		case "CXXFLAGS":
+			di.CgoCXXFLAGS = append(di.CgoCXXFLAGS, args...)
+		case "LDFLAGS":
+			di.CgoLDFLAGS = append(di.CgoLDFLAGS, args...)
+		case "pkg-config":
+			di.CgoPkgConfig = append(di.CgoPkgConfig, args...)
+		default:
+			return fmt.Errorf("%s: invalid #cgo verb: %s", filename, orig)
+		}
+	}
+	return nil
+}
+
+// NOTE: $ is not safe for the shell, but it is allowed here because of linker options like -Wl,$ORIGIN.
+// We never pass these arguments to a shell (just to programs we construct argv for), so this should be okay.
+// See golang.org/issue/6038.
+var safeBytes = []byte("+-.,/0123456789=ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz:$")
+
+func safeCgoName(s string) bool {
+	if s == "" {
+		return false
+	}
+	for i := 0; i < len(s); i++ {
+		if c := s[i]; c < 0x80 && bytes.IndexByte(safeBytes, c) < 0 {
+			return false
+		}
+	}
+	return true
+}
+
+// splitQuoted splits the string s around each instance of one or more consecutive
+// white space characters while taking into account quotes and escaping, and
+// returns an array of substrings of s or an empty list if s contains only white space.
+// Single quotes and double quotes are recognized to prevent splitting within the
+// quoted region, and are removed from the resulting substrings. If a quote in s
+// isn't closed err will be set and r will have the unclosed argument as the
+// last element.  The backslash is used for escaping.
+//
+// For example, the following string:
+//
+//     a b:"c d" 'e''f'  "g\""
+//
+// Would be parsed as:
+//
+//     []string{"a", "b:c d", "ef", `g"`}
+//
+func splitQuoted(s string) (r []string, err error) {
+	var args []string
+	arg := make([]rune, len(s))
+	escaped := false
+	quoted := false
+	quote := '\x00'
+	i := 0
+	for _, rune := range s {
+		switch {
+		case escaped:
+			escaped = false
+		case rune == '\\':
+			escaped = true
+			continue
+		case quote != '\x00':
+			if rune == quote {
+				quote = '\x00'
+				continue
+			}
+		case rune == '"' || rune == '\'':
+			quoted = true
+			quote = rune
+			continue
+		case unicode.IsSpace(rune):
+			if quoted || i > 0 {
+				quoted = false
+				args = append(args, string(arg[:i]))
+				i = 0
+			}
+			continue
+		}
+		arg[i] = rune
+		i++
+	}
+	if quoted || i > 0 {
+		args = append(args, string(arg[:i]))
+	}
+	if quote != 0 {
+		err = errors.New("unclosed quote")
+	} else if escaped {
+		err = errors.New("unfinished escaping")
+	}
+	return args, err
+}
+
+// match returns true if the name is one of:
+//
+//	$GOOS
+//	$GOARCH
+//	cgo (if cgo is enabled)
+//	!cgo (if cgo is disabled)
+//	ctxt.Compiler
+//	!ctxt.Compiler
+//	tag (if tag is listed in ctxt.BuildTags or ctxt.ReleaseTags)
+//	!tag (if tag is not listed in ctxt.BuildTags or ctxt.ReleaseTags)
+//	a comma-separated list of any of these
+//
+func (ctxt *Context) match(name string, allTags map[string]bool) bool {
+	if name == "" {
+		if allTags != nil {
+			allTags[name] = true
+		}
+		return false
+	}
+	if i := strings.Index(name, ","); i >= 0 {
+		// comma-separated list
+		ok1 := ctxt.match(name[:i], allTags)
+		ok2 := ctxt.match(name[i+1:], allTags)
+		return ok1 && ok2
+	}
+	if strings.HasPrefix(name, "!!") { // bad syntax, reject always
+		return false
+	}
+	if strings.HasPrefix(name, "!") { // negation
+		return len(name) > 1 && !ctxt.match(name[1:], allTags)
+	}
+
+	if allTags != nil {
+		allTags[name] = true
+	}
+
+	// Tags must be letters, digits, underscores or dots.
+	// Unlike in Go identifiers, all digits are fine (e.g., "386").
+	for _, c := range name {
+		if !unicode.IsLetter(c) && !unicode.IsDigit(c) && c != '_' && c != '.' {
+			return false
+		}
+	}
+
+	// special tags
+	if ctxt.CgoEnabled && name == "cgo" {
+		return true
+	}
+	if name == ctxt.GOOS || name == ctxt.GOARCH || name == ctxt.Compiler {
+		return true
+	}
+	if ctxt.GOOS == "android" && name == "linux" {
+		return true
+	}
+
+	// other tags
+	for _, tag := range ctxt.BuildTags {
+		if tag == name {
+			return true
+		}
+	}
+	for _, tag := range ctxt.ReleaseTags {
+		if tag == name {
+			return true
+		}
+	}
+
+	return false
+}
+
+// goodOSArchFile returns false if the name contains a $GOOS or $GOARCH
+// suffix which does not match the current system.
+// The recognized name formats are:
+//
+//     name_$(GOOS).*
+//     name_$(GOARCH).*
+//     name_$(GOOS)_$(GOARCH).*
+//     name_$(GOOS)_test.*
+//     name_$(GOARCH)_test.*
+//     name_$(GOOS)_$(GOARCH)_test.*
+//
+// An exception: if GOOS=android, then files with GOOS=linux are also matched.
+func (ctxt *Context) goodOSArchFile(name string, allTags map[string]bool) bool {
+	if dot := strings.Index(name, "."); dot != -1 {
+		name = name[:dot]
+	}
+	l := strings.Split(name, "_")
+	if n := len(l); n > 0 && l[n-1] == "test" {
+		l = l[:n-1]
+	}
+	n := len(l)
+	if n >= 2 && knownOS[l[n-2]] && knownArch[l[n-1]] {
+		if allTags != nil {
+			allTags[l[n-2]] = true
+			allTags[l[n-1]] = true
+		}
+		if l[n-1] != ctxt.GOARCH {
+			return false
+		}
+		if ctxt.GOOS == "android" && l[n-2] == "linux" {
+			return true
+		}
+		return l[n-2] == ctxt.GOOS
+	}
+	if n >= 1 && knownOS[l[n-1]] {
+		if allTags != nil {
+			allTags[l[n-1]] = true
+		}
+		if ctxt.GOOS == "android" && l[n-1] == "linux" {
+			return true
+		}
+		return l[n-1] == ctxt.GOOS
+	}
+	if n >= 1 && knownArch[l[n-1]] {
+		if allTags != nil {
+			allTags[l[n-1]] = true
+		}
+		return l[n-1] == ctxt.GOARCH
+	}
+	return true
+}
+
+var knownOS = make(map[string]bool)
+var knownArch = make(map[string]bool)
+
+func init() {
+	for _, v := range strings.Fields(goosList) {
+		knownOS[v] = true
+	}
+	for _, v := range strings.Fields(goarchList) {
+		knownArch[v] = true
+	}
+}
+
+// ToolDir is the directory containing build tools.
+var ToolDir = filepath.Join(runtime.GOROOT(), "pkg/tool/"+runtime.GOOS+"_"+runtime.GOARCH)
+
+// IsLocalImport reports whether the import path is
+// a local import path, like ".", "..", "./foo", or "../foo".
+func IsLocalImport(path string) bool {
+	return path == "." || path == ".." ||
+		strings.HasPrefix(path, "./") || strings.HasPrefix(path, "../")
+}
+
+// ArchChar returns the architecture character for the given goarch.
+// For example, ArchChar("amd64") returns "6".
+func ArchChar(goarch string) (string, error) {
+	switch goarch {
+	case "386":
+		return "8", nil
+	case "amd64", "amd64p32":
+		return "6", nil
+	case "arm":
+		return "5", nil
+	}
+	return "", errors.New("unsupported GOARCH " + goarch)
+}
diff --git a/src/go/build/build_test.go b/src/go/build/build_test.go
new file mode 100644
index 000000000..004010113
--- /dev/null
+++ b/src/go/build/build_test.go
@@ -0,0 +1,205 @@
+// Copyright 2011 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package build
+
+import (
+	"io"
+	"os"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+func TestMatch(t *testing.T) {
+	ctxt := Default
+	what := "default"
+	match := func(tag string, want map[string]bool) {
+		m := make(map[string]bool)
+		if !ctxt.match(tag, m) {
+			t.Errorf("%s context should match %s, does not", what, tag)
+		}
+		if !reflect.DeepEqual(m, want) {
+			t.Errorf("%s tags = %v, want %v", tag, m, want)
+		}
+	}
+	nomatch := func(tag string, want map[string]bool) {
+		m := make(map[string]bool)
+		if ctxt.match(tag, m) {
+			t.Errorf("%s context should NOT match %s, does", what, tag)
+		}
+		if !reflect.DeepEqual(m, want) {
+			t.Errorf("%s tags = %v, want %v", tag, m, want)
+		}
+	}
+
+	match(runtime.GOOS+","+runtime.GOARCH, map[string]bool{runtime.GOOS: true, runtime.GOARCH: true})
+	match(runtime.GOOS+","+runtime.GOARCH+",!foo", map[string]bool{runtime.GOOS: true, runtime.GOARCH: true, "foo": true})
+	nomatch(runtime.GOOS+","+runtime.GOARCH+",foo", map[string]bool{runtime.GOOS: true, runtime.GOARCH: true, "foo": true})
+
+	what = "modified"
+	ctxt.BuildTags = []string{"foo"}
+	match(runtime.GOOS+","+runtime.GOARCH, map[string]bool{runtime.GOOS: true, runtime.GOARCH: true})
+	match(runtime.GOOS+","+runtime.GOARCH+",foo", map[string]bool{runtime.GOOS: true, runtime.GOARCH: true, "foo": true})
+	nomatch(runtime.GOOS+","+runtime.GOARCH+",!foo", map[string]bool{runtime.GOOS: true, runtime.GOARCH: true, "foo": true})
+	match(runtime.GOOS+","+runtime.GOARCH+",!bar", map[string]bool{runtime.GOOS: true, runtime.GOARCH: true, "bar": true})
+	nomatch(runtime.GOOS+","+runtime.GOARCH+",bar", map[string]bool{runtime.GOOS: true, runtime.GOARCH: true, "bar": true})
+	nomatch("!", map[string]bool{})
+}
+
+func TestDotSlashImport(t *testing.T) {
+	p, err := ImportDir("testdata/other", 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(p.Imports) != 1 || p.Imports[0] != "./file" {
+		t.Fatalf("testdata/other: Imports=%v, want [./file]", p.Imports)
+	}
+
+	p1, err := Import("./file", "testdata/other", 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if p1.Name != "file" {
+		t.Fatalf("./file: Name=%q, want %q", p1.Name, "file")
+	}
+	dir := filepath.Clean("testdata/other/file") // Clean to use \ on Windows
+	if p1.Dir != dir {
+		t.Fatalf("./file: Dir=%q, want %q", p1.Name, dir)
+	}
+}
+
+func TestEmptyImport(t *testing.T) {
+	p, err := Import("", Default.GOROOT, FindOnly)
+	if err == nil {
+		t.Fatal(`Import("") returned nil error.`)
+	}
+	if p == nil {
+		t.Fatal(`Import("") returned nil package.`)
+	}
+	if p.ImportPath != "" {
+		t.Fatalf("ImportPath=%q, want %q.", p.ImportPath, "")
+	}
+}
+
+func TestLocalDirectory(t *testing.T) {
+	cwd, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	p, err := ImportDir(cwd, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if p.ImportPath != "go/build" {
+		t.Fatalf("ImportPath=%q, want %q", p.ImportPath, "go/build")
+	}
+}
+
+func TestShouldBuild(t *testing.T) {
+	const file1 = "// +build tag1\n\n" +
+		"package main\n"
+	want1 := map[string]bool{"tag1": true}
+
+	const file2 = "// +build cgo\n\n" +
+		"// This package implements parsing of tags like\n" +
+		"// +build tag1\n" +
+		"package build"
+	want2 := map[string]bool{"cgo": true}
+
+	const file3 = "// Copyright The Go Authors.\n\n" +
+		"package build\n\n" +
+		"// shouldBuild checks tags given by lines of the form\n" +
+		"// +build tag\n" +
+		"func shouldBuild(content []byte)\n"
+	want3 := map[string]bool{}
+
+	ctx := &Context{BuildTags: []string{"tag1"}}
+	m := map[string]bool{}
+	if !ctx.shouldBuild([]byte(file1), m) {
+		t.Errorf("shouldBuild(file1) = false, want true")
+	}
+	if !reflect.DeepEqual(m, want1) {
+		t.Errorf("shoudBuild(file1) tags = %v, want %v", m, want1)
+	}
+
+	m = map[string]bool{}
+	if ctx.shouldBuild([]byte(file2), m) {
+		t.Errorf("shouldBuild(file2) = true, want fakse")
+	}
+	if !reflect.DeepEqual(m, want2) {
+		t.Errorf("shoudBuild(file2) tags = %v, want %v", m, want2)
+	}
+
+	m = map[string]bool{}
+	ctx = &Context{BuildTags: nil}
+	if !ctx.shouldBuild([]byte(file3), m) {
+		t.Errorf("shouldBuild(file3) = false, want true")
+	}
+	if !reflect.DeepEqual(m, want3) {
+		t.Errorf("shoudBuild(file3) tags = %v, want %v", m, want3)
+	}
+}
+
+type readNopCloser struct {
+	io.Reader
+}
+
+func (r readNopCloser) Close() error {
+	return nil
+}
+
+var (
+	ctxtP9      = Context{GOARCH: "arm", GOOS: "plan9"}
+	ctxtAndroid = Context{GOARCH: "arm", GOOS: "android"}
+)
+
+var matchFileTests = []struct {
+	ctxt  Context
+	name  string
+	data  string
+	match bool
+}{
+	{ctxtP9, "foo_arm.go", "", true},
+	{ctxtP9, "foo1_arm.go", "// +build linux\n\npackage main\n", false},
+	{ctxtP9, "foo_darwin.go", "", false},
+	{ctxtP9, "foo.go", "", true},
+	{ctxtP9, "foo1.go", "// +build linux\n\npackage main\n", false},
+	{ctxtP9, "foo.badsuffix", "", false},
+	{ctxtAndroid, "foo_linux.go", "", true},
+	{ctxtAndroid, "foo_android.go", "", true},
+	{ctxtAndroid, "foo_plan9.go", "", false},
+}
+
+func TestMatchFile(t *testing.T) {
+	for _, tt := range matchFileTests {
+		ctxt := tt.ctxt
+		ctxt.OpenFile = func(path string) (r io.ReadCloser, err error) {
+			if path != "x+"+tt.name {
+				t.Fatalf("OpenFile asked for %q, expected %q", path, "x+"+tt.name)
+			}
+			return &readNopCloser{strings.NewReader(tt.data)}, nil
+		}
+		ctxt.JoinPath = func(elem ...string) string {
+			return strings.Join(elem, "+")
+		}
+		match, err := ctxt.MatchFile("x", tt.name)
+		if match != tt.match || err != nil {
+			t.Fatalf("MatchFile(%q) = %v, %v, want %v, nil", tt.name, match, err, tt.match)
+		}
+	}
+}
+
+func TestImportCmd(t *testing.T) {
+	p, err := Import("cmd/internal/objfile", "", 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.HasSuffix(filepath.ToSlash(p.Dir), "src/cmd/internal/objfile") {
+		t.Fatalf("Import cmd/internal/objfile returned Dir=%q, want %q", filepath.ToSlash(p.Dir), ".../src/cmd/internal/objfile")
+	}
+}
diff --git a/src/go/build/deps_test.go b/src/go/build/deps_test.go
new file mode 100644
index 000000000..b74595ea8
--- /dev/null
+++ b/src/go/build/deps_test.go
@@ -0,0 +1,443 @@
+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file exercises the import parser but also checks that
+// some low-level packages do not have new dependencies added.
+
+package build
+
+import (
+	"runtime"
+	"sort"
+	"testing"
+)
+
+// pkgDeps defines the expected dependencies between packages in
+// the Go source tree.  It is a statement of policy.
+// Changes should not be made to this map without prior discussion.
+//
+// The map contains two kinds of entries:
+// 1) Lower-case keys are standard import paths and list the
+// allowed imports in that package.
+// 2) Upper-case keys define aliases for package sets, which can then
+// be used as dependencies by other rules.
+//
+// DO NOT CHANGE THIS DATA TO FIX BUILDS.
+//
+var pkgDeps = map[string][]string{
+	// L0 is the lowest level, core, nearly unavoidable packages.
+	"errors":      {},
+	"io":          {"errors", "sync"},
+	"runtime":     {"unsafe"},
+	"sync":        {"runtime", "sync/atomic", "unsafe"},
+	"sync/atomic": {"unsafe"},
+	"unsafe":      {},
+
+	"L0": {
+		"errors",
+		"io",
+		"runtime",
+		"sync",
+		"sync/atomic",
+		"unsafe",
+	},
+
+	// L1 adds simple functions and strings processing,
+	// but not Unicode tables.
+	"math":          {"unsafe"},
+	"math/cmplx":    {"math"},
+	"math/rand":     {"L0", "math"},
+	"sort":          {},
+	"strconv":       {"L0", "unicode/utf8", "math"},
+	"unicode/utf16": {},
+	"unicode/utf8":  {},
+
+	"L1": {
+		"L0",
+		"math",
+		"math/cmplx",
+		"math/rand",
+		"sort",
+		"strconv",
+		"unicode/utf16",
+		"unicode/utf8",
+	},
+
+	// L2 adds Unicode and strings processing.
+	"bufio":   {"L0", "unicode/utf8", "bytes"},
+	"bytes":   {"L0", "unicode", "unicode/utf8"},
+	"path":    {"L0", "unicode/utf8", "strings"},
+	"strings": {"L0", "unicode", "unicode/utf8"},
+	"unicode": {},
+
+	"L2": {
+		"L1",
+		"bufio",
+		"bytes",
+		"path",
+		"strings",
+		"unicode",
+	},
+
+	// L3 adds reflection and some basic utility packages
+	// and interface definitions, but nothing that makes
+	// system calls.
+	"crypto":              {"L2", "hash"},          // interfaces
+	"crypto/cipher":       {"L2", "crypto/subtle"}, // interfaces
+	"crypto/subtle":       {},
+	"encoding/base32":     {"L2"},
+	"encoding/base64":     {"L2"},
+	"encoding/binary":     {"L2", "reflect"},
+	"hash":                {"L2"}, // interfaces
+	"hash/adler32":        {"L2", "hash"},
+	"hash/crc32":          {"L2", "hash"},
+	"hash/crc64":          {"L2", "hash"},
+	"hash/fnv":            {"L2", "hash"},
+	"image":               {"L2", "image/color"}, // interfaces
+	"image/color":         {"L2"},                // interfaces
+	"image/color/palette": {"L2", "image/color"},
+	"reflect":             {"L2"},
+
+	"L3": {
+		"L2",
+		"crypto",
+		"crypto/cipher",
+		"crypto/subtle",
+		"encoding/base32",
+		"encoding/base64",
+		"encoding/binary",
+		"hash",
+		"hash/adler32",
+		"hash/crc32",
+		"hash/crc64",
+		"hash/fnv",
+		"image",
+		"image/color",
+		"image/color/palette",
+		"reflect",
+	},
+
+	// End of linear dependency definitions.
+
+	// Operating system access.
+	"syscall":       {"L0", "unicode/utf16"},
+	"time":          {"L0", "syscall"},
+	"os":            {"L1", "os", "syscall", "time"},
+	"path/filepath": {"L2", "os", "syscall"},
+	"io/ioutil":     {"L2", "os", "path/filepath", "time"},
+	"os/exec":       {"L2", "os", "path/filepath", "syscall"},
+	"os/signal":     {"L2", "os", "syscall"},
+
+	// OS enables basic operating system functionality,
+	// but not direct use of package syscall, nor os/signal.
+	"OS": {
+		"io/ioutil",
+		"os",
+		"os/exec",
+		"path/filepath",
+		"time",
+	},
+
+	// Formatted I/O: few dependencies (L1) but we must add reflect.
+	"fmt": {"L1", "os", "reflect"},
+	"log": {"L1", "os", "fmt", "time"},
+
+	// Packages used by testing must be low-level (L2+fmt).
+	"regexp":         {"L2", "regexp/syntax"},
+	"regexp/syntax":  {"L2"},
+	"runtime/debug":  {"L2", "fmt", "io/ioutil", "os", "time"},
+	"runtime/pprof":  {"L2", "fmt", "text/tabwriter"},
+	"text/tabwriter": {"L2"},
+
+	"testing":        {"L2", "flag", "fmt", "os", "runtime/pprof", "time"},
+	"testing/iotest": {"L2", "log"},
+	"testing/quick":  {"L2", "flag", "fmt", "reflect"},
+
+	// L4 is defined as L3+fmt+log+time, because in general once
+	// you're using L3 packages, use of fmt, log, or time is not a big deal.
+	"L4": {
+		"L3",
+		"fmt",
+		"log",
+		"time",
+	},
+
+	// Go parser.
+	"go/ast":     {"L4", "OS", "go/scanner", "go/token"},
+	"go/doc":     {"L4", "go/ast", "go/token", "regexp", "text/template"},
+	"go/parser":  {"L4", "OS", "go/ast", "go/scanner", "go/token"},
+	"go/printer": {"L4", "OS", "go/ast", "go/scanner", "go/token", "text/tabwriter"},
+	"go/scanner": {"L4", "OS", "go/token"},
+	"go/token":   {"L4"},
+
+	"GOPARSER": {
+		"go/ast",
+		"go/doc",
+		"go/parser",
+		"go/printer",
+		"go/scanner",
+		"go/token",
+	},
+
+	// One of a kind.
+	"archive/tar":         {"L4", "OS", "syscall"},
+	"archive/zip":         {"L4", "OS", "compress/flate"},
+	"compress/bzip2":      {"L4"},
+	"compress/flate":      {"L4"},
+	"compress/gzip":       {"L4", "compress/flate"},
+	"compress/lzw":        {"L4"},
+	"compress/zlib":       {"L4", "compress/flate"},
+	"database/sql":        {"L4", "container/list", "database/sql/driver"},
+	"database/sql/driver": {"L4", "time"},
+	"debug/dwarf":         {"L4"},
+	"debug/elf":           {"L4", "OS", "debug/dwarf"},
+	"debug/gosym":         {"L4"},
+	"debug/macho":         {"L4", "OS", "debug/dwarf"},
+	"debug/pe":            {"L4", "OS", "debug/dwarf"},
+	"encoding":            {"L4"},
+	"encoding/ascii85":    {"L4"},
+	"encoding/asn1":       {"L4", "math/big"},
+	"encoding/csv":        {"L4"},
+	"encoding/gob":        {"L4", "OS", "encoding"},
+	"encoding/hex":        {"L4"},
+	"encoding/json":       {"L4", "encoding"},
+	"encoding/pem":        {"L4"},
+	"encoding/xml":        {"L4", "encoding"},
+	"flag":                {"L4", "OS"},
+	"go/build":            {"L4", "OS", "GOPARSER"},
+	"html":                {"L4"},
+	"image/draw":          {"L4"},
+	"image/gif":           {"L4", "compress/lzw", "image/color/palette", "image/draw"},
+	"image/jpeg":          {"L4"},
+	"image/png":           {"L4", "compress/zlib"},
+	"index/suffixarray":   {"L4", "regexp"},
+	"math/big":            {"L4"},
+	"mime":                {"L4", "OS", "syscall"},
+	"net/url":             {"L4"},
+	"text/scanner":        {"L4", "OS"},
+	"text/template/parse": {"L4"},
+
+	"html/template": {
+		"L4", "OS", "encoding/json", "html", "text/template",
+		"text/template/parse",
+	},
+	"text/template": {
+		"L4", "OS", "net/url", "text/template/parse",
+	},
+
+	// Cgo.
+	"runtime/cgo": {"L0", "C"},
+	"CGO":         {"C", "runtime/cgo"},
+
+	// Fake entry to satisfy the pseudo-import "C"
+	// that shows up in programs that use cgo.
+	"C": {},
+
+	// Plan 9 alone needs io/ioutil and os.
+	"os/user": {"L4", "CGO", "io/ioutil", "os", "syscall"},
+
+	// Basic networking.
+	// Because net must be used by any package that wants to
+	// do networking portably, it must have a small dependency set: just L1+basic os.
+	"net": {"L1", "CGO", "os", "syscall", "time"},
+
+	// NET enables use of basic network-related packages.
+	"NET": {
+		"net",
+		"mime",
+		"net/textproto",
+		"net/url",
+	},
+
+	// Uses of networking.
+	"log/syslog":    {"L4", "OS", "net"},
+	"net/mail":      {"L4", "NET", "OS"},
+	"net/textproto": {"L4", "OS", "net"},
+
+	// Core crypto.
+	"crypto/aes":    {"L3"},
+	"crypto/des":    {"L3"},
+	"crypto/hmac":   {"L3"},
+	"crypto/md5":    {"L3"},
+	"crypto/rc4":    {"L3"},
+	"crypto/sha1":   {"L3"},
+	"crypto/sha256": {"L3"},
+	"crypto/sha512": {"L3"},
+
+	"CRYPTO": {
+		"crypto/aes",
+		"crypto/des",
+		"crypto/hmac",
+		"crypto/md5",
+		"crypto/rc4",
+		"crypto/sha1",
+		"crypto/sha256",
+		"crypto/sha512",
+	},
+
+	// Random byte, number generation.
+	// This would be part of core crypto except that it imports
+	// math/big, which imports fmt.
+	"crypto/rand": {"L4", "CRYPTO", "OS", "math/big", "syscall", "internal/syscall"},
+
+	// Mathematical crypto: dependencies on fmt (L4) and math/big.
+	// We could avoid some of the fmt, but math/big imports fmt anyway.
+	"crypto/dsa":      {"L4", "CRYPTO", "math/big"},
+	"crypto/ecdsa":    {"L4", "CRYPTO", "crypto/elliptic", "math/big", "encoding/asn1"},
+	"crypto/elliptic": {"L4", "CRYPTO", "math/big"},
+	"crypto/rsa":      {"L4", "CRYPTO", "crypto/rand", "math/big"},
+
+	"CRYPTO-MATH": {
+		"CRYPTO",
+		"crypto/dsa",
+		"crypto/ecdsa",
+		"crypto/elliptic",
+		"crypto/rand",
+		"crypto/rsa",
+		"encoding/asn1",
+		"math/big",
+	},
+
+	// SSL/TLS.
+	"crypto/tls": {
+		"L4", "CRYPTO-MATH", "CGO", "OS",
+		"container/list", "crypto/x509", "encoding/pem", "net", "syscall",
+	},
+	"crypto/x509": {
+		"L4", "CRYPTO-MATH", "OS", "CGO",
+		"crypto/x509/pkix", "encoding/pem", "encoding/hex", "net", "syscall",
+	},
+	"crypto/x509/pkix": {"L4", "CRYPTO-MATH"},
+
+	// Simple net+crypto-aware packages.
+	"mime/multipart": {"L4", "OS", "mime", "crypto/rand", "net/textproto"},
+	"net/smtp":       {"L4", "CRYPTO", "NET", "crypto/tls"},
+
+	// HTTP, kingpin of dependencies.
+	"net/http": {
+		"L4", "NET", "OS",
+		"compress/gzip", "crypto/tls", "mime/multipart", "runtime/debug",
+		"net/http/internal",
+	},
+
+	// HTTP-using packages.
+	"expvar":            {"L4", "OS", "encoding/json", "net/http"},
+	"net/http/cgi":      {"L4", "NET", "OS", "crypto/tls", "net/http", "regexp"},
+	"net/http/fcgi":     {"L4", "NET", "OS", "net/http", "net/http/cgi"},
+	"net/http/httptest": {"L4", "NET", "OS", "crypto/tls", "flag", "net/http"},
+	"net/http/httputil": {"L4", "NET", "OS", "net/http", "net/http/internal"},
+	"net/http/pprof":    {"L4", "OS", "html/template", "net/http", "runtime/pprof"},
+	"net/rpc":           {"L4", "NET", "encoding/gob", "html/template", "net/http"},
+	"net/rpc/jsonrpc":   {"L4", "NET", "encoding/json", "net/rpc"},
+}
+
+// isMacro reports whether p is a package dependency macro
+// (uppercase name).
+func isMacro(p string) bool {
+	return 'A' <= p[0] && p[0] <= 'Z'
+}
+
+func allowed(pkg string) map[string]bool {
+	m := map[string]bool{}
+	var allow func(string)
+	allow = func(p string) {
+		if m[p] {
+			return
+		}
+		m[p] = true // set even for macros, to avoid loop on cycle
+
+		// Upper-case names are macro-expanded.
+		if isMacro(p) {
+			for _, pp := range pkgDeps[p] {
+				allow(pp)
+			}
+		}
+	}
+	for _, pp := range pkgDeps[pkg] {
+		allow(pp)
+	}
+	return m
+}
+
+var bools = []bool{false, true}
+var geese = []string{"android", "darwin", "dragonfly", "freebsd", "linux", "nacl", "netbsd", "openbsd", "plan9", "solaris", "windows"}
+var goarches = []string{"386", "amd64", "arm"}
+
+type osPkg struct {
+	goos, pkg string
+}
+
+// allowedErrors are the operating systems and packages known to contain errors
+// (currently just "no Go source files")
+var allowedErrors = map[osPkg]bool{
+	osPkg{"windows", "log/syslog"}: true,
+	osPkg{"plan9", "log/syslog"}:   true,
+}
+
+func TestDependencies(t *testing.T) {
+	if runtime.GOOS == "nacl" {
+		// NaCl tests run in a limited file system and we do not
+		// provide access to every source file.
+		t.Skip("skipping on NaCl")
+	}
+	var all []string
+
+	for k := range pkgDeps {
+		all = append(all, k)
+	}
+	sort.Strings(all)
+
+	ctxt := Default
+	test := func(mustImport bool) {
+		for _, pkg := range all {
+			if isMacro(pkg) {
+				continue
+			}
+			if pkg == "runtime/cgo" && !ctxt.CgoEnabled {
+				continue
+			}
+			p, err := ctxt.Import(pkg, "", 0)
+			if err != nil {
+				if allowedErrors[osPkg{ctxt.GOOS, pkg}] {
+					continue
+				}
+				if !ctxt.CgoEnabled && pkg == "runtime/cgo" {
+					continue
+				}
+				// Some of the combinations we try might not
+				// be reasonable (like arm,plan9,cgo), so ignore
+				// errors for the auto-generated combinations.
+				if !mustImport {
+					continue
+				}
+				t.Errorf("%s/%s/cgo=%v %v", ctxt.GOOS, ctxt.GOARCH, ctxt.CgoEnabled, err)
+				continue
+			}
+			ok := allowed(pkg)
+			var bad []string
+			for _, imp := range p.Imports {
+				if !ok[imp] {
+					bad = append(bad, imp)
+				}
+			}
+			if bad != nil {
+				t.Errorf("%s/%s/cgo=%v unexpected dependency: %s imports %v", ctxt.GOOS, ctxt.GOARCH, ctxt.CgoEnabled, pkg, bad)
+			}
+		}
+	}
+	test(true)
+
+	if testing.Short() {
+		t.Logf("skipping other systems")
+		return
+	}
+
+	for _, ctxt.GOOS = range geese {
+		for _, ctxt.GOARCH = range goarches {
+			for _, ctxt.CgoEnabled = range bools {
+				test(false)
+			}
+		}
+	}
+}
diff --git a/src/go/build/doc.go b/src/go/build/doc.go
new file mode 100644
index 000000000..d78ef3f1c
--- /dev/null
+++ b/src/go/build/doc.go
@@ -0,0 +1,140 @@
+// Copyright 2011 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package build gathers information about Go packages.
+//
+// Go Path
+//
+// The Go path is a list of directory trees containing Go source code.
+// It is consulted to resolve imports that cannot be found in the standard
+// Go tree.  The default path is the value of the GOPATH environment
+// variable, interpreted as a path list appropriate to the operating system
+// (on Unix, the variable is a colon-separated string;
+// on Windows, a semicolon-separated string;
+// on Plan 9, a list).
+//
+// Each directory listed in the Go path must have a prescribed structure:
+//
+// The src/ directory holds source code.  The path below 'src' determines
+// the import path or executable name.
+//
+// The pkg/ directory holds installed package objects.
+// As in the Go tree, each target operating system and
+// architecture pair has its own subdirectory of pkg
+// (pkg/GOOS_GOARCH).
+//
+// If DIR is a directory listed in the Go path, a package with
+// source in DIR/src/foo/bar can be imported as "foo/bar" and
+// has its compiled form installed to "DIR/pkg/GOOS_GOARCH/foo/bar.a"
+// (or, for gccgo, "DIR/pkg/gccgo/foo/libbar.a").
+//
+// The bin/ directory holds compiled commands.
+// Each command is named for its source directory, but only
+// using the final element, not the entire path.  That is, the
+// command with source in DIR/src/foo/quux is installed into
+// DIR/bin/quux, not DIR/bin/foo/quux.  The foo/ is stripped
+// so that you can add DIR/bin to your PATH to get at the
+// installed commands.
+//
+// Here's an example directory layout:
+//
+//	GOPATH=/home/user/gocode
+//
+//	/home/user/gocode/
+//	    src/
+//	        foo/
+//	            bar/               (go code in package bar)
+//	                x.go
+//	            quux/              (go code in package main)
+//	                y.go
+//	    bin/
+//	        quux                   (installed command)
+//	    pkg/
+//	        linux_amd64/
+//	            foo/
+//	                bar.a          (installed package object)
+//
+// Build Constraints
+//
+// A build constraint, also known as a build tag, is a line comment that begins
+//
+//	// +build
+//
+// that lists the conditions under which a file should be included in the package.
+// Constraints may appear in any kind of source file (not just Go), but
+// they must appear near the top of the file, preceded
+// only by blank lines and other line comments. These rules mean that in Go
+// files a build constraint must appear before the package clause.
+//
+// To distinguish build constraints from package documentation, a series of
+// build constraints must be followed by a blank line.
+//
+// A build constraint is evaluated as the OR of space-separated options;
+// each option evaluates as the AND of its comma-separated terms;
+// and each term is an alphanumeric word or, preceded by !, its negation.
+// That is, the build constraint:
+//
+//	// +build linux,386 darwin,!cgo
+//
+// corresponds to the boolean formula:
+//
+//	(linux AND 386) OR (darwin AND (NOT cgo))
+//
+// A file may have multiple build constraints. The overall constraint is the AND
+// of the individual constraints. That is, the build constraints:
+//
+//	// +build linux darwin
+//	// +build 386
+//
+// corresponds to the boolean formula:
+//
+//	(linux OR darwin) AND 386
+//
+// During a particular build, the following words are satisfied:
+//
+//	- the target operating system, as spelled by runtime.GOOS
+//	- the target architecture, as spelled by runtime.GOARCH
+//	- the compiler being used, either "gc" or "gccgo"
+//	- "cgo", if ctxt.CgoEnabled is true
+//	- "go1.1", from Go version 1.1 onward
+//	- "go1.2", from Go version 1.2 onward
+//	- "go1.3", from Go version 1.3 onward
+//	- any additional words listed in ctxt.BuildTags
+//
+// If a file's name, after stripping the extension and a possible _test suffix,
+// matches any of the following patterns:
+//	*_GOOS
+// 	*_GOARCH
+// 	*_GOOS_GOARCH
+// (example: source_windows_amd64.go) or the literals:
+//	GOOS
+// 	GOARCH
+// (example: windows.go) where GOOS and GOARCH represent any known operating
+// system and architecture values respectively, then the file is considered to
+// have an implicit build constraint requiring those terms.
+//
+// To keep a file from being considered for the build:
+//
+//	// +build ignore
+//
+// (any other unsatisfied word will work as well, but ``ignore'' is conventional.)
+//
+// To build a file only when using cgo, and only on Linux and OS X:
+//
+//	// +build linux,cgo darwin,cgo
+//
+// Such a file is usually paired with another file implementing the
+// default functionality for other systems, which in this case would
+// carry the constraint:
+//
+//	// +build !linux,!darwin !cgo
+//
+// Naming a file dns_windows.go will cause it to be included only when
+// building the package for Windows; similarly, math_386.s will be included
+// only when building the package for 32-bit x86.
+//
+// Using GOOS=android matches build tags and files as for GOOS=linux
+// in addition to android tags and files.
+//
+package build
diff --git a/src/go/build/read.go b/src/go/build/read.go
new file mode 100644
index 000000000..c8079dfd1
--- /dev/null
+++ b/src/go/build/read.go
@@ -0,0 +1,238 @@
+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package build
+
+import (
+	"bufio"
+	"errors"
+	"io"
+)
+
+type importReader struct {
+	b    *bufio.Reader
+	buf  []byte
+	peek byte
+	err  error
+	eof  bool
+	nerr int
+}
+
+func isIdent(c byte) bool {
+	return 'A' <= c && c <= 'Z' || 'a' <= c && c <= 'z' || '0' <= c && c <= '9' || c == '_' || c >= 0x80
+}
+
+var (
+	errSyntax = errors.New("syntax error")
+	errNUL    = errors.New("unexpected NUL in input")
+)
+
+// syntaxError records a syntax error, but only if an I/O error has not already been recorded.
+func (r *importReader) syntaxError() {
+	if r.err == nil {
+		r.err = errSyntax
+	}
+}
+
+// readByte reads the next byte from the input, saves it in buf, and returns it.
+// If an error occurs, readByte records the error in r.err and returns 0.
+func (r *importReader) readByte() byte {
+	c, err := r.b.ReadByte()
+	if err == nil {
+		r.buf = append(r.buf, c)
+		if c == 0 {
+			err = errNUL
+		}
+	}
+	if err != nil {
+		if err == io.EOF {
+			r.eof = true
+		} else if r.err == nil {
+			r.err = err
+		}
+		c = 0
+	}
+	return c
+}
+
+// peekByte returns the next byte from the input reader but does not advance beyond it.
+// If skipSpace is set, peekByte skips leading spaces and comments.
+func (r *importReader) peekByte(skipSpace bool) byte {
+	if r.err != nil {
+		if r.nerr++; r.nerr > 10000 {
+			panic("go/build: import reader looping")
+		}
+		return 0
+	}
+
+	// Use r.peek as first input byte.
+	// Don't just return r.peek here: it might have been left by peekByte(false)
+	// and this might be peekByte(true).
+	c := r.peek
+	if c == 0 {
+		c = r.readByte()
+	}
+	for r.err == nil && !r.eof {
+		if skipSpace {
+			// For the purposes of this reader, semicolons are never necessary to
+			// understand the input and are treated as spaces.
+			switch c {
+			case ' ', '\f', '\t', '\r', '\n', ';':
+				c = r.readByte()
+				continue
+
+			case '/':
+				c = r.readByte()
+				if c == '/' {
+					for c != '\n' && r.err == nil && !r.eof {
+						c = r.readByte()
+					}
+				} else if c == '*' {
+					var c1 byte
+					for (c != '*' || c1 != '/') && r.err == nil {
+						if r.eof {
+							r.syntaxError()
+						}
+						c, c1 = c1, r.readByte()
+					}
+				} else {
+					r.syntaxError()
+				}
+				c = r.readByte()
+				continue
+			}
+		}
+		break
+	}
+	r.peek = c
+	return r.peek
+}
+
+// nextByte is like peekByte but advances beyond the returned byte.
+func (r *importReader) nextByte(skipSpace bool) byte {
+	c := r.peekByte(skipSpace)
+	r.peek = 0
+	return c
+}
+
+// readKeyword reads the given keyword from the input.
+// If the keyword is not present, readKeyword records a syntax error.
+func (r *importReader) readKeyword(kw string) {
+	r.peekByte(true)
+	for i := 0; i < len(kw); i++ {
+		if r.nextByte(false) != kw[i] {
+			r.syntaxError()
+			return
+		}
+	}
+	if isIdent(r.peekByte(false)) {
+		r.syntaxError()
+	}
+}
+
+// readIdent reads an identifier from the input.
+// If an identifier is not present, readIdent records a syntax error.
+func (r *importReader) readIdent() {
+	c := r.peekByte(true)
+	if !isIdent(c) {
+		r.syntaxError()
+		return
+	}
+	for isIdent(r.peekByte(false)) {
+		r.peek = 0
+	}
+}
+
+// readString reads a quoted string literal from the input.
+// If an identifier is not present, readString records a syntax error.
+func (r *importReader) readString() {
+	switch r.nextByte(true) {
+	case '`':
+		for r.err == nil {
+			if r.nextByte(false) == '`' {
+				break
+			}
+			if r.eof {
+				r.syntaxError()
+			}
+		}
+	case '"':
+		for r.err == nil {
+			c := r.nextByte(false)
+			if c == '"' {
+				break
+			}
+			if r.eof || c == '\n' {
+				r.syntaxError()
+			}
+			if c == '\\' {
+				r.nextByte(false)
+			}
+		}
+	default:
+		r.syntaxError()
+	}
+}
+
+// readImport reads an import clause - optional identifier followed by quoted string -
+// from the input.
+func (r *importReader) readImport() {
+	c := r.peekByte(true)
+	if c == '.' {
+		r.peek = 0
+	} else if isIdent(c) {
+		r.readIdent()
+	}
+	r.readString()
+}
+
+// readComments is like ioutil.ReadAll, except that it only reads the leading
+// block of comments in the file.
+func readComments(f io.Reader) ([]byte, error) {
+	r := &importReader{b: bufio.NewReader(f)}
+	r.peekByte(true)
+	if r.err == nil && !r.eof {
+		// Didn't reach EOF, so must have found a non-space byte. Remove it.
+		r.buf = r.buf[:len(r.buf)-1]
+	}
+	return r.buf, r.err
+}
+
+// readImports is like ioutil.ReadAll, except that it expects a Go file as input
+// and stops reading the input once the imports have completed.
+func readImports(f io.Reader, reportSyntaxError bool) ([]byte, error) {
+	r := &importReader{b: bufio.NewReader(f)}
+
+	r.readKeyword("package")
+	r.readIdent()
+	for r.peekByte(true) == 'i' {
+		r.readKeyword("import")
+		if r.peekByte(true) == '(' {
+			r.nextByte(false)
+			for r.peekByte(true) != ')' && r.err == nil {
+				r.readImport()
+			}
+			r.nextByte(false)
+		} else {
+			r.readImport()
+		}
+	}
+
+	// If we stopped successfully before EOF, we read a byte that told us we were done.
+	// Return all but that last byte, which would cause a syntax error if we let it through.
+	if r.err == nil && !r.eof {
+		return r.buf[:len(r.buf)-1], nil
+	}
+
+	// If we stopped for a syntax error, consume the whole file so that
+	// we are sure we don't change the errors that go/parser returns.
+	if r.err == errSyntax && !reportSyntaxError {
+		r.err = nil
+		for r.err == nil && !r.eof {
+			r.readByte()
+		}
+	}
+
+	return r.buf, r.err
+}
diff --git a/src/go/build/read_test.go b/src/go/build/read_test.go
new file mode 100644
index 000000000..2dcc1208f
--- /dev/null
+++ b/src/go/build/read_test.go
@@ -0,0 +1,226 @@
+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package build
+
+import (
+	"io"
+	"strings"
+	"testing"
+)
+
+const quote = "`"
+
+type readTest struct {
+	// Test input contains ℙ where readImports should stop.
+	in  string
+	err string
+}
+
+var readImportsTests = []readTest{
+	{
+		`package p`,
+		"",
+	},
+	{
+		`package p; import "x"`,
+		"",
+	},
+	{
+		`package p; import . "x"`,
+		"",
+	},
+	{
+		`package p; import "x";ℙvar x = 1`,
+		"",
+	},
+	{
+		`package p
+		
+		// comment
+		
+		import "x"
+		import _ "x"
+		import a "x"
+		
+		/* comment */
+		
+		import (
+			"x" /* comment */
+			_ "x"
+			a "x" // comment
+			` + quote + `x` + quote + `
+			_ /*comment*/ ` + quote + `x` + quote + `
+			a ` + quote + `x` + quote + `
+		)
+		import (
+		)
+		import ()
+		import()import()import()
+		import();import();import()
+		
+		ℙvar x = 1
+		`,
+		"",
+	},
+}
+
+var readCommentsTests = []readTest{
+	{
+		`ℙpackage p`,
+		"",
+	},
+	{
+		`ℙpackage p; import "x"`,
+		"",
+	},
+	{
+		`ℙpackage p; import . "x"`,
+		"",
+	},
+	{
+		`// foo
+
+		/* bar */
+
+		/* quux */ // baz
+		
+		/*/ zot */
+
+		// asdf
+		ℙHello, world`,
+		"",
+	},
+}
+
+func testRead(t *testing.T, tests []readTest, read func(io.Reader) ([]byte, error)) {
+	for i, tt := range tests {
+		var in, testOut string
+		j := strings.Index(tt.in, "ℙ")
+		if j < 0 {
+			in = tt.in
+			testOut = tt.in
+		} else {
+			in = tt.in[:j] + tt.in[j+len("ℙ"):]
+			testOut = tt.in[:j]
+		}
+		r := strings.NewReader(in)
+		buf, err := read(r)
+		if err != nil {
+			if tt.err == "" {
+				t.Errorf("#%d: err=%q, expected success (%q)", i, err, string(buf))
+				continue
+			}
+			if !strings.Contains(err.Error(), tt.err) {
+				t.Errorf("#%d: err=%q, expected %q", i, err, tt.err)
+				continue
+			}
+			continue
+		}
+		if err == nil && tt.err != "" {
+			t.Errorf("#%d: success, expected %q", i, tt.err)
+			continue
+		}
+
+		out := string(buf)
+		if out != testOut {
+			t.Errorf("#%d: wrong output:\nhave %q\nwant %q\n", i, out, testOut)
+		}
+	}
+}
+
+func TestReadImports(t *testing.T) {
+	testRead(t, readImportsTests, func(r io.Reader) ([]byte, error) { return readImports(r, true) })
+}
+
+func TestReadComments(t *testing.T) {
+	testRead(t, readCommentsTests, readComments)
+}
+
+var readFailuresTests = []readTest{
+	{
+		`package`,
+		"syntax error",
+	},
+	{
+		"package p\n\x00\nimport `math`\n",
+		"unexpected NUL in input",
+	},
+	{
+		`package p; import`,
+		"syntax error",
+	},
+	{
+		`package p; import "`,
+		"syntax error",
+	},
+	{
+		"package p; import ` \n\n",
+		"syntax error",
+	},
+	{
+		`package p; import "x`,
+		"syntax error",
+	},
+	{
+		`package p; import _`,
+		"syntax error",
+	},
+	{
+		`package p; import _ "`,
+		"syntax error",
+	},
+	{
+		`package p; import _ "x`,
+		"syntax error",
+	},
+	{
+		`package p; import .`,
+		"syntax error",
+	},
+	{
+		`package p; import . "`,
+		"syntax error",
+	},
+	{
+		`package p; import . "x`,
+		"syntax error",
+	},
+	{
+		`package p; import (`,
+		"syntax error",
+	},
+	{
+		`package p; import ("`,
+		"syntax error",
+	},
+	{
+		`package p; import ("x`,
+		"syntax error",
+	},
+	{
+		`package p; import ("x"`,
+		"syntax error",
+	},
+}
+
+func TestReadFailures(t *testing.T) {
+	// Errors should be reported (true arg to readImports).
+	testRead(t, readFailuresTests, func(r io.Reader) ([]byte, error) { return readImports(r, true) })
+}
+
+func TestReadFailuresIgnored(t *testing.T) {
+	// Syntax errors should not be reported (false arg to readImports).
+	// Instead, entire file should be the output and no error.
+	// Convert tests not to return syntax errors.
+	tests := make([]readTest, len(readFailuresTests))
+	copy(tests, readFailuresTests)
+	for i := range tests {
+		tt := &tests[i]
+		if !strings.Contains(tt.err, "NUL") {
+			tt.err = ""
+		}
+	}
+	testRead(t, tests, func(r io.Reader) ([]byte, error) { return readImports(r, false) })
+}
diff --git a/src/go/build/syslist.go b/src/go/build/syslist.go
new file mode 100644
index 000000000..965f873df
--- /dev/null
+++ b/src/go/build/syslist.go
@@ -0,0 +1,8 @@
+// Copyright 2011 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package build
+
+const goosList = "android darwin dragonfly freebsd linux nacl netbsd openbsd plan9 solaris windows "
+const goarchList = "386 amd64 amd64p32 arm "
diff --git a/src/go/build/syslist_test.go b/src/go/build/syslist_test.go
new file mode 100644
index 000000000..3be2928f5
--- /dev/null
+++ b/src/go/build/syslist_test.go
@@ -0,0 +1,62 @@
+// Copyright 2011 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package build
+
+import (
+	"runtime"
+	"testing"
+)
+
+var (
+	thisOS    = runtime.GOOS
+	thisArch  = runtime.GOARCH
+	otherOS   = anotherOS()
+	otherArch = anotherArch()
+)
+
+func anotherOS() string {
+	if thisOS != "darwin" {
+		return "darwin"
+	}
+	return "linux"
+}
+
+func anotherArch() string {
+	if thisArch != "amd64" {
+		return "amd64"
+	}
+	return "386"
+}
+
+type GoodFileTest struct {
+	name   string
+	result bool
+}
+
+var tests = []GoodFileTest{
+	{"file.go", true},
+	{"file.c", true},
+	{"file_foo.go", true},
+	{"file_" + thisArch + ".go", true},
+	{"file_" + otherArch + ".go", false},
+	{"file_" + thisOS + ".go", true},
+	{"file_" + otherOS + ".go", false},
+	{"file_" + thisOS + "_" + thisArch + ".go", true},
+	{"file_" + otherOS + "_" + thisArch + ".go", false},
+	{"file_" + thisOS + "_" + otherArch + ".go", false},
+	{"file_" + otherOS + "_" + otherArch + ".go", false},
+	{"file_foo_" + thisArch + ".go", true},
+	{"file_foo_" + otherArch + ".go", false},
+	{"file_" + thisOS + ".c", true},
+	{"file_" + otherOS + ".c", false},
+}
+
+func TestGoodOSArch(t *testing.T) {
+	for _, test := range tests {
+		if Default.goodOSArchFile(test.name, make(map[string]bool)) != test.result {
+			t.Fatalf("goodOSArchFile(%q) != %v", test.name, test.result)
+		}
+	}
+}
diff --git a/src/go/build/testdata/other/file/file.go b/src/go/build/testdata/other/file/file.go
new file mode 100644
index 000000000..bbfd3e9e5
--- /dev/null
+++ b/src/go/build/testdata/other/file/file.go
@@ -0,0 +1,5 @@
+// Test data - not compiled.
+
+package file
+
+func F() {}
diff --git a/src/go/build/testdata/other/main.go b/src/go/build/testdata/other/main.go
new file mode 100644
index 000000000..e0904357c
--- /dev/null
+++ b/src/go/build/testdata/other/main.go
@@ -0,0 +1,11 @@
+// Test data - not compiled.
+
+package main
+
+import (
+	"./file"
+)
+
+func main() {
+	file.F()
+}
diff --git a/src/go/doc/Makefile b/src/go/doc/Makefile
new file mode 100644
index 000000000..ca4948f91
--- /dev/null
+++ b/src/go/doc/Makefile
@@ -0,0 +1,7 @@
+# Copyright 2009 The Go Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file.
+
+# Script to test heading detection heuristic
+headscan: headscan.go
+	go build headscan.go
diff --git a/src/go/doc/comment.go b/src/go/doc/comment.go
new file mode 100644
index 000000000..f414ca409
--- /dev/null
+++ b/src/go/doc/comment.go
@@ -0,0 +1,480 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Godoc comment extraction and comment -> HTML formatting.
+
+package doc
+
+import (
+	"io"
+	"regexp"
+	"strings"
+	"text/template" // for HTMLEscape
+	"unicode"
+	"unicode/utf8"
+)
+
+var (
+	ldquo = []byte("&ldquo;")
+	rdquo = []byte("&rdquo;")
+)
+
+// Escape comment text for HTML. If nice is set,
+// also turn `` into &ldquo; and '' into &rdquo;.
+func commentEscape(w io.Writer, text string, nice bool) {
+	last := 0
+	if nice {
+		for i := 0; i < len(text)-1; i++ {
+			ch := text[i]
+			if ch == text[i+1] && (ch == '`' || ch == '\'') {
+				template.HTMLEscape(w, []byte(text[last:i]))
+				last = i + 2
+				switch ch {
+				case '`':
+					w.Write(ldquo)
+				case '\'':
+					w.Write(rdquo)
+				}
+				i++ // loop will add one more
+			}
+		}
+	}
+	template.HTMLEscape(w, []byte(text[last:]))
+}
+
+const (
+	// Regexp for Go identifiers
+	identRx = `[\pL_][\pL_0-9]*`
+
+	// Regexp for URLs
+	protocol = `https?|ftp|file|gopher|mailto|news|nntp|telnet|wais|prospero`
+	hostPart = `[a-zA-Z0-9_@\-]+`
+	filePart = `[a-zA-Z0-9_?%#~&/\-+=()]+` // parentheses may not be matching; see pairedParensPrefixLen
+	urlRx    = `(` + protocol + `)://` +   // http://
+		hostPart + `([.:]` + hostPart + `)*/?` + // //www.google.com:8080/
+		filePart + `([:.,]` + filePart + `)*`
+)
+
+var matchRx = regexp.MustCompile(`(` + urlRx + `)|(` + identRx + `)`)
+
+var (
+	html_a      = []byte(`<a href="`)
+	html_aq     = []byte(`">`)
+	html_enda   = []byte("</a>")
+	html_i      = []byte("<i>")
+	html_endi   = []byte("</i>")
+	html_p      = []byte("<p>\n")
+	html_endp   = []byte("</p>\n")
+	html_pre    = []byte("<pre>")
+	html_endpre = []byte("</pre>\n")
+	html_h      = []byte(`<h3 id="`)
+	html_hq     = []byte(`">`)
+	html_endh   = []byte("</h3>\n")
+)
+
+// pairedParensPrefixLen returns the length of the longest prefix of s containing paired parentheses.
+func pairedParensPrefixLen(s string) int {
+	parens := 0
+	l := len(s)
+	for i, ch := range s {
+		switch ch {
+		case '(':
+			if parens == 0 {
+				l = i
+			}
+			parens++
+		case ')':
+			parens--
+			if parens == 0 {
+				l = len(s)
+			} else if parens < 0 {
+				return i
+			}
+		}
+	}
+	return l
+}
+
+// Emphasize and escape a line of text for HTML. URLs are converted into links;
+// if the URL also appears in the words map, the link is taken from the map (if
+// the corresponding map value is the empty string, the URL is not converted
+// into a link). Go identifiers that appear in the words map are italicized; if
+// the corresponding map value is not the empty string, it is considered a URL
+// and the word is converted into a link. If nice is set, the remaining text's
+// appearance is improved where it makes sense (e.g., `` is turned into &ldquo;
+// and '' into &rdquo;).
+func emphasize(w io.Writer, line string, words map[string]string, nice bool) {
+	for {
+		m := matchRx.FindStringSubmatchIndex(line)
+		if m == nil {
+			break
+		}
+		// m >= 6 (two parenthesized sub-regexps in matchRx, 1st one is urlRx)
+
+		// write text before match
+		commentEscape(w, line[0:m[0]], nice)
+
+		// adjust match if necessary
+		match := line[m[0]:m[1]]
+		if n := pairedParensPrefixLen(match); n < len(match) {
+			// match contains unpaired parentheses (rare);
+			// redo matching with shortened line for correct indices
+			m = matchRx.FindStringSubmatchIndex(line[:m[0]+n])
+			match = match[:n]
+		}
+
+		// analyze match
+		url := ""
+		italics := false
+		if words != nil {
+			url, italics = words[match]
+		}
+		if m[2] >= 0 {
+			// match against first parenthesized sub-regexp; must be match against urlRx
+			if !italics {
+				// no alternative URL in words list, use match instead
+				url = match
+			}
+			italics = false // don't italicize URLs
+		}
+
+		// write match
+		if len(url) > 0 {
+			w.Write(html_a)
+			template.HTMLEscape(w, []byte(url))
+			w.Write(html_aq)
+		}
+		if italics {
+			w.Write(html_i)
+		}
+		commentEscape(w, match, nice)
+		if italics {
+			w.Write(html_endi)
+		}
+		if len(url) > 0 {
+			w.Write(html_enda)
+		}
+
+		// advance
+		line = line[m[1]:]
+	}
+	commentEscape(w, line, nice)
+}
+
+func indentLen(s string) int {
+	i := 0
+	for i < len(s) && (s[i] == ' ' || s[i] == '\t') {
+		i++
+	}
+	return i
+}
+
+func isBlank(s string) bool {
+	return len(s) == 0 || (len(s) == 1 && s[0] == '\n')
+}
+
+func commonPrefix(a, b string) string {
+	i := 0
+	for i < len(a) && i < len(b) && a[i] == b[i] {
+		i++
+	}
+	return a[0:i]
+}
+
+func unindent(block []string) {
+	if len(block) == 0 {
+		return
+	}
+
+	// compute maximum common white prefix
+	prefix := block[0][0:indentLen(block[0])]
+	for _, line := range block {
+		if !isBlank(line) {
+			prefix = commonPrefix(prefix, line[0:indentLen(line)])
+		}
+	}
+	n := len(prefix)
+
+	// remove
+	for i, line := range block {
+		if !isBlank(line) {
+			block[i] = line[n:]
+		}
+	}
+}
+
+// heading returns the trimmed line if it passes as a section heading;
+// otherwise it returns the empty string.
+func heading(line string) string {
+	line = strings.TrimSpace(line)
+	if len(line) == 0 {
+		return ""
+	}
+
+	// a heading must start with an uppercase letter
+	r, _ := utf8.DecodeRuneInString(line)
+	if !unicode.IsLetter(r) || !unicode.IsUpper(r) {
+		return ""
+	}
+
+	// it must end in a letter or digit:
+	r, _ = utf8.DecodeLastRuneInString(line)
+	if !unicode.IsLetter(r) && !unicode.IsDigit(r) {
+		return ""
+	}
+
+	// exclude lines with illegal characters
+	if strings.IndexAny(line, ",.;:!?+*/=()[]{}_^°&§~%#@<\">\\") >= 0 {
+		return ""
+	}
+
+	// allow "'" for possessive "'s" only
+	for b := line; ; {
+		i := strings.IndexRune(b, '\'')
+		if i < 0 {
+			break
+		}
+		if i+1 >= len(b) || b[i+1] != 's' || (i+2 < len(b) && b[i+2] != ' ') {
+			return "" // not followed by "s "
+		}
+		b = b[i+2:]
+	}
+
+	return line
+}
+
+type op int
+
+const (
+	opPara op = iota
+	opHead
+	opPre
+)
+
+type block struct {
+	op    op
+	lines []string
+}
+
+var nonAlphaNumRx = regexp.MustCompile(`[^a-zA-Z0-9]`)
+
+func anchorID(line string) string {
+	// Add a "hdr-" prefix to avoid conflicting with IDs used for package symbols.
+	return "hdr-" + nonAlphaNumRx.ReplaceAllString(line, "_")
+}
+
+// ToHTML converts comment text to formatted HTML.
+// The comment was prepared by DocReader,
+// so it is known not to have leading, trailing blank lines
+// nor to have trailing spaces at the end of lines.
+// The comment markers have already been removed.
+//
+// Each span of unindented non-blank lines is converted into
+// a single paragraph. There is one exception to the rule: a span that
+// consists of a single line, is followed by another paragraph span,
+// begins with a capital letter, and contains no punctuation
+// is formatted as a heading.
+//
+// A span of indented lines is converted into a <pre> block,
+// with the common indent prefix removed.
+//
+// URLs in the comment text are converted into links; if the URL also appears
+// in the words map, the link is taken from the map (if the corresponding map
+// value is the empty string, the URL is not converted into a link).
+//
+// Go identifiers that appear in the words map are italicized; if the corresponding
+// map value is not the empty string, it is considered a URL and the word is converted
+// into a link.
+func ToHTML(w io.Writer, text string, words map[string]string) {
+	for _, b := range blocks(text) {
+		switch b.op {
+		case opPara:
+			w.Write(html_p)
+			for _, line := range b.lines {
+				emphasize(w, line, words, true)
+			}
+			w.Write(html_endp)
+		case opHead:
+			w.Write(html_h)
+			id := ""
+			for _, line := range b.lines {
+				if id == "" {
+					id = anchorID(line)
+					w.Write([]byte(id))
+					w.Write(html_hq)
+				}
+				commentEscape(w, line, true)
+			}
+			if id == "" {
+				w.Write(html_hq)
+			}
+			w.Write(html_endh)
+		case opPre:
+			w.Write(html_pre)
+			for _, line := range b.lines {
+				emphasize(w, line, nil, false)
+			}
+			w.Write(html_endpre)
+		}
+	}
+}
+
+func blocks(text string) []block {
+	var (
+		out  []block
+		para []string
+
+		lastWasBlank   = false
+		lastWasHeading = false
+	)
+
+	close := func() {
+		if para != nil {
+			out = append(out, block{opPara, para})
+			para = nil
+		}
+	}
+
+	lines := strings.SplitAfter(text, "\n")
+	unindent(lines)
+	for i := 0; i < len(lines); {
+		line := lines[i]
+		if isBlank(line) {
+			// close paragraph
+			close()
+			i++
+			lastWasBlank = true
+			continue
+		}
+		if indentLen(line) > 0 {
+			// close paragraph
+			close()
+
+			// count indented or blank lines
+			j := i + 1
+			for j < len(lines) && (isBlank(lines[j]) || indentLen(lines[j]) > 0) {
+				j++
+			}
+			// but not trailing blank lines
+			for j > i && isBlank(lines[j-1]) {
+				j--
+			}
+			pre := lines[i:j]
+			i = j
+
+			unindent(pre)
+
+			// put those lines in a pre block
+			out = append(out, block{opPre, pre})
+			lastWasHeading = false
+			continue
+		}
+
+		if lastWasBlank && !lastWasHeading && i+2 < len(lines) &&
+			isBlank(lines[i+1]) && !isBlank(lines[i+2]) && indentLen(lines[i+2]) == 0 {
+			// current line is non-blank, surrounded by blank lines
+			// and the next non-blank line is not indented: this
+			// might be a heading.
+			if head := heading(line); head != "" {
+				close()
+				out = append(out, block{opHead, []string{head}})
+				i += 2
+				lastWasHeading = true
+				continue
+			}
+		}
+
+		// open paragraph
+		lastWasBlank = false
+		lastWasHeading = false
+		para = append(para, lines[i])
+		i++
+	}
+	close()
+
+	return out
+}
+
+// ToText prepares comment text for presentation in textual output.
+// It wraps paragraphs of text to width or fewer Unicode code points
+// and then prefixes each line with the indent.  In preformatted sections
+// (such as program text), it prefixes each non-blank line with preIndent.
+func ToText(w io.Writer, text string, indent, preIndent string, width int) {
+	l := lineWrapper{
+		out:    w,
+		width:  width,
+		indent: indent,
+	}
+	for _, b := range blocks(text) {
+		switch b.op {
+		case opPara:
+			// l.write will add leading newline if required
+			for _, line := range b.lines {
+				l.write(line)
+			}
+			l.flush()
+		case opHead:
+			w.Write(nl)
+			for _, line := range b.lines {
+				l.write(line + "\n")
+			}
+			l.flush()
+		case opPre:
+			w.Write(nl)
+			for _, line := range b.lines {
+				if isBlank(line) {
+					w.Write([]byte("\n"))
+				} else {
+					w.Write([]byte(preIndent))
+					w.Write([]byte(line))
+				}
+			}
+		}
+	}
+}
+
+type lineWrapper struct {
+	out       io.Writer
+	printed   bool
+	width     int
+	indent    string
+	n         int
+	pendSpace int
+}
+
+var nl = []byte("\n")
+var space = []byte(" ")
+
+func (l *lineWrapper) write(text string) {
+	if l.n == 0 && l.printed {
+		l.out.Write(nl) // blank line before new paragraph
+	}
+	l.printed = true
+
+	for _, f := range strings.Fields(text) {
+		w := utf8.RuneCountInString(f)
+		// wrap if line is too long
+		if l.n > 0 && l.n+l.pendSpace+w > l.width {
+			l.out.Write(nl)
+			l.n = 0
+			l.pendSpace = 0
+		}
+		if l.n == 0 {
+			l.out.Write([]byte(l.indent))
+		}
+		l.out.Write(space[:l.pendSpace])
+		l.out.Write([]byte(f))
+		l.n += l.pendSpace + w
+		l.pendSpace = 1
+	}
+}
+
+func (l *lineWrapper) flush() {
+	if l.n == 0 {
+		return
+	}
+	l.out.Write(nl)
+	l.pendSpace = 0
+	l.n = 0
+}
diff --git a/src/go/doc/comment_test.go b/src/go/doc/comment_test.go
new file mode 100644
index 000000000..ad65c2a27
--- /dev/null
+++ b/src/go/doc/comment_test.go
@@ -0,0 +1,207 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import (
+	"bytes"
+	"reflect"
+	"testing"
+)
+
+var headingTests = []struct {
+	line string
+	ok   bool
+}{
+	{"Section", true},
+	{"A typical usage", true},
+	{"ΔΛΞ is Greek", true},
+	{"Foo 42", true},
+	{"", false},
+	{"section", false},
+	{"A typical usage:", false},
+	{"This code:", false},
+	{"δ is Greek", false},
+	{"Foo §", false},
+	{"Fermat's Last Sentence", true},
+	{"Fermat's", true},
+	{"'sX", false},
+	{"Ted 'Too' Bar", false},
+	{"Use n+m", false},
+	{"Scanning:", false},
+	{"N:M", false},
+}
+
+func TestIsHeading(t *testing.T) {
+	for _, tt := range headingTests {
+		if h := heading(tt.line); (len(h) > 0) != tt.ok {
+			t.Errorf("isHeading(%q) = %v, want %v", tt.line, h, tt.ok)
+		}
+	}
+}
+
+var blocksTests = []struct {
+	in   string
+	out  []block
+	text string
+}{
+	{
+		in: `Para 1.
+Para 1 line 2.
+
+Para 2.
+
+Section
+
+Para 3.
+
+	pre
+	pre1
+
+Para 4.
+
+	pre
+	pre1
+
+	pre2
+
+Para 5.
+
+
+	pre
+
+
+	pre1
+	pre2
+
+Para 6.
+	pre
+	pre2
+`,
+		out: []block{
+			{opPara, []string{"Para 1.\n", "Para 1 line 2.\n"}},
+			{opPara, []string{"Para 2.\n"}},
+			{opHead, []string{"Section"}},
+			{opPara, []string{"Para 3.\n"}},
+			{opPre, []string{"pre\n", "pre1\n"}},
+			{opPara, []string{"Para 4.\n"}},
+			{opPre, []string{"pre\n", "pre1\n", "\n", "pre2\n"}},
+			{opPara, []string{"Para 5.\n"}},
+			{opPre, []string{"pre\n", "\n", "\n", "pre1\n", "pre2\n"}},
+			{opPara, []string{"Para 6.\n"}},
+			{opPre, []string{"pre\n", "pre2\n"}},
+		},
+		text: `.   Para 1. Para 1 line 2.
+
+.   Para 2.
+
+
+.   Section
+
+.   Para 3.
+
+$	pre
+$	pre1
+
+.   Para 4.
+
+$	pre
+$	pre1
+
+$	pre2
+
+.   Para 5.
+
+$	pre
+
+
+$	pre1
+$	pre2
+
+.   Para 6.
+
+$	pre
+$	pre2
+`,
+	},
+}
+
+func TestBlocks(t *testing.T) {
+	for i, tt := range blocksTests {
+		b := blocks(tt.in)
+		if !reflect.DeepEqual(b, tt.out) {
+			t.Errorf("#%d: mismatch\nhave: %v\nwant: %v", i, b, tt.out)
+		}
+	}
+}
+
+func TestToText(t *testing.T) {
+	var buf bytes.Buffer
+	for i, tt := range blocksTests {
+		ToText(&buf, tt.in, ".   ", "$\t", 40)
+		if have := buf.String(); have != tt.text {
+			t.Errorf("#%d: mismatch\nhave: %s\nwant: %s\nhave vs want:\n%q\n%q", i, have, tt.text, have, tt.text)
+		}
+		buf.Reset()
+	}
+}
+
+var emphasizeTests = []struct {
+	in, out string
+}{
+	{"http://www.google.com/", `<a href="http://www.google.com/">http://www.google.com/</a>`},
+	{"https://www.google.com/", `<a href="https://www.google.com/">https://www.google.com/</a>`},
+	{"http://www.google.com/path.", `<a href="http://www.google.com/path">http://www.google.com/path</a>.`},
+	{"http://en.wikipedia.org/wiki/Camellia_(cipher)", `<a href="http://en.wikipedia.org/wiki/Camellia_(cipher)">http://en.wikipedia.org/wiki/Camellia_(cipher)</a>`},
+	{"(http://www.google.com/)", `(<a href="http://www.google.com/">http://www.google.com/</a>)`},
+	{"http://gmail.com)", `<a href="http://gmail.com">http://gmail.com</a>)`},
+	{"((http://gmail.com))", `((<a href="http://gmail.com">http://gmail.com</a>))`},
+	{"http://gmail.com ((http://gmail.com)) ()", `<a href="http://gmail.com">http://gmail.com</a> ((<a href="http://gmail.com">http://gmail.com</a>)) ()`},
+	{"Foo bar http://example.com/ quux!", `Foo bar <a href="http://example.com/">http://example.com/</a> quux!`},
+	{"Hello http://example.com/%2f/ /world.", `Hello <a href="http://example.com/%2f/">http://example.com/%2f/</a> /world.`},
+	{"Lorem http: ipsum //host/path", "Lorem http: ipsum //host/path"},
+	{"javascript://is/not/linked", "javascript://is/not/linked"},
+}
+
+func TestEmphasize(t *testing.T) {
+	for i, tt := range emphasizeTests {
+		var buf bytes.Buffer
+		emphasize(&buf, tt.in, nil, true)
+		out := buf.String()
+		if out != tt.out {
+			t.Errorf("#%d: mismatch\nhave: %v\nwant: %v", i, out, tt.out)
+		}
+	}
+}
+
+var pairedParensPrefixLenTests = []struct {
+	in, out string
+}{
+	{"", ""},
+	{"foo", "foo"},
+	{"()", "()"},
+	{"foo()", "foo()"},
+	{"foo()()()", "foo()()()"},
+	{"foo()((()()))", "foo()((()()))"},
+	{"foo()((()()))bar", "foo()((()()))bar"},
+	{"foo)", "foo"},
+	{"foo))", "foo"},
+	{"foo)))))", "foo"},
+	{"(foo", ""},
+	{"((foo", ""},
+	{"(((((foo", ""},
+	{"(foo)", "(foo)"},
+	{"((((foo))))", "((((foo))))"},
+	{"foo()())", "foo()()"},
+	{"foo((()())", "foo"},
+	{"foo((()())) (() foo ", "foo((()())) "},
+}
+
+func TestPairedParensPrefixLen(t *testing.T) {
+	for i, tt := range pairedParensPrefixLenTests {
+		if out := tt.in[:pairedParensPrefixLen(tt.in)]; out != tt.out {
+			t.Errorf("#%d: mismatch\nhave: %q\nwant: %q", i, out, tt.out)
+		}
+	}
+}
diff --git a/src/go/doc/doc.go b/src/go/doc/doc.go
new file mode 100644
index 000000000..4264940a0
--- /dev/null
+++ b/src/go/doc/doc.go
@@ -0,0 +1,111 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package doc extracts source code documentation from a Go AST.
+package doc
+
+import (
+	"go/ast"
+	"go/token"
+)
+
+// Package is the documentation for an entire package.
+type Package struct {
+	Doc        string
+	Name       string
+	ImportPath string
+	Imports    []string
+	Filenames  []string
+	Notes      map[string][]*Note
+	// DEPRECATED. For backward compatibility Bugs is still populated,
+	// but all new code should use Notes instead.
+	Bugs []string
+
+	// declarations
+	Consts []*Value
+	Types  []*Type
+	Vars   []*Value
+	Funcs  []*Func
+}
+
+// Value is the documentation for a (possibly grouped) var or const declaration.
+type Value struct {
+	Doc   string
+	Names []string // var or const names in declaration order
+	Decl  *ast.GenDecl
+
+	order int
+}
+
+// Type is the documentation for a type declaration.
+type Type struct {
+	Doc  string
+	Name string
+	Decl *ast.GenDecl
+
+	// associated declarations
+	Consts  []*Value // sorted list of constants of (mostly) this type
+	Vars    []*Value // sorted list of variables of (mostly) this type
+	Funcs   []*Func  // sorted list of functions returning this type
+	Methods []*Func  // sorted list of methods (including embedded ones) of this type
+}
+
+// Func is the documentation for a func declaration.
+type Func struct {
+	Doc  string
+	Name string
+	Decl *ast.FuncDecl
+
+	// methods
+	// (for functions, these fields have the respective zero value)
+	Recv  string // actual   receiver "T" or "*T"
+	Orig  string // original receiver "T" or "*T"
+	Level int    // embedding level; 0 means not embedded
+}
+
+// A Note represents a marked comment starting with "MARKER(uid): note body".
+// Any note with a marker of 2 or more upper case [A-Z] letters and a uid of
+// at least one character is recognized. The ":" following the uid is optional.
+// Notes are collected in the Package.Notes map indexed by the notes marker.
+type Note struct {
+	Pos, End token.Pos // position range of the comment containing the marker
+	UID      string    // uid found with the marker
+	Body     string    // note body text
+}
+
+// Mode values control the operation of New.
+type Mode int
+
+const (
+	// extract documentation for all package-level declarations,
+	// not just exported ones
+	AllDecls Mode = 1 << iota
+
+	// show all embedded methods, not just the ones of
+	// invisible (unexported) anonymous fields
+	AllMethods
+)
+
+// New computes the package documentation for the given package AST.
+// New takes ownership of the AST pkg and may edit or overwrite it.
+//
+func New(pkg *ast.Package, importPath string, mode Mode) *Package {
+	var r reader
+	r.readPackage(pkg, mode)
+	r.computeMethodSets()
+	r.cleanupTypes()
+	return &Package{
+		Doc:        r.doc,
+		Name:       pkg.Name,
+		ImportPath: importPath,
+		Imports:    sortedKeys(r.imports),
+		Filenames:  r.filenames,
+		Notes:      r.notes,
+		Bugs:       noteBodies(r.notes["BUG"]),
+		Consts:     sortedValues(r.values, token.CONST),
+		Types:      sortedTypes(r.types, mode&AllMethods != 0),
+		Vars:       sortedValues(r.values, token.VAR),
+		Funcs:      sortedFuncs(r.funcs, true),
+	}
+}
diff --git a/src/go/doc/doc_test.go b/src/go/doc/doc_test.go
new file mode 100644
index 000000000..ad8ba5378
--- /dev/null
+++ b/src/go/doc/doc_test.go
@@ -0,0 +1,146 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"go/parser"
+	"go/printer"
+	"go/token"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+	"text/template"
+)
+
+var update = flag.Bool("update", false, "update golden (.out) files")
+var files = flag.String("files", "", "consider only Go test files matching this regular expression")
+
+const dataDir = "testdata"
+
+var templateTxt = readTemplate("template.txt")
+
+func readTemplate(filename string) *template.Template {
+	t := template.New(filename)
+	t.Funcs(template.FuncMap{
+		"node":     nodeFmt,
+		"synopsis": synopsisFmt,
+		"indent":   indentFmt,
+	})
+	return template.Must(t.ParseFiles(filepath.Join(dataDir, filename)))
+}
+
+func nodeFmt(node interface{}, fset *token.FileSet) string {
+	var buf bytes.Buffer
+	printer.Fprint(&buf, fset, node)
+	return strings.Replace(strings.TrimSpace(buf.String()), "\n", "\n\t", -1)
+}
+
+func synopsisFmt(s string) string {
+	const n = 64
+	if len(s) > n {
+		// cut off excess text and go back to a word boundary
+		s = s[0:n]
+		if i := strings.LastIndexAny(s, "\t\n "); i >= 0 {
+			s = s[0:i]
+		}
+		s = strings.TrimSpace(s) + " ..."
+	}
+	return "// " + strings.Replace(s, "\n", " ", -1)
+}
+
+func indentFmt(indent, s string) string {
+	end := ""
+	if strings.HasSuffix(s, "\n") {
+		end = "\n"
+		s = s[:len(s)-1]
+	}
+	return indent + strings.Replace(s, "\n", "\n"+indent, -1) + end
+}
+
+func isGoFile(fi os.FileInfo) bool {
+	name := fi.Name()
+	return !fi.IsDir() &&
+		len(name) > 0 && name[0] != '.' && // ignore .files
+		filepath.Ext(name) == ".go"
+}
+
+type bundle struct {
+	*Package
+	FSet *token.FileSet
+}
+
+func test(t *testing.T, mode Mode) {
+	// determine file filter
+	filter := isGoFile
+	if *files != "" {
+		rx, err := regexp.Compile(*files)
+		if err != nil {
+			t.Fatal(err)
+		}
+		filter = func(fi os.FileInfo) bool {
+			return isGoFile(fi) && rx.MatchString(fi.Name())
+		}
+	}
+
+	// get packages
+	fset := token.NewFileSet()
+	pkgs, err := parser.ParseDir(fset, dataDir, filter, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// test packages
+	for _, pkg := range pkgs {
+		importpath := dataDir + "/" + pkg.Name
+		doc := New(pkg, importpath, mode)
+
+		// golden files always use / in filenames - canonicalize them
+		for i, filename := range doc.Filenames {
+			doc.Filenames[i] = filepath.ToSlash(filename)
+		}
+
+		// print documentation
+		var buf bytes.Buffer
+		if err := templateTxt.Execute(&buf, bundle{doc, fset}); err != nil {
+			t.Error(err)
+			continue
+		}
+		got := buf.Bytes()
+
+		// update golden file if necessary
+		golden := filepath.Join(dataDir, fmt.Sprintf("%s.%d.golden", pkg.Name, mode))
+		if *update {
+			err := ioutil.WriteFile(golden, got, 0644)
+			if err != nil {
+				t.Error(err)
+			}
+			continue
+		}
+
+		// get golden file
+		want, err := ioutil.ReadFile(golden)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+
+		// compare
+		if !bytes.Equal(got, want) {
+			t.Errorf("package %s\n\tgot:\n%s\n\twant:\n%s", pkg.Name, got, want)
+		}
+	}
+}
+
+func Test(t *testing.T) {
+	test(t, 0)
+	test(t, AllDecls)
+	test(t, AllMethods)
+}
diff --git a/src/go/doc/example.go b/src/go/doc/example.go
new file mode 100644
index 000000000..c414e548c
--- /dev/null
+++ b/src/go/doc/example.go
@@ -0,0 +1,355 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Extract example functions from file ASTs.
+
+package doc
+
+import (
+	"go/ast"
+	"go/token"
+	"path"
+	"regexp"
+	"sort"
+	"strconv"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+// An Example represents an example function found in a source files.
+type Example struct {
+	Name        string // name of the item being exemplified
+	Doc         string // example function doc string
+	Code        ast.Node
+	Play        *ast.File // a whole program version of the example
+	Comments    []*ast.CommentGroup
+	Output      string // expected output
+	EmptyOutput bool   // expect empty output
+	Order       int    // original source code order
+}
+
+// Examples returns the examples found in the files, sorted by Name field.
+// The Order fields record the order in which the examples were encountered.
+//
+// Playable Examples must be in a package whose name ends in "_test".
+// An Example is "playable" (the Play field is non-nil) in either of these
+// circumstances:
+//   - The example function is self-contained: the function references only
+//     identifiers from other packages (or predeclared identifiers, such as
+//     "int") and the test file does not include a dot import.
+//   - The entire test file is the example: the file contains exactly one
+//     example function, zero test or benchmark functions, and at least one
+//     top-level function, type, variable, or constant declaration other
+//     than the example function.
+func Examples(files ...*ast.File) []*Example {
+	var list []*Example
+	for _, file := range files {
+		hasTests := false // file contains tests or benchmarks
+		numDecl := 0      // number of non-import declarations in the file
+		var flist []*Example
+		for _, decl := range file.Decls {
+			if g, ok := decl.(*ast.GenDecl); ok && g.Tok != token.IMPORT {
+				numDecl++
+				continue
+			}
+			f, ok := decl.(*ast.FuncDecl)
+			if !ok {
+				continue
+			}
+			numDecl++
+			name := f.Name.Name
+			if isTest(name, "Test") || isTest(name, "Benchmark") {
+				hasTests = true
+				continue
+			}
+			if !isTest(name, "Example") {
+				continue
+			}
+			var doc string
+			if f.Doc != nil {
+				doc = f.Doc.Text()
+			}
+			output, hasOutput := exampleOutput(f.Body, file.Comments)
+			flist = append(flist, &Example{
+				Name:        name[len("Example"):],
+				Doc:         doc,
+				Code:        f.Body,
+				Play:        playExample(file, f.Body),
+				Comments:    file.Comments,
+				Output:      output,
+				EmptyOutput: output == "" && hasOutput,
+				Order:       len(flist),
+			})
+		}
+		if !hasTests && numDecl > 1 && len(flist) == 1 {
+			// If this file only has one example function, some
+			// other top-level declarations, and no tests or
+			// benchmarks, use the whole file as the example.
+			flist[0].Code = file
+			flist[0].Play = playExampleFile(file)
+		}
+		list = append(list, flist...)
+	}
+	sort.Sort(exampleByName(list))
+	return list
+}
+
+var outputPrefix = regexp.MustCompile(`(?i)^[[:space:]]*output:`)
+
+// Extracts the expected output and whether there was a valid output comment
+func exampleOutput(b *ast.BlockStmt, comments []*ast.CommentGroup) (output string, ok bool) {
+	if _, last := lastComment(b, comments); last != nil {
+		// test that it begins with the correct prefix
+		text := last.Text()
+		if loc := outputPrefix.FindStringIndex(text); loc != nil {
+			text = text[loc[1]:]
+			// Strip zero or more spaces followed by \n or a single space.
+			text = strings.TrimLeft(text, " ")
+			if len(text) > 0 && text[0] == '\n' {
+				text = text[1:]
+			}
+			return text, true
+		}
+	}
+	return "", false // no suitable comment found
+}
+
+// isTest tells whether name looks like a test, example, or benchmark.
+// It is a Test (say) if there is a character after Test that is not a
+// lower-case letter. (We don't want Testiness.)
+func isTest(name, prefix string) bool {
+	if !strings.HasPrefix(name, prefix) {
+		return false
+	}
+	if len(name) == len(prefix) { // "Test" is ok
+		return true
+	}
+	rune, _ := utf8.DecodeRuneInString(name[len(prefix):])
+	return !unicode.IsLower(rune)
+}
+
+type exampleByName []*Example
+
+func (s exampleByName) Len() int           { return len(s) }
+func (s exampleByName) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
+func (s exampleByName) Less(i, j int) bool { return s[i].Name < s[j].Name }
+
+// playExample synthesizes a new *ast.File based on the provided
+// file with the provided function body as the body of main.
+func playExample(file *ast.File, body *ast.BlockStmt) *ast.File {
+	if !strings.HasSuffix(file.Name.Name, "_test") {
+		// We don't support examples that are part of the
+		// greater package (yet).
+		return nil
+	}
+
+	// Find top-level declarations in the file.
+	topDecls := make(map[*ast.Object]bool)
+	for _, decl := range file.Decls {
+		switch d := decl.(type) {
+		case *ast.FuncDecl:
+			topDecls[d.Name.Obj] = true
+		case *ast.GenDecl:
+			for _, spec := range d.Specs {
+				switch s := spec.(type) {
+				case *ast.TypeSpec:
+					topDecls[s.Name.Obj] = true
+				case *ast.ValueSpec:
+					for _, id := range s.Names {
+						topDecls[id.Obj] = true
+					}
+				}
+			}
+		}
+	}
+
+	// Find unresolved identifiers and uses of top-level declarations.
+	unresolved := make(map[string]bool)
+	usesTopDecl := false
+	var inspectFunc func(ast.Node) bool
+	inspectFunc = func(n ast.Node) bool {
+		// For selector expressions, only inspect the left hand side.
+		// (For an expression like fmt.Println, only add "fmt" to the
+		// set of unresolved names, not "Println".)
+		if e, ok := n.(*ast.SelectorExpr); ok {
+			ast.Inspect(e.X, inspectFunc)
+			return false
+		}
+		// For key value expressions, only inspect the value
+		// as the key should be resolved by the type of the
+		// composite literal.
+		if e, ok := n.(*ast.KeyValueExpr); ok {
+			ast.Inspect(e.Value, inspectFunc)
+			return false
+		}
+		if id, ok := n.(*ast.Ident); ok {
+			if id.Obj == nil {
+				unresolved[id.Name] = true
+			} else if topDecls[id.Obj] {
+				usesTopDecl = true
+			}
+		}
+		return true
+	}
+	ast.Inspect(body, inspectFunc)
+	if usesTopDecl {
+		// We don't support examples that are not self-contained (yet).
+		return nil
+	}
+
+	// Remove predeclared identifiers from unresolved list.
+	for n := range unresolved {
+		if predeclaredTypes[n] || predeclaredConstants[n] || predeclaredFuncs[n] {
+			delete(unresolved, n)
+		}
+	}
+
+	// Use unresolved identifiers to determine the imports used by this
+	// example. The heuristic assumes package names match base import
+	// paths for imports w/o renames (should be good enough most of the time).
+	namedImports := make(map[string]string) // [name]path
+	var blankImports []ast.Spec             // _ imports
+	for _, s := range file.Imports {
+		p, err := strconv.Unquote(s.Path.Value)
+		if err != nil {
+			continue
+		}
+		n := path.Base(p)
+		if s.Name != nil {
+			n = s.Name.Name
+			switch n {
+			case "_":
+				blankImports = append(blankImports, s)
+				continue
+			case ".":
+				// We can't resolve dot imports (yet).
+				return nil
+			}
+		}
+		if unresolved[n] {
+			namedImports[n] = p
+			delete(unresolved, n)
+		}
+	}
+
+	// If there are other unresolved identifiers, give up because this
+	// synthesized file is not going to build.
+	if len(unresolved) > 0 {
+		return nil
+	}
+
+	// Include documentation belonging to blank imports.
+	var comments []*ast.CommentGroup
+	for _, s := range blankImports {
+		if c := s.(*ast.ImportSpec).Doc; c != nil {
+			comments = append(comments, c)
+		}
+	}
+
+	// Include comments that are inside the function body.
+	for _, c := range file.Comments {
+		if body.Pos() <= c.Pos() && c.End() <= body.End() {
+			comments = append(comments, c)
+		}
+	}
+
+	// Strip "Output:" comment and adjust body end position.
+	body, comments = stripOutputComment(body, comments)
+
+	// Synthesize import declaration.
+	importDecl := &ast.GenDecl{
+		Tok:    token.IMPORT,
+		Lparen: 1, // Need non-zero Lparen and Rparen so that printer
+		Rparen: 1, // treats this as a factored import.
+	}
+	for n, p := range namedImports {
+		s := &ast.ImportSpec{Path: &ast.BasicLit{Value: strconv.Quote(p)}}
+		if path.Base(p) != n {
+			s.Name = ast.NewIdent(n)
+		}
+		importDecl.Specs = append(importDecl.Specs, s)
+	}
+	importDecl.Specs = append(importDecl.Specs, blankImports...)
+
+	// Synthesize main function.
+	funcDecl := &ast.FuncDecl{
+		Name: ast.NewIdent("main"),
+		Type: &ast.FuncType{Params: &ast.FieldList{}}, // FuncType.Params must be non-nil
+		Body: body,
+	}
+
+	// Synthesize file.
+	return &ast.File{
+		Name:     ast.NewIdent("main"),
+		Decls:    []ast.Decl{importDecl, funcDecl},
+		Comments: comments,
+	}
+}
+
+// playExampleFile takes a whole file example and synthesizes a new *ast.File
+// such that the example is function main in package main.
+func playExampleFile(file *ast.File) *ast.File {
+	// Strip copyright comment if present.
+	comments := file.Comments
+	if len(comments) > 0 && strings.HasPrefix(comments[0].Text(), "Copyright") {
+		comments = comments[1:]
+	}
+
+	// Copy declaration slice, rewriting the ExampleX function to main.
+	var decls []ast.Decl
+	for _, d := range file.Decls {
+		if f, ok := d.(*ast.FuncDecl); ok && isTest(f.Name.Name, "Example") {
+			// Copy the FuncDecl, as it may be used elsewhere.
+			newF := *f
+			newF.Name = ast.NewIdent("main")
+			newF.Body, comments = stripOutputComment(f.Body, comments)
+			d = &newF
+		}
+		decls = append(decls, d)
+	}
+
+	// Copy the File, as it may be used elsewhere.
+	f := *file
+	f.Name = ast.NewIdent("main")
+	f.Decls = decls
+	f.Comments = comments
+	return &f
+}
+
+// stripOutputComment finds and removes an "Output:" comment from body
+// and comments, and adjusts the body block's end position.
+func stripOutputComment(body *ast.BlockStmt, comments []*ast.CommentGroup) (*ast.BlockStmt, []*ast.CommentGroup) {
+	// Do nothing if no "Output:" comment found.
+	i, last := lastComment(body, comments)
+	if last == nil || !outputPrefix.MatchString(last.Text()) {
+		return body, comments
+	}
+
+	// Copy body and comments, as the originals may be used elsewhere.
+	newBody := &ast.BlockStmt{
+		Lbrace: body.Lbrace,
+		List:   body.List,
+		Rbrace: last.Pos(),
+	}
+	newComments := make([]*ast.CommentGroup, len(comments)-1)
+	copy(newComments, comments[:i])
+	copy(newComments[i:], comments[i+1:])
+	return newBody, newComments
+}
+
+// lastComment returns the last comment inside the provided block.
+func lastComment(b *ast.BlockStmt, c []*ast.CommentGroup) (i int, last *ast.CommentGroup) {
+	pos, end := b.Pos(), b.End()
+	for j, cg := range c {
+		if cg.Pos() < pos {
+			continue
+		}
+		if cg.End() > end {
+			break
+		}
+		i, last = j, cg
+	}
+	return
+}
diff --git a/src/go/doc/example_test.go b/src/go/doc/example_test.go
new file mode 100644
index 000000000..e154ea8bf
--- /dev/null
+++ b/src/go/doc/example_test.go
@@ -0,0 +1,191 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc_test
+
+import (
+	"bytes"
+	"go/doc"
+	"go/format"
+	"go/parser"
+	"go/token"
+	"strings"
+	"testing"
+)
+
+const exampleTestFile = `
+package foo_test
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"os/exec"
+)
+
+func ExampleHello() {
+	fmt.Println("Hello, world!")
+	// Output: Hello, world!
+}
+
+func ExampleImport() {
+	out, err := exec.Command("date").Output()
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("The date is %s\n", out)
+}
+
+func ExampleKeyValue() {
+	v := struct {
+		a string
+		b int
+	}{
+		a: "A",
+		b: 1,
+	}
+	fmt.Print(v)
+	// Output: a: "A", b: 1
+}
+
+func ExampleKeyValueImport() {
+	f := flag.Flag{
+		Name: "play",
+	}
+	fmt.Print(f)
+	// Output: Name: "play"
+}
+
+var keyValueTopDecl = struct {
+	a string
+	b int
+}{
+	a: "B",
+	b: 2,
+}
+
+func ExampleKeyValueTopDecl() {
+	fmt.Print(keyValueTopDecl)
+}
+`
+
+var exampleTestCases = []struct {
+	Name, Play, Output string
+}{
+	{
+		Name:   "Hello",
+		Play:   exampleHelloPlay,
+		Output: "Hello, world!\n",
+	},
+	{
+		Name: "Import",
+		Play: exampleImportPlay,
+	},
+	{
+		Name:   "KeyValue",
+		Play:   exampleKeyValuePlay,
+		Output: "a: \"A\", b: 1\n",
+	},
+	{
+		Name:   "KeyValueImport",
+		Play:   exampleKeyValueImportPlay,
+		Output: "Name: \"play\"\n",
+	},
+	{
+		Name: "KeyValueTopDecl",
+		Play: "<nil>",
+	},
+}
+
+const exampleHelloPlay = `package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	fmt.Println("Hello, world!")
+}
+`
+const exampleImportPlay = `package main
+
+import (
+	"fmt"
+	"log"
+	"os/exec"
+)
+
+func main() {
+	out, err := exec.Command("date").Output()
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("The date is %s\n", out)
+}
+`
+
+const exampleKeyValuePlay = `package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	v := struct {
+		a string
+		b int
+	}{
+		a: "A",
+		b: 1,
+	}
+	fmt.Print(v)
+}
+`
+
+const exampleKeyValueImportPlay = `package main
+
+import (
+	"flag"
+	"fmt"
+)
+
+func main() {
+	f := flag.Flag{
+		Name: "play",
+	}
+	fmt.Print(f)
+}
+`
+
+func TestExamples(t *testing.T) {
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, "test.go", strings.NewReader(exampleTestFile), parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, e := range doc.Examples(file) {
+		c := exampleTestCases[i]
+		if e.Name != c.Name {
+			t.Errorf("got Name == %q, want %q", e.Name, c.Name)
+		}
+		if w := c.Play; w != "" {
+			var g string // hah
+			if e.Play == nil {
+				g = "<nil>"
+			} else {
+				var buf bytes.Buffer
+				if err := format.Node(&buf, fset, e.Play); err != nil {
+					t.Fatal(err)
+				}
+				g = buf.String()
+			}
+			if g != w {
+				t.Errorf("%s: got Play == %q, want %q", c.Name, g, w)
+			}
+		}
+		if g, w := e.Output, c.Output; g != w {
+			t.Errorf("%s: got Output == %q, want %q", c.Name, g, w)
+		}
+	}
+}
diff --git a/src/go/doc/exports.go b/src/go/doc/exports.go
new file mode 100644
index 000000000..ff01285d4
--- /dev/null
+++ b/src/go/doc/exports.go
@@ -0,0 +1,199 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements export filtering of an AST.
+
+package doc
+
+import "go/ast"
+
+// filterIdentList removes unexported names from list in place
+// and returns the resulting list.
+//
+func filterIdentList(list []*ast.Ident) []*ast.Ident {
+	j := 0
+	for _, x := range list {
+		if ast.IsExported(x.Name) {
+			list[j] = x
+			j++
+		}
+	}
+	return list[0:j]
+}
+
+// removeErrorField removes anonymous fields named "error" from an interface.
+// This is called when "error" has been determined to be a local name,
+// not the predeclared type.
+//
+func removeErrorField(ityp *ast.InterfaceType) {
+	list := ityp.Methods.List // we know that ityp.Methods != nil
+	j := 0
+	for _, field := range list {
+		keepField := true
+		if n := len(field.Names); n == 0 {
+			// anonymous field
+			if fname, _ := baseTypeName(field.Type); fname == "error" {
+				keepField = false
+			}
+		}
+		if keepField {
+			list[j] = field
+			j++
+		}
+	}
+	if j < len(list) {
+		ityp.Incomplete = true
+	}
+	ityp.Methods.List = list[0:j]
+}
+
+// filterFieldList removes unexported fields (field names) from the field list
+// in place and returns true if fields were removed. Anonymous fields are
+// recorded with the parent type. filterType is called with the types of
+// all remaining fields.
+//
+func (r *reader) filterFieldList(parent *namedType, fields *ast.FieldList, ityp *ast.InterfaceType) (removedFields bool) {
+	if fields == nil {
+		return
+	}
+	list := fields.List
+	j := 0
+	for _, field := range list {
+		keepField := false
+		if n := len(field.Names); n == 0 {
+			// anonymous field
+			fname := r.recordAnonymousField(parent, field.Type)
+			if ast.IsExported(fname) {
+				keepField = true
+			} else if ityp != nil && fname == "error" {
+				// possibly the predeclared error interface; keep
+				// it for now but remember this interface so that
+				// it can be fixed if error is also defined locally
+				keepField = true
+				r.remember(ityp)
+			}
+		} else {
+			field.Names = filterIdentList(field.Names)
+			if len(field.Names) < n {
+				removedFields = true
+			}
+			if len(field.Names) > 0 {
+				keepField = true
+			}
+		}
+		if keepField {
+			r.filterType(nil, field.Type)
+			list[j] = field
+			j++
+		}
+	}
+	if j < len(list) {
+		removedFields = true
+	}
+	fields.List = list[0:j]
+	return
+}
+
+// filterParamList applies filterType to each parameter type in fields.
+//
+func (r *reader) filterParamList(fields *ast.FieldList) {
+	if fields != nil {
+		for _, f := range fields.List {
+			r.filterType(nil, f.Type)
+		}
+	}
+}
+
+// filterType strips any unexported struct fields or method types from typ
+// in place. If fields (or methods) have been removed, the corresponding
+// struct or interface type has the Incomplete field set to true.
+//
+func (r *reader) filterType(parent *namedType, typ ast.Expr) {
+	switch t := typ.(type) {
+	case *ast.Ident:
+		// nothing to do
+	case *ast.ParenExpr:
+		r.filterType(nil, t.X)
+	case *ast.ArrayType:
+		r.filterType(nil, t.Elt)
+	case *ast.StructType:
+		if r.filterFieldList(parent, t.Fields, nil) {
+			t.Incomplete = true
+		}
+	case *ast.FuncType:
+		r.filterParamList(t.Params)
+		r.filterParamList(t.Results)
+	case *ast.InterfaceType:
+		if r.filterFieldList(parent, t.Methods, t) {
+			t.Incomplete = true
+		}
+	case *ast.MapType:
+		r.filterType(nil, t.Key)
+		r.filterType(nil, t.Value)
+	case *ast.ChanType:
+		r.filterType(nil, t.Value)
+	}
+}
+
+func (r *reader) filterSpec(spec ast.Spec) bool {
+	switch s := spec.(type) {
+	case *ast.ImportSpec:
+		// always keep imports so we can collect them
+		return true
+	case *ast.ValueSpec:
+		s.Names = filterIdentList(s.Names)
+		if len(s.Names) > 0 {
+			r.filterType(nil, s.Type)
+			return true
+		}
+	case *ast.TypeSpec:
+		if name := s.Name.Name; ast.IsExported(name) {
+			r.filterType(r.lookupType(s.Name.Name), s.Type)
+			return true
+		} else if name == "error" {
+			// special case: remember that error is declared locally
+			r.errorDecl = true
+		}
+	}
+	return false
+}
+
+func (r *reader) filterSpecList(list []ast.Spec) []ast.Spec {
+	j := 0
+	for _, s := range list {
+		if r.filterSpec(s) {
+			list[j] = s
+			j++
+		}
+	}
+	return list[0:j]
+}
+
+func (r *reader) filterDecl(decl ast.Decl) bool {
+	switch d := decl.(type) {
+	case *ast.GenDecl:
+		d.Specs = r.filterSpecList(d.Specs)
+		return len(d.Specs) > 0
+	case *ast.FuncDecl:
+		// ok to filter these methods early because any
+		// conflicting method will be filtered here, too -
+		// thus, removing these methods early will not lead
+		// to the false removal of possible conflicts
+		return ast.IsExported(d.Name.Name)
+	}
+	return false
+}
+
+// fileExports removes unexported declarations from src in place.
+//
+func (r *reader) fileExports(src *ast.File) {
+	j := 0
+	for _, d := range src.Decls {
+		if r.filterDecl(d) {
+			src.Decls[j] = d
+			j++
+		}
+	}
+	src.Decls = src.Decls[0:j]
+}
diff --git a/src/go/doc/filter.go b/src/go/doc/filter.go
new file mode 100644
index 000000000..a6f243f33
--- /dev/null
+++ b/src/go/doc/filter.go
@@ -0,0 +1,105 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import "go/ast"
+
+type Filter func(string) bool
+
+func matchFields(fields *ast.FieldList, f Filter) bool {
+	if fields != nil {
+		for _, field := range fields.List {
+			for _, name := range field.Names {
+				if f(name.Name) {
+					return true
+				}
+			}
+		}
+	}
+	return false
+}
+
+func matchDecl(d *ast.GenDecl, f Filter) bool {
+	for _, d := range d.Specs {
+		switch v := d.(type) {
+		case *ast.ValueSpec:
+			for _, name := range v.Names {
+				if f(name.Name) {
+					return true
+				}
+			}
+		case *ast.TypeSpec:
+			if f(v.Name.Name) {
+				return true
+			}
+			switch t := v.Type.(type) {
+			case *ast.StructType:
+				if matchFields(t.Fields, f) {
+					return true
+				}
+			case *ast.InterfaceType:
+				if matchFields(t.Methods, f) {
+					return true
+				}
+			}
+		}
+	}
+	return false
+}
+
+func filterValues(a []*Value, f Filter) []*Value {
+	w := 0
+	for _, vd := range a {
+		if matchDecl(vd.Decl, f) {
+			a[w] = vd
+			w++
+		}
+	}
+	return a[0:w]
+}
+
+func filterFuncs(a []*Func, f Filter) []*Func {
+	w := 0
+	for _, fd := range a {
+		if f(fd.Name) {
+			a[w] = fd
+			w++
+		}
+	}
+	return a[0:w]
+}
+
+func filterTypes(a []*Type, f Filter) []*Type {
+	w := 0
+	for _, td := range a {
+		n := 0 // number of matches
+		if matchDecl(td.Decl, f) {
+			n = 1
+		} else {
+			// type name doesn't match, but we may have matching consts, vars, factories or methods
+			td.Consts = filterValues(td.Consts, f)
+			td.Vars = filterValues(td.Vars, f)
+			td.Funcs = filterFuncs(td.Funcs, f)
+			td.Methods = filterFuncs(td.Methods, f)
+			n += len(td.Consts) + len(td.Vars) + len(td.Funcs) + len(td.Methods)
+		}
+		if n > 0 {
+			a[w] = td
+			w++
+		}
+	}
+	return a[0:w]
+}
+
+// Filter eliminates documentation for names that don't pass through the filter f.
+// TODO(gri): Recognize "Type.Method" as a name.
+//
+func (p *Package) Filter(f Filter) {
+	p.Consts = filterValues(p.Consts, f)
+	p.Vars = filterValues(p.Vars, f)
+	p.Types = filterTypes(p.Types, f)
+	p.Funcs = filterFuncs(p.Funcs, f)
+	p.Doc = "" // don't show top-level package doc
+}
diff --git a/src/go/doc/headscan.go b/src/go/doc/headscan.go
new file mode 100644
index 000000000..1ccaa1581
--- /dev/null
+++ b/src/go/doc/headscan.go
@@ -0,0 +1,114 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build ignore
+
+/*
+	The headscan command extracts comment headings from package files;
+	it is used to detect false positives which may require an adjustment
+	to the comment formatting heuristics in comment.go.
+
+	Usage: headscan [-root root_directory]
+
+	By default, the $GOROOT/src directory is scanned.
+*/
+package main
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"go/doc"
+	"go/parser"
+	"go/token"
+	"os"
+	"path/filepath"
+	"regexp"
+	"runtime"
+	"strings"
+)
+
+var (
+	root    = flag.String("root", filepath.Join(runtime.GOROOT(), "src"), "root of filesystem tree to scan")
+	verbose = flag.Bool("v", false, "verbose mode")
+)
+
+// ToHTML in comment.go assigns a (possibly blank) ID to each heading
+var html_h = regexp.MustCompile(`<h3 id="[^"]*">`)
+
+const html_endh = "</h3>\n"
+
+func isGoFile(fi os.FileInfo) bool {
+	return strings.HasSuffix(fi.Name(), ".go") &&
+		!strings.HasSuffix(fi.Name(), "_test.go")
+}
+
+func appendHeadings(list []string, comment string) []string {
+	var buf bytes.Buffer
+	doc.ToHTML(&buf, comment, nil)
+	for s := buf.String(); ; {
+		loc := html_h.FindStringIndex(s)
+		if len(loc) == 0 {
+			break
+		}
+		i := loc[1]
+		j := strings.Index(s, html_endh)
+		if j < 0 {
+			list = append(list, s[i:]) // incorrect HTML
+			break
+		}
+		list = append(list, s[i:j])
+		s = s[j+len(html_endh):]
+	}
+	return list
+}
+
+func main() {
+	flag.Parse()
+	fset := token.NewFileSet()
+	nheadings := 0
+	err := filepath.Walk(*root, func(path string, fi os.FileInfo, err error) error {
+		if !fi.IsDir() {
+			return nil
+		}
+		pkgs, err := parser.ParseDir(fset, path, isGoFile, parser.ParseComments)
+		if err != nil {
+			if *verbose {
+				fmt.Fprintln(os.Stderr, err)
+			}
+			return nil
+		}
+		for _, pkg := range pkgs {
+			d := doc.New(pkg, path, doc.Mode(0))
+			list := appendHeadings(nil, d.Doc)
+			for _, d := range d.Consts {
+				list = appendHeadings(list, d.Doc)
+			}
+			for _, d := range d.Types {
+				list = appendHeadings(list, d.Doc)
+			}
+			for _, d := range d.Vars {
+				list = appendHeadings(list, d.Doc)
+			}
+			for _, d := range d.Funcs {
+				list = appendHeadings(list, d.Doc)
+			}
+			if len(list) > 0 {
+				// directories may contain multiple packages;
+				// print path and package name
+				fmt.Printf("%s (package %s)\n", path, pkg.Name)
+				for _, h := range list {
+					fmt.Printf("\t%s\n", h)
+				}
+				nheadings += len(list)
+			}
+		}
+		return nil
+	})
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	fmt.Println(nheadings, "headings found")
+}
diff --git a/src/go/doc/reader.go b/src/go/doc/reader.go
new file mode 100644
index 000000000..ed82c47cd
--- /dev/null
+++ b/src/go/doc/reader.go
@@ -0,0 +1,853 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import (
+	"go/ast"
+	"go/token"
+	"regexp"
+	"sort"
+	"strconv"
+)
+
+// ----------------------------------------------------------------------------
+// function/method sets
+//
+// Internally, we treat functions like methods and collect them in method sets.
+
+// A methodSet describes a set of methods. Entries where Decl == nil are conflict
+// entries (more then one method with the same name at the same embedding level).
+//
+type methodSet map[string]*Func
+
+// recvString returns a string representation of recv of the
+// form "T", "*T", or "BADRECV" (if not a proper receiver type).
+//
+func recvString(recv ast.Expr) string {
+	switch t := recv.(type) {
+	case *ast.Ident:
+		return t.Name
+	case *ast.StarExpr:
+		return "*" + recvString(t.X)
+	}
+	return "BADRECV"
+}
+
+// set creates the corresponding Func for f and adds it to mset.
+// If there are multiple f's with the same name, set keeps the first
+// one with documentation; conflicts are ignored.
+//
+func (mset methodSet) set(f *ast.FuncDecl) {
+	name := f.Name.Name
+	if g := mset[name]; g != nil && g.Doc != "" {
+		// A function with the same name has already been registered;
+		// since it has documentation, assume f is simply another
+		// implementation and ignore it. This does not happen if the
+		// caller is using go/build.ScanDir to determine the list of
+		// files implementing a package.
+		return
+	}
+	// function doesn't exist or has no documentation; use f
+	recv := ""
+	if f.Recv != nil {
+		var typ ast.Expr
+		// be careful in case of incorrect ASTs
+		if list := f.Recv.List; len(list) == 1 {
+			typ = list[0].Type
+		}
+		recv = recvString(typ)
+	}
+	mset[name] = &Func{
+		Doc:  f.Doc.Text(),
+		Name: name,
+		Decl: f,
+		Recv: recv,
+		Orig: recv,
+	}
+	f.Doc = nil // doc consumed - remove from AST
+}
+
+// add adds method m to the method set; m is ignored if the method set
+// already contains a method with the same name at the same or a higher
+// level then m.
+//
+func (mset methodSet) add(m *Func) {
+	old := mset[m.Name]
+	if old == nil || m.Level < old.Level {
+		mset[m.Name] = m
+		return
+	}
+	if old != nil && m.Level == old.Level {
+		// conflict - mark it using a method with nil Decl
+		mset[m.Name] = &Func{
+			Name:  m.Name,
+			Level: m.Level,
+		}
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Named types
+
+// baseTypeName returns the name of the base type of x (or "")
+// and whether the type is imported or not.
+//
+func baseTypeName(x ast.Expr) (name string, imported bool) {
+	switch t := x.(type) {
+	case *ast.Ident:
+		return t.Name, false
+	case *ast.SelectorExpr:
+		if _, ok := t.X.(*ast.Ident); ok {
+			// only possible for qualified type names;
+			// assume type is imported
+			return t.Sel.Name, true
+		}
+	case *ast.StarExpr:
+		return baseTypeName(t.X)
+	}
+	return
+}
+
+// An embeddedSet describes a set of embedded types.
+type embeddedSet map[*namedType]bool
+
+// A namedType represents a named unqualified (package local, or possibly
+// predeclared) type. The namedType for a type name is always found via
+// reader.lookupType.
+//
+type namedType struct {
+	doc  string       // doc comment for type
+	name string       // type name
+	decl *ast.GenDecl // nil if declaration hasn't been seen yet
+
+	isEmbedded bool        // true if this type is embedded
+	isStruct   bool        // true if this type is a struct
+	embedded   embeddedSet // true if the embedded type is a pointer
+
+	// associated declarations
+	values  []*Value // consts and vars
+	funcs   methodSet
+	methods methodSet
+}
+
+// ----------------------------------------------------------------------------
+// AST reader
+
+// reader accumulates documentation for a single package.
+// It modifies the AST: Comments (declaration documentation)
+// that have been collected by the reader are set to nil
+// in the respective AST nodes so that they are not printed
+// twice (once when printing the documentation and once when
+// printing the corresponding AST node).
+//
+type reader struct {
+	mode Mode
+
+	// package properties
+	doc       string // package documentation, if any
+	filenames []string
+	notes     map[string][]*Note
+
+	// declarations
+	imports map[string]int
+	values  []*Value // consts and vars
+	types   map[string]*namedType
+	funcs   methodSet
+
+	// support for package-local error type declarations
+	errorDecl bool                 // if set, type "error" was declared locally
+	fixlist   []*ast.InterfaceType // list of interfaces containing anonymous field "error"
+}
+
+func (r *reader) isVisible(name string) bool {
+	return r.mode&AllDecls != 0 || ast.IsExported(name)
+}
+
+// lookupType returns the base type with the given name.
+// If the base type has not been encountered yet, a new
+// type with the given name but no associated declaration
+// is added to the type map.
+//
+func (r *reader) lookupType(name string) *namedType {
+	if name == "" || name == "_" {
+		return nil // no type docs for anonymous types
+	}
+	if typ, found := r.types[name]; found {
+		return typ
+	}
+	// type not found - add one without declaration
+	typ := &namedType{
+		name:     name,
+		embedded: make(embeddedSet),
+		funcs:    make(methodSet),
+		methods:  make(methodSet),
+	}
+	r.types[name] = typ
+	return typ
+}
+
+// recordAnonymousField registers fieldType as the type of an
+// anonymous field in the parent type. If the field is imported
+// (qualified name) or the parent is nil, the field is ignored.
+// The function returns the field name.
+//
+func (r *reader) recordAnonymousField(parent *namedType, fieldType ast.Expr) (fname string) {
+	fname, imp := baseTypeName(fieldType)
+	if parent == nil || imp {
+		return
+	}
+	if ftype := r.lookupType(fname); ftype != nil {
+		ftype.isEmbedded = true
+		_, ptr := fieldType.(*ast.StarExpr)
+		parent.embedded[ftype] = ptr
+	}
+	return
+}
+
+func (r *reader) readDoc(comment *ast.CommentGroup) {
+	// By convention there should be only one package comment
+	// but collect all of them if there are more then one.
+	text := comment.Text()
+	if r.doc == "" {
+		r.doc = text
+		return
+	}
+	r.doc += "\n" + text
+}
+
+func (r *reader) remember(typ *ast.InterfaceType) {
+	r.fixlist = append(r.fixlist, typ)
+}
+
+func specNames(specs []ast.Spec) []string {
+	names := make([]string, 0, len(specs)) // reasonable estimate
+	for _, s := range specs {
+		// s guaranteed to be an *ast.ValueSpec by readValue
+		for _, ident := range s.(*ast.ValueSpec).Names {
+			names = append(names, ident.Name)
+		}
+	}
+	return names
+}
+
+// readValue processes a const or var declaration.
+//
+func (r *reader) readValue(decl *ast.GenDecl) {
+	// determine if decl should be associated with a type
+	// Heuristic: For each typed entry, determine the type name, if any.
+	//            If there is exactly one type name that is sufficiently
+	//            frequent, associate the decl with the respective type.
+	domName := ""
+	domFreq := 0
+	prev := ""
+	n := 0
+	for _, spec := range decl.Specs {
+		s, ok := spec.(*ast.ValueSpec)
+		if !ok {
+			continue // should not happen, but be conservative
+		}
+		name := ""
+		switch {
+		case s.Type != nil:
+			// a type is present; determine its name
+			if n, imp := baseTypeName(s.Type); !imp {
+				name = n
+			}
+		case decl.Tok == token.CONST:
+			// no type is present but we have a constant declaration;
+			// use the previous type name (w/o more type information
+			// we cannot handle the case of unnamed variables with
+			// initializer expressions except for some trivial cases)
+			name = prev
+		}
+		if name != "" {
+			// entry has a named type
+			if domName != "" && domName != name {
+				// more than one type name - do not associate
+				// with any type
+				domName = ""
+				break
+			}
+			domName = name
+			domFreq++
+		}
+		prev = name
+		n++
+	}
+
+	// nothing to do w/o a legal declaration
+	if n == 0 {
+		return
+	}
+
+	// determine values list with which to associate the Value for this decl
+	values := &r.values
+	const threshold = 0.75
+	if domName != "" && r.isVisible(domName) && domFreq >= int(float64(len(decl.Specs))*threshold) {
+		// typed entries are sufficiently frequent
+		if typ := r.lookupType(domName); typ != nil {
+			values = &typ.values // associate with that type
+		}
+	}
+
+	*values = append(*values, &Value{
+		Doc:   decl.Doc.Text(),
+		Names: specNames(decl.Specs),
+		Decl:  decl,
+		order: len(*values),
+	})
+	decl.Doc = nil // doc consumed - remove from AST
+}
+
+// fields returns a struct's fields or an interface's methods.
+//
+func fields(typ ast.Expr) (list []*ast.Field, isStruct bool) {
+	var fields *ast.FieldList
+	switch t := typ.(type) {
+	case *ast.StructType:
+		fields = t.Fields
+		isStruct = true
+	case *ast.InterfaceType:
+		fields = t.Methods
+	}
+	if fields != nil {
+		list = fields.List
+	}
+	return
+}
+
+// readType processes a type declaration.
+//
+func (r *reader) readType(decl *ast.GenDecl, spec *ast.TypeSpec) {
+	typ := r.lookupType(spec.Name.Name)
+	if typ == nil {
+		return // no name or blank name - ignore the type
+	}
+
+	// A type should be added at most once, so typ.decl
+	// should be nil - if it is not, simply overwrite it.
+	typ.decl = decl
+
+	// compute documentation
+	doc := spec.Doc
+	spec.Doc = nil // doc consumed - remove from AST
+	if doc == nil {
+		// no doc associated with the spec, use the declaration doc, if any
+		doc = decl.Doc
+	}
+	decl.Doc = nil // doc consumed - remove from AST
+	typ.doc = doc.Text()
+
+	// record anonymous fields (they may contribute methods)
+	// (some fields may have been recorded already when filtering
+	// exports, but that's ok)
+	var list []*ast.Field
+	list, typ.isStruct = fields(spec.Type)
+	for _, field := range list {
+		if len(field.Names) == 0 {
+			r.recordAnonymousField(typ, field.Type)
+		}
+	}
+}
+
+// readFunc processes a func or method declaration.
+//
+func (r *reader) readFunc(fun *ast.FuncDecl) {
+	// strip function body
+	fun.Body = nil
+
+	// associate methods with the receiver type, if any
+	if fun.Recv != nil {
+		// method
+		recvTypeName, imp := baseTypeName(fun.Recv.List[0].Type)
+		if imp {
+			// should not happen (incorrect AST);
+			// don't show this method
+			return
+		}
+		if typ := r.lookupType(recvTypeName); typ != nil {
+			typ.methods.set(fun)
+		}
+		// otherwise ignore the method
+		// TODO(gri): There may be exported methods of non-exported types
+		// that can be called because of exported values (consts, vars, or
+		// function results) of that type. Could determine if that is the
+		// case and then show those methods in an appropriate section.
+		return
+	}
+
+	// associate factory functions with the first visible result type, if any
+	if fun.Type.Results.NumFields() >= 1 {
+		res := fun.Type.Results.List[0]
+		if len(res.Names) <= 1 {
+			// exactly one (named or anonymous) result associated
+			// with the first type in result signature (there may
+			// be more than one result)
+			if n, imp := baseTypeName(res.Type); !imp && r.isVisible(n) {
+				if typ := r.lookupType(n); typ != nil {
+					// associate function with typ
+					typ.funcs.set(fun)
+					return
+				}
+			}
+		}
+	}
+
+	// just an ordinary function
+	r.funcs.set(fun)
+}
+
+var (
+	noteMarker    = `([A-Z][A-Z]+)\(([^)]+)\):?`                    // MARKER(uid), MARKER at least 2 chars, uid at least 1 char
+	noteMarkerRx  = regexp.MustCompile(`^[ \t]*` + noteMarker)      // MARKER(uid) at text start
+	noteCommentRx = regexp.MustCompile(`^/[/*][ \t]*` + noteMarker) // MARKER(uid) at comment start
+)
+
+// readNote collects a single note from a sequence of comments.
+//
+func (r *reader) readNote(list []*ast.Comment) {
+	text := (&ast.CommentGroup{List: list}).Text()
+	if m := noteMarkerRx.FindStringSubmatchIndex(text); m != nil {
+		// The note body starts after the marker.
+		// We remove any formatting so that we don't
+		// get spurious line breaks/indentation when
+		// showing the TODO body.
+		body := clean(text[m[1]:], keepNL)
+		if body != "" {
+			marker := text[m[2]:m[3]]
+			r.notes[marker] = append(r.notes[marker], &Note{
+				Pos:  list[0].Pos(),
+				End:  list[len(list)-1].End(),
+				UID:  text[m[4]:m[5]],
+				Body: body,
+			})
+		}
+	}
+}
+
+// readNotes extracts notes from comments.
+// A note must start at the beginning of a comment with "MARKER(uid):"
+// and is followed by the note body (e.g., "// BUG(gri): fix this").
+// The note ends at the end of the comment group or at the start of
+// another note in the same comment group, whichever comes first.
+//
+func (r *reader) readNotes(comments []*ast.CommentGroup) {
+	for _, group := range comments {
+		i := -1 // comment index of most recent note start, valid if >= 0
+		list := group.List
+		for j, c := range list {
+			if noteCommentRx.MatchString(c.Text) {
+				if i >= 0 {
+					r.readNote(list[i:j])
+				}
+				i = j
+			}
+		}
+		if i >= 0 {
+			r.readNote(list[i:])
+		}
+	}
+}
+
+// readFile adds the AST for a source file to the reader.
+//
+func (r *reader) readFile(src *ast.File) {
+	// add package documentation
+	if src.Doc != nil {
+		r.readDoc(src.Doc)
+		src.Doc = nil // doc consumed - remove from AST
+	}
+
+	// add all declarations
+	for _, decl := range src.Decls {
+		switch d := decl.(type) {
+		case *ast.GenDecl:
+			switch d.Tok {
+			case token.IMPORT:
+				// imports are handled individually
+				for _, spec := range d.Specs {
+					if s, ok := spec.(*ast.ImportSpec); ok {
+						if import_, err := strconv.Unquote(s.Path.Value); err == nil {
+							r.imports[import_] = 1
+						}
+					}
+				}
+			case token.CONST, token.VAR:
+				// constants and variables are always handled as a group
+				r.readValue(d)
+			case token.TYPE:
+				// types are handled individually
+				if len(d.Specs) == 1 && !d.Lparen.IsValid() {
+					// common case: single declaration w/o parentheses
+					// (if a single declaration is parenthesized,
+					// create a new fake declaration below, so that
+					// go/doc type declarations always appear w/o
+					// parentheses)
+					if s, ok := d.Specs[0].(*ast.TypeSpec); ok {
+						r.readType(d, s)
+					}
+					break
+				}
+				for _, spec := range d.Specs {
+					if s, ok := spec.(*ast.TypeSpec); ok {
+						// use an individual (possibly fake) declaration
+						// for each type; this also ensures that each type
+						// gets to (re-)use the declaration documentation
+						// if there's none associated with the spec itself
+						fake := &ast.GenDecl{
+							Doc: d.Doc,
+							// don't use the existing TokPos because it
+							// will lead to the wrong selection range for
+							// the fake declaration if there are more
+							// than one type in the group (this affects
+							// src/cmd/godoc/godoc.go's posLink_urlFunc)
+							TokPos: s.Pos(),
+							Tok:    token.TYPE,
+							Specs:  []ast.Spec{s},
+						}
+						r.readType(fake, s)
+					}
+				}
+			}
+		case *ast.FuncDecl:
+			r.readFunc(d)
+		}
+	}
+
+	// collect MARKER(...): annotations
+	r.readNotes(src.Comments)
+	src.Comments = nil // consumed unassociated comments - remove from AST
+}
+
+func (r *reader) readPackage(pkg *ast.Package, mode Mode) {
+	// initialize reader
+	r.filenames = make([]string, len(pkg.Files))
+	r.imports = make(map[string]int)
+	r.mode = mode
+	r.types = make(map[string]*namedType)
+	r.funcs = make(methodSet)
+	r.notes = make(map[string][]*Note)
+
+	// sort package files before reading them so that the
+	// result does not depend on map iteration order
+	i := 0
+	for filename := range pkg.Files {
+		r.filenames[i] = filename
+		i++
+	}
+	sort.Strings(r.filenames)
+
+	// process files in sorted order
+	for _, filename := range r.filenames {
+		f := pkg.Files[filename]
+		if mode&AllDecls == 0 {
+			r.fileExports(f)
+		}
+		r.readFile(f)
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Types
+
+func customizeRecv(f *Func, recvTypeName string, embeddedIsPtr bool, level int) *Func {
+	if f == nil || f.Decl == nil || f.Decl.Recv == nil || len(f.Decl.Recv.List) != 1 {
+		return f // shouldn't happen, but be safe
+	}
+
+	// copy existing receiver field and set new type
+	newField := *f.Decl.Recv.List[0]
+	origPos := newField.Type.Pos()
+	_, origRecvIsPtr := newField.Type.(*ast.StarExpr)
+	newIdent := &ast.Ident{NamePos: origPos, Name: recvTypeName}
+	var typ ast.Expr = newIdent
+	if !embeddedIsPtr && origRecvIsPtr {
+		newIdent.NamePos++ // '*' is one character
+		typ = &ast.StarExpr{Star: origPos, X: newIdent}
+	}
+	newField.Type = typ
+
+	// copy existing receiver field list and set new receiver field
+	newFieldList := *f.Decl.Recv
+	newFieldList.List = []*ast.Field{&newField}
+
+	// copy existing function declaration and set new receiver field list
+	newFuncDecl := *f.Decl
+	newFuncDecl.Recv = &newFieldList
+
+	// copy existing function documentation and set new declaration
+	newF := *f
+	newF.Decl = &newFuncDecl
+	newF.Recv = recvString(typ)
+	// the Orig field never changes
+	newF.Level = level
+
+	return &newF
+}
+
+// collectEmbeddedMethods collects the embedded methods of typ in mset.
+//
+func (r *reader) collectEmbeddedMethods(mset methodSet, typ *namedType, recvTypeName string, embeddedIsPtr bool, level int, visited embeddedSet) {
+	visited[typ] = true
+	for embedded, isPtr := range typ.embedded {
+		// Once an embedded type is embedded as a pointer type
+		// all embedded types in those types are treated like
+		// pointer types for the purpose of the receiver type
+		// computation; i.e., embeddedIsPtr is sticky for this
+		// embedding hierarchy.
+		thisEmbeddedIsPtr := embeddedIsPtr || isPtr
+		for _, m := range embedded.methods {
+			// only top-level methods are embedded
+			if m.Level == 0 {
+				mset.add(customizeRecv(m, recvTypeName, thisEmbeddedIsPtr, level))
+			}
+		}
+		if !visited[embedded] {
+			r.collectEmbeddedMethods(mset, embedded, recvTypeName, thisEmbeddedIsPtr, level+1, visited)
+		}
+	}
+	delete(visited, typ)
+}
+
+// computeMethodSets determines the actual method sets for each type encountered.
+//
+func (r *reader) computeMethodSets() {
+	for _, t := range r.types {
+		// collect embedded methods for t
+		if t.isStruct {
+			// struct
+			r.collectEmbeddedMethods(t.methods, t, t.name, false, 1, make(embeddedSet))
+		} else {
+			// interface
+			// TODO(gri) fix this
+		}
+	}
+
+	// if error was declared locally, don't treat it as exported field anymore
+	if r.errorDecl {
+		for _, ityp := range r.fixlist {
+			removeErrorField(ityp)
+		}
+	}
+}
+
+// cleanupTypes removes the association of functions and methods with
+// types that have no declaration. Instead, these functions and methods
+// are shown at the package level. It also removes types with missing
+// declarations or which are not visible.
+//
+func (r *reader) cleanupTypes() {
+	for _, t := range r.types {
+		visible := r.isVisible(t.name)
+		if t.decl == nil && (predeclaredTypes[t.name] || t.isEmbedded && visible) {
+			// t.name is a predeclared type (and was not redeclared in this package),
+			// or it was embedded somewhere but its declaration is missing (because
+			// the AST is incomplete): move any associated values, funcs, and methods
+			// back to the top-level so that they are not lost.
+			// 1) move values
+			r.values = append(r.values, t.values...)
+			// 2) move factory functions
+			for name, f := range t.funcs {
+				// in a correct AST, package-level function names
+				// are all different - no need to check for conflicts
+				r.funcs[name] = f
+			}
+			// 3) move methods
+			for name, m := range t.methods {
+				// don't overwrite functions with the same name - drop them
+				if _, found := r.funcs[name]; !found {
+					r.funcs[name] = m
+				}
+			}
+		}
+		// remove types w/o declaration or which are not visible
+		if t.decl == nil || !visible {
+			delete(r.types, t.name)
+		}
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Sorting
+
+type data struct {
+	n    int
+	swap func(i, j int)
+	less func(i, j int) bool
+}
+
+func (d *data) Len() int           { return d.n }
+func (d *data) Swap(i, j int)      { d.swap(i, j) }
+func (d *data) Less(i, j int) bool { return d.less(i, j) }
+
+// sortBy is a helper function for sorting
+func sortBy(less func(i, j int) bool, swap func(i, j int), n int) {
+	sort.Sort(&data{n, swap, less})
+}
+
+func sortedKeys(m map[string]int) []string {
+	list := make([]string, len(m))
+	i := 0
+	for key := range m {
+		list[i] = key
+		i++
+	}
+	sort.Strings(list)
+	return list
+}
+
+// sortingName returns the name to use when sorting d into place.
+//
+func sortingName(d *ast.GenDecl) string {
+	if len(d.Specs) == 1 {
+		if s, ok := d.Specs[0].(*ast.ValueSpec); ok {
+			return s.Names[0].Name
+		}
+	}
+	return ""
+}
+
+func sortedValues(m []*Value, tok token.Token) []*Value {
+	list := make([]*Value, len(m)) // big enough in any case
+	i := 0
+	for _, val := range m {
+		if val.Decl.Tok == tok {
+			list[i] = val
+			i++
+		}
+	}
+	list = list[0:i]
+
+	sortBy(
+		func(i, j int) bool {
+			if ni, nj := sortingName(list[i].Decl), sortingName(list[j].Decl); ni != nj {
+				return ni < nj
+			}
+			return list[i].order < list[j].order
+		},
+		func(i, j int) { list[i], list[j] = list[j], list[i] },
+		len(list),
+	)
+
+	return list
+}
+
+func sortedTypes(m map[string]*namedType, allMethods bool) []*Type {
+	list := make([]*Type, len(m))
+	i := 0
+	for _, t := range m {
+		list[i] = &Type{
+			Doc:     t.doc,
+			Name:    t.name,
+			Decl:    t.decl,
+			Consts:  sortedValues(t.values, token.CONST),
+			Vars:    sortedValues(t.values, token.VAR),
+			Funcs:   sortedFuncs(t.funcs, true),
+			Methods: sortedFuncs(t.methods, allMethods),
+		}
+		i++
+	}
+
+	sortBy(
+		func(i, j int) bool { return list[i].Name < list[j].Name },
+		func(i, j int) { list[i], list[j] = list[j], list[i] },
+		len(list),
+	)
+
+	return list
+}
+
+func removeStar(s string) string {
+	if len(s) > 0 && s[0] == '*' {
+		return s[1:]
+	}
+	return s
+}
+
+func sortedFuncs(m methodSet, allMethods bool) []*Func {
+	list := make([]*Func, len(m))
+	i := 0
+	for _, m := range m {
+		// determine which methods to include
+		switch {
+		case m.Decl == nil:
+			// exclude conflict entry
+		case allMethods, m.Level == 0, !ast.IsExported(removeStar(m.Orig)):
+			// forced inclusion, method not embedded, or method
+			// embedded but original receiver type not exported
+			list[i] = m
+			i++
+		}
+	}
+	list = list[0:i]
+	sortBy(
+		func(i, j int) bool { return list[i].Name < list[j].Name },
+		func(i, j int) { list[i], list[j] = list[j], list[i] },
+		len(list),
+	)
+	return list
+}
+
+// noteBodies returns a list of note body strings given a list of notes.
+// This is only used to populate the deprecated Package.Bugs field.
+//
+func noteBodies(notes []*Note) []string {
+	var list []string
+	for _, n := range notes {
+		list = append(list, n.Body)
+	}
+	return list
+}
+
+// ----------------------------------------------------------------------------
+// Predeclared identifiers
+
+var predeclaredTypes = map[string]bool{
+	"bool":       true,
+	"byte":       true,
+	"complex64":  true,
+	"complex128": true,
+	"error":      true,
+	"float32":    true,
+	"float64":    true,
+	"int":        true,
+	"int8":       true,
+	"int16":      true,
+	"int32":      true,
+	"int64":      true,
+	"rune":       true,
+	"string":     true,
+	"uint":       true,
+	"uint8":      true,
+	"uint16":     true,
+	"uint32":     true,
+	"uint64":     true,
+	"uintptr":    true,
+}
+
+var predeclaredFuncs = map[string]bool{
+	"append":  true,
+	"cap":     true,
+	"close":   true,
+	"complex": true,
+	"copy":    true,
+	"delete":  true,
+	"imag":    true,
+	"len":     true,
+	"make":    true,
+	"new":     true,
+	"panic":   true,
+	"print":   true,
+	"println": true,
+	"real":    true,
+	"recover": true,
+}
+
+var predeclaredConstants = map[string]bool{
+	"false": true,
+	"iota":  true,
+	"nil":   true,
+	"true":  true,
+}
diff --git a/src/go/doc/synopsis.go b/src/go/doc/synopsis.go
new file mode 100644
index 000000000..c90080b7c
--- /dev/null
+++ b/src/go/doc/synopsis.go
@@ -0,0 +1,82 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import (
+	"strings"
+	"unicode"
+)
+
+// firstSentenceLen returns the length of the first sentence in s.
+// The sentence ends after the first period followed by space and
+// not preceded by exactly one uppercase letter.
+//
+func firstSentenceLen(s string) int {
+	var ppp, pp, p rune
+	for i, q := range s {
+		if q == '\n' || q == '\r' || q == '\t' {
+			q = ' '
+		}
+		if q == ' ' && p == '.' && (!unicode.IsUpper(pp) || unicode.IsUpper(ppp)) {
+			return i
+		}
+		if p == '。' || p == '．' {
+			return i
+		}
+		ppp, pp, p = pp, p, q
+	}
+	return len(s)
+}
+
+const (
+	keepNL = 1 << iota
+)
+
+// clean replaces each sequence of space, \n, \r, or \t characters
+// with a single space and removes any trailing and leading spaces.
+// If the keepNL flag is set, newline characters are passed through
+// instead of being change to spaces.
+func clean(s string, flags int) string {
+	var b []byte
+	p := byte(' ')
+	for i := 0; i < len(s); i++ {
+		q := s[i]
+		if (flags&keepNL) == 0 && q == '\n' || q == '\r' || q == '\t' {
+			q = ' '
+		}
+		if q != ' ' || p != ' ' {
+			b = append(b, q)
+			p = q
+		}
+	}
+	// remove trailing blank, if any
+	if n := len(b); n > 0 && p == ' ' {
+		b = b[0 : n-1]
+	}
+	return string(b)
+}
+
+// Synopsis returns a cleaned version of the first sentence in s.
+// That sentence ends after the first period followed by space and
+// not preceded by exactly one uppercase letter. The result string
+// has no \n, \r, or \t characters and uses only single spaces between
+// words. If s starts with any of the IllegalPrefixes, the result
+// is the empty string.
+//
+func Synopsis(s string) string {
+	s = clean(s[0:firstSentenceLen(s)], 0)
+	for _, prefix := range IllegalPrefixes {
+		if strings.HasPrefix(strings.ToLower(s), prefix) {
+			return ""
+		}
+	}
+	return s
+}
+
+var IllegalPrefixes = []string{
+	"copyright",
+	"all rights",
+	"author",
+}
diff --git a/src/go/doc/synopsis_test.go b/src/go/doc/synopsis_test.go
new file mode 100644
index 000000000..59b253cb8
--- /dev/null
+++ b/src/go/doc/synopsis_test.go
@@ -0,0 +1,51 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import "testing"
+
+var tests = []struct {
+	txt string
+	fsl int
+	syn string
+}{
+	{"", 0, ""},
+	{"foo", 3, "foo"},
+	{"foo.", 4, "foo."},
+	{"foo.bar", 7, "foo.bar"},
+	{"  foo.  ", 6, "foo."},
+	{"  foo\t  bar.\n", 12, "foo bar."},
+	{"  foo\t  bar.\n", 12, "foo bar."},
+	{"a  b\n\nc\r\rd\t\t", 12, "a b c d"},
+	{"a  b\n\nc\r\rd\t\t  . BLA", 15, "a b c d ."},
+	{"Package poems by T.S.Eliot. To rhyme...", 27, "Package poems by T.S.Eliot."},
+	{"Package poems by T. S. Eliot. To rhyme...", 29, "Package poems by T. S. Eliot."},
+	{"foo implements the foo ABI. The foo ABI is...", 27, "foo implements the foo ABI."},
+	{"Package\nfoo. ..", 12, "Package foo."},
+	{"P . Q.", 3, "P ."},
+	{"P. Q.   ", 8, "P. Q."},
+	{"Package Καλημέρα κόσμε.", 36, "Package Καλημέρα κόσμε."},
+	{"Package こんにちは 世界\n", 31, "Package こんにちは 世界"},
+	{"Package こんにちは。世界", 26, "Package こんにちは。"},
+	{"Package 안녕．世界", 17, "Package 안녕．"},
+	{"Package foo does bar.", 21, "Package foo does bar."},
+	{"Copyright 2012 Google, Inc. Package foo does bar.", 27, ""},
+	{"All Rights reserved. Package foo does bar.", 20, ""},
+	{"All rights reserved. Package foo does bar.", 20, ""},
+	{"Authors: foo@bar.com. Package foo does bar.", 21, ""},
+}
+
+func TestSynopsis(t *testing.T) {
+	for _, e := range tests {
+		fsl := firstSentenceLen(e.txt)
+		if fsl != e.fsl {
+			t.Errorf("got fsl = %d; want %d for %q\n", fsl, e.fsl, e.txt)
+		}
+		syn := Synopsis(e.txt)
+		if syn != e.syn {
+			t.Errorf("got syn = %q; want %q for %q\n", syn, e.syn, e.txt)
+		}
+	}
+}
diff --git a/src/go/doc/testdata/a.0.golden b/src/go/doc/testdata/a.0.golden
new file mode 100644
index 000000000..7e680b80b
--- /dev/null
+++ b/src/go/doc/testdata/a.0.golden
@@ -0,0 +1,52 @@
+// comment 0  comment 1 
+PACKAGE a
+
+IMPORTPATH
+	testdata/a
+
+FILENAMES
+	testdata/a0.go
+	testdata/a1.go
+
+BUGS .Bugs is now deprecated, please use .Notes instead
+	bug0
+
+	bug1
+
+
+BUGS
+BUG(uid)	bug0
+
+BUG(uid)	bug1
+
+
+NOTES
+NOTE(uid)	
+
+NOTE(foo)	1 of 4 - this is the first line of note 1
+	- note 1 continues on this 2nd line
+	- note 1 continues on this 3rd line
+
+NOTE(foo)	2 of 4
+
+NOTE(bar)	3 of 4
+
+NOTE(bar)	4 of 4
+	- this is the last line of note 4
+
+NOTE(bam)	This note which contains a (parenthesized) subphrase
+	 must appear in its entirety.
+
+NOTE(xxx)	The ':' after the marker and uid is optional.
+
+
+SECBUGS
+SECBUG(uid)	sec hole 0
+	need to fix asap
+
+
+TODOS
+TODO(uid)	todo0
+
+TODO(uid)	todo1
+
diff --git a/src/go/doc/testdata/a.1.golden b/src/go/doc/testdata/a.1.golden
new file mode 100644
index 000000000..7e680b80b
--- /dev/null
+++ b/src/go/doc/testdata/a.1.golden
@@ -0,0 +1,52 @@
+// comment 0  comment 1 
+PACKAGE a
+
+IMPORTPATH
+	testdata/a
+
+FILENAMES
+	testdata/a0.go
+	testdata/a1.go
+
+BUGS .Bugs is now deprecated, please use .Notes instead
+	bug0
+
+	bug1
+
+
+BUGS
+BUG(uid)	bug0
+
+BUG(uid)	bug1
+
+
+NOTES
+NOTE(uid)	
+
+NOTE(foo)	1 of 4 - this is the first line of note 1
+	- note 1 continues on this 2nd line
+	- note 1 continues on this 3rd line
+
+NOTE(foo)	2 of 4
+
+NOTE(bar)	3 of 4
+
+NOTE(bar)	4 of 4
+	- this is the last line of note 4
+
+NOTE(bam)	This note which contains a (parenthesized) subphrase
+	 must appear in its entirety.
+
+NOTE(xxx)	The ':' after the marker and uid is optional.
+
+
+SECBUGS
+SECBUG(uid)	sec hole 0
+	need to fix asap
+
+
+TODOS
+TODO(uid)	todo0
+
+TODO(uid)	todo1
+
diff --git a/src/go/doc/testdata/a.2.golden b/src/go/doc/testdata/a.2.golden
new file mode 100644
index 000000000..7e680b80b
--- /dev/null
+++ b/src/go/doc/testdata/a.2.golden
@@ -0,0 +1,52 @@
+// comment 0  comment 1 
+PACKAGE a
+
+IMPORTPATH
+	testdata/a
+
+FILENAMES
+	testdata/a0.go
+	testdata/a1.go
+
+BUGS .Bugs is now deprecated, please use .Notes instead
+	bug0
+
+	bug1
+
+
+BUGS
+BUG(uid)	bug0
+
+BUG(uid)	bug1
+
+
+NOTES
+NOTE(uid)	
+
+NOTE(foo)	1 of 4 - this is the first line of note 1
+	- note 1 continues on this 2nd line
+	- note 1 continues on this 3rd line
+
+NOTE(foo)	2 of 4
+
+NOTE(bar)	3 of 4
+
+NOTE(bar)	4 of 4
+	- this is the last line of note 4
+
+NOTE(bam)	This note which contains a (parenthesized) subphrase
+	 must appear in its entirety.
+
+NOTE(xxx)	The ':' after the marker and uid is optional.
+
+
+SECBUGS
+SECBUG(uid)	sec hole 0
+	need to fix asap
+
+
+TODOS
+TODO(uid)	todo0
+
+TODO(uid)	todo1
+
diff --git a/src/go/doc/testdata/a0.go b/src/go/doc/testdata/a0.go
new file mode 100644
index 000000000..2420c8a48
--- /dev/null
+++ b/src/go/doc/testdata/a0.go
@@ -0,0 +1,40 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// comment 0
+package a
+
+//BUG(uid): bug0
+
+//TODO(uid): todo0
+
+// A note with some spaces after it, should be ignored (watch out for
+// emacs modes that remove trailing whitespace).
+//NOTE(uid):
+
+// SECBUG(uid): sec hole 0
+// need to fix asap
+
+// Multiple notes may be in the same comment group and should be
+// recognized individually. Notes may start in the middle of a
+// comment group as long as they start at the beginning of an
+// individual comment.
+//
+// NOTE(foo): 1 of 4 - this is the first line of note 1
+// - note 1 continues on this 2nd line
+// - note 1 continues on this 3rd line
+// NOTE(foo): 2 of 4
+// NOTE(bar): 3 of 4
+/* NOTE(bar): 4 of 4 */
+// - this is the last line of note 4
+//
+//
+
+// NOTE(bam): This note which contains a (parenthesized) subphrase
+//            must appear in its entirety.
+
+// NOTE(xxx) The ':' after the marker and uid is optional.
+
+// NOTE(): NO uid - should not show up.
+// NOTE()  NO uid - should not show up.
diff --git a/src/go/doc/testdata/a1.go b/src/go/doc/testdata/a1.go
new file mode 100644
index 000000000..9fad1e09b
--- /dev/null
+++ b/src/go/doc/testdata/a1.go
@@ -0,0 +1,12 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// comment 1
+package a
+
+//BUG(uid): bug1
+
+//TODO(uid): todo1
+
+//TODO(): ignored
diff --git a/src/go/doc/testdata/b.0.golden b/src/go/doc/testdata/b.0.golden
new file mode 100644
index 000000000..9d93392ea
--- /dev/null
+++ b/src/go/doc/testdata/b.0.golden
@@ -0,0 +1,71 @@
+// 
+PACKAGE b
+
+IMPORTPATH
+	testdata/b
+
+IMPORTS
+	a
+
+FILENAMES
+	testdata/b.go
+
+CONSTANTS
+	// 
+	const (
+		C1	notExported	= iota
+		C2
+	
+		C4
+		C5
+	)
+
+	// 
+	const C notExported = 0
+
+	// 
+	const Pi = 3.14	// Pi
+
+
+VARIABLES
+	// 
+	var (
+		U1, U2, U4, U5	notExported
+	
+		U7	notExported	= 7
+	)
+
+	// 
+	var MaxInt int	// MaxInt
+
+	// 
+	var V notExported
+
+	// 
+	var V1, V2, V4, V5 notExported
+
+
+FUNCTIONS
+	// 
+	func F(x int) int
+
+	// 
+	func F1() notExported
+
+	// Always under the package functions list. 
+	func NotAFactory() int
+
+	// Associated with uint type if AllDecls is set. 
+	func UintFactory() uint
+
+
+TYPES
+	// 
+	type T struct{}	// T
+
+	// 
+	var V T	// v
+
+	// 
+	func (x *T) M()
+
diff --git a/src/go/doc/testdata/b.1.golden b/src/go/doc/testdata/b.1.golden
new file mode 100644
index 000000000..66c47b5c2
--- /dev/null
+++ b/src/go/doc/testdata/b.1.golden
@@ -0,0 +1,83 @@
+// 
+PACKAGE b
+
+IMPORTPATH
+	testdata/b
+
+IMPORTS
+	a
+
+FILENAMES
+	testdata/b.go
+
+CONSTANTS
+	// 
+	const Pi = 3.14	// Pi
+
+
+VARIABLES
+	// 
+	var MaxInt int	// MaxInt
+
+
+FUNCTIONS
+	// 
+	func F(x int) int
+
+	// Always under the package functions list. 
+	func NotAFactory() int
+
+
+TYPES
+	// 
+	type T struct{}	// T
+
+	// 
+	var V T	// v
+
+	// 
+	func (x *T) M()
+
+	// 
+	type notExported int
+
+	// 
+	const (
+		C1	notExported	= iota
+		C2
+		c3
+		C4
+		C5
+	)
+
+	// 
+	const C notExported = 0
+
+	// 
+	var (
+		U1, U2, u3, U4, U5	notExported
+		u6			notExported
+		U7			notExported	= 7
+	)
+
+	// 
+	var V notExported
+
+	// 
+	var V1, V2, v3, V4, V5 notExported
+
+	// 
+	func F1() notExported
+
+	// 
+	func f2() notExported
+
+	// Should only appear if AllDecls is set. 
+	type uint struct{}	// overrides a predeclared type uint
+
+	// Associated with uint type if AllDecls is set. 
+	func UintFactory() uint
+
+	// Associated with uint type if AllDecls is set. 
+	func uintFactory() uint
+
diff --git a/src/go/doc/testdata/b.2.golden b/src/go/doc/testdata/b.2.golden
new file mode 100644
index 000000000..9d93392ea
--- /dev/null
+++ b/src/go/doc/testdata/b.2.golden
@@ -0,0 +1,71 @@
+// 
+PACKAGE b
+
+IMPORTPATH
+	testdata/b
+
+IMPORTS
+	a
+
+FILENAMES
+	testdata/b.go
+
+CONSTANTS
+	// 
+	const (
+		C1	notExported	= iota
+		C2
+	
+		C4
+		C5
+	)
+
+	// 
+	const C notExported = 0
+
+	// 
+	const Pi = 3.14	// Pi
+
+
+VARIABLES
+	// 
+	var (
+		U1, U2, U4, U5	notExported
+	
+		U7	notExported	= 7
+	)
+
+	// 
+	var MaxInt int	// MaxInt
+
+	// 
+	var V notExported
+
+	// 
+	var V1, V2, V4, V5 notExported
+
+
+FUNCTIONS
+	// 
+	func F(x int) int
+
+	// 
+	func F1() notExported
+
+	// Always under the package functions list. 
+	func NotAFactory() int
+
+	// Associated with uint type if AllDecls is set. 
+	func UintFactory() uint
+
+
+TYPES
+	// 
+	type T struct{}	// T
+
+	// 
+	var V T	// v
+
+	// 
+	func (x *T) M()
+
diff --git a/src/go/doc/testdata/b.go b/src/go/doc/testdata/b.go
new file mode 100644
index 000000000..e50663b3d
--- /dev/null
+++ b/src/go/doc/testdata/b.go
@@ -0,0 +1,58 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package b
+
+import "a"
+
+// ----------------------------------------------------------------------------
+// Basic declarations
+
+const Pi = 3.14   // Pi
+var MaxInt int    // MaxInt
+type T struct{}   // T
+var V T           // v
+func F(x int) int {} // F
+func (x *T) M()   {} // M
+
+// Corner cases: association with (presumed) predeclared types
+
+// Always under the package functions list.
+func NotAFactory() int {}
+
+// Associated with uint type if AllDecls is set.
+func UintFactory() uint {}
+
+// Associated with uint type if AllDecls is set.
+func uintFactory() uint {}
+
+// Should only appear if AllDecls is set.
+type uint struct{} // overrides a predeclared type uint
+
+// ----------------------------------------------------------------------------
+// Exported declarations associated with non-exported types must always be shown.
+
+type notExported int
+
+const C notExported = 0
+
+const (
+	C1 notExported = iota
+	C2
+	c3
+	C4
+	C5
+)
+
+var V notExported
+var V1, V2, v3, V4, V5 notExported
+
+var (
+	U1, U2, u3, U4, U5 notExported
+	u6                 notExported
+	U7                 notExported = 7
+)
+
+func F1() notExported {}
+func f2() notExported {}
diff --git a/src/go/doc/testdata/benchmark.go b/src/go/doc/testdata/benchmark.go
new file mode 100644
index 000000000..905e49644
--- /dev/null
+++ b/src/go/doc/testdata/benchmark.go
@@ -0,0 +1,293 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"runtime"
+	"time"
+)
+
+var matchBenchmarks = flag.String("test.bench", "", "regular expression to select benchmarks to run")
+var benchTime = flag.Duration("test.benchtime", 1*time.Second, "approximate run time for each benchmark")
+
+// An internal type but exported because it is cross-package; part of the implementation
+// of go test.
+type InternalBenchmark struct {
+	Name string
+	F    func(b *B)
+}
+
+// B is a type passed to Benchmark functions to manage benchmark
+// timing and to specify the number of iterations to run.
+type B struct {
+	common
+	N         int
+	benchmark InternalBenchmark
+	bytes     int64
+	timerOn   bool
+	result    BenchmarkResult
+}
+
+// StartTimer starts timing a test.  This function is called automatically
+// before a benchmark starts, but it can also used to resume timing after
+// a call to StopTimer.
+func (b *B) StartTimer() {
+	if !b.timerOn {
+		b.start = time.Now()
+		b.timerOn = true
+	}
+}
+
+// StopTimer stops timing a test.  This can be used to pause the timer
+// while performing complex initialization that you don't
+// want to measure.
+func (b *B) StopTimer() {
+	if b.timerOn {
+		b.duration += time.Now().Sub(b.start)
+		b.timerOn = false
+	}
+}
+
+// ResetTimer sets the elapsed benchmark time to zero.
+// It does not affect whether the timer is running.
+func (b *B) ResetTimer() {
+	if b.timerOn {
+		b.start = time.Now()
+	}
+	b.duration = 0
+}
+
+// SetBytes records the number of bytes processed in a single operation.
+// If this is called, the benchmark will report ns/op and MB/s.
+func (b *B) SetBytes(n int64) { b.bytes = n }
+
+func (b *B) nsPerOp() int64 {
+	if b.N <= 0 {
+		return 0
+	}
+	return b.duration.Nanoseconds() / int64(b.N)
+}
+
+// runN runs a single benchmark for the specified number of iterations.
+func (b *B) runN(n int) {
+	// Try to get a comparable environment for each run
+	// by clearing garbage from previous runs.
+	runtime.GC()
+	b.N = n
+	b.ResetTimer()
+	b.StartTimer()
+	b.benchmark.F(b)
+	b.StopTimer()
+}
+
+func min(x, y int) int {
+	if x > y {
+		return y
+	}
+	return x
+}
+
+func max(x, y int) int {
+	if x < y {
+		return y
+	}
+	return x
+}
+
+// roundDown10 rounds a number down to the nearest power of 10.
+func roundDown10(n int) int {
+	var tens = 0
+	// tens = floor(log_10(n))
+	for n > 10 {
+		n = n / 10
+		tens++
+	}
+	// result = 10^tens
+	result := 1
+	for i := 0; i < tens; i++ {
+		result *= 10
+	}
+	return result
+}
+
+// roundUp rounds x up to a number of the form [1eX, 2eX, 5eX].
+func roundUp(n int) int {
+	base := roundDown10(n)
+	if n < (2 * base) {
+		return 2 * base
+	}
+	if n < (5 * base) {
+		return 5 * base
+	}
+	return 10 * base
+}
+
+// run times the benchmark function in a separate goroutine.
+func (b *B) run() BenchmarkResult {
+	go b.launch()
+	<-b.signal
+	return b.result
+}
+
+// launch launches the benchmark function.  It gradually increases the number
+// of benchmark iterations until the benchmark runs for a second in order
+// to get a reasonable measurement.  It prints timing information in this form
+//		testing.BenchmarkHello	100000		19 ns/op
+// launch is run by the fun function as a separate goroutine.
+func (b *B) launch() {
+	// Run the benchmark for a single iteration in case it's expensive.
+	n := 1
+
+	// Signal that we're done whether we return normally
+	// or by FailNow's runtime.Goexit.
+	defer func() {
+		b.signal <- b
+	}()
+
+	b.runN(n)
+	// Run the benchmark for at least the specified amount of time.
+	d := *benchTime
+	for !b.failed && b.duration < d && n < 1e9 {
+		last := n
+		// Predict iterations/sec.
+		if b.nsPerOp() == 0 {
+			n = 1e9
+		} else {
+			n = int(d.Nanoseconds() / b.nsPerOp())
+		}
+		// Run more iterations than we think we'll need for a second (1.5x).
+		// Don't grow too fast in case we had timing errors previously.
+		// Be sure to run at least one more than last time.
+		n = max(min(n+n/2, 100*last), last+1)
+		// Round up to something easy to read.
+		n = roundUp(n)
+		b.runN(n)
+	}
+	b.result = BenchmarkResult{b.N, b.duration, b.bytes}
+}
+
+// The results of a benchmark run.
+type BenchmarkResult struct {
+	N     int           // The number of iterations.
+	T     time.Duration // The total time taken.
+	Bytes int64         // Bytes processed in one iteration.
+}
+
+func (r BenchmarkResult) NsPerOp() int64 {
+	if r.N <= 0 {
+		return 0
+	}
+	return r.T.Nanoseconds() / int64(r.N)
+}
+
+func (r BenchmarkResult) mbPerSec() float64 {
+	if r.Bytes <= 0 || r.T <= 0 || r.N <= 0 {
+		return 0
+	}
+	return (float64(r.Bytes) * float64(r.N) / 1e6) / r.T.Seconds()
+}
+
+func (r BenchmarkResult) String() string {
+	mbs := r.mbPerSec()
+	mb := ""
+	if mbs != 0 {
+		mb = fmt.Sprintf("\t%7.2f MB/s", mbs)
+	}
+	nsop := r.NsPerOp()
+	ns := fmt.Sprintf("%10d ns/op", nsop)
+	if r.N > 0 && nsop < 100 {
+		// The format specifiers here make sure that
+		// the ones digits line up for all three possible formats.
+		if nsop < 10 {
+			ns = fmt.Sprintf("%13.2f ns/op", float64(r.T.Nanoseconds())/float64(r.N))
+		} else {
+			ns = fmt.Sprintf("%12.1f ns/op", float64(r.T.Nanoseconds())/float64(r.N))
+		}
+	}
+	return fmt.Sprintf("%8d\t%s%s", r.N, ns, mb)
+}
+
+// An internal function but exported because it is cross-package; part of the implementation
+// of go test.
+func RunBenchmarks(matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark) {
+	// If no flag was specified, don't run benchmarks.
+	if len(*matchBenchmarks) == 0 {
+		return
+	}
+	for _, Benchmark := range benchmarks {
+		matched, err := matchString(*matchBenchmarks, Benchmark.Name)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "testing: invalid regexp for -test.bench: %s\n", err)
+			os.Exit(1)
+		}
+		if !matched {
+			continue
+		}
+		for _, procs := range cpuList {
+			runtime.GOMAXPROCS(procs)
+			b := &B{
+				common: common{
+					signal: make(chan interface{}),
+				},
+				benchmark: Benchmark,
+			}
+			benchName := Benchmark.Name
+			if procs != 1 {
+				benchName = fmt.Sprintf("%s-%d", Benchmark.Name, procs)
+			}
+			fmt.Printf("%s\t", benchName)
+			r := b.run()
+			if b.failed {
+				// The output could be very long here, but probably isn't.
+				// We print it all, regardless, because we don't want to trim the reason
+				// the benchmark failed.
+				fmt.Printf("--- FAIL: %s\n%s", benchName, b.output)
+				continue
+			}
+			fmt.Printf("%v\n", r)
+			// Unlike with tests, we ignore the -chatty flag and always print output for
+			// benchmarks since the output generation time will skew the results.
+			if len(b.output) > 0 {
+				b.trimOutput()
+				fmt.Printf("--- BENCH: %s\n%s", benchName, b.output)
+			}
+			if p := runtime.GOMAXPROCS(-1); p != procs {
+				fmt.Fprintf(os.Stderr, "testing: %s left GOMAXPROCS set to %d\n", benchName, p)
+			}
+		}
+	}
+}
+
+// trimOutput shortens the output from a benchmark, which can be very long.
+func (b *B) trimOutput() {
+	// The output is likely to appear multiple times because the benchmark
+	// is run multiple times, but at least it will be seen. This is not a big deal
+	// because benchmarks rarely print, but just in case, we trim it if it's too long.
+	const maxNewlines = 10
+	for nlCount, j := 0, 0; j < len(b.output); j++ {
+		if b.output[j] == '\n' {
+			nlCount++
+			if nlCount >= maxNewlines {
+				b.output = append(b.output[:j], "\n\t... [output truncated]\n"...)
+				break
+			}
+		}
+	}
+}
+
+// Benchmark benchmarks a single function. Useful for creating
+// custom benchmarks that do not use go test.
+func Benchmark(f func(b *B)) BenchmarkResult {
+	b := &B{
+		common: common{
+			signal: make(chan interface{}),
+		},
+		benchmark: InternalBenchmark{"", f},
+	}
+	return b.run()
+}
diff --git a/src/go/doc/testdata/bugpara.0.golden b/src/go/doc/testdata/bugpara.0.golden
new file mode 100644
index 000000000..580485950
--- /dev/null
+++ b/src/go/doc/testdata/bugpara.0.golden
@@ -0,0 +1,20 @@
+// 
+PACKAGE bugpara
+
+IMPORTPATH
+	testdata/bugpara
+
+FILENAMES
+	testdata/bugpara.go
+
+BUGS .Bugs is now deprecated, please use .Notes instead
+	Sometimes bugs have multiple paragraphs.
+	
+	Like this one.
+
+
+BUGS
+BUG(rsc)	Sometimes bugs have multiple paragraphs.
+	
+	Like this one.
+
diff --git a/src/go/doc/testdata/bugpara.1.golden b/src/go/doc/testdata/bugpara.1.golden
new file mode 100644
index 000000000..580485950
--- /dev/null
+++ b/src/go/doc/testdata/bugpara.1.golden
@@ -0,0 +1,20 @@
+// 
+PACKAGE bugpara
+
+IMPORTPATH
+	testdata/bugpara
+
+FILENAMES
+	testdata/bugpara.go
+
+BUGS .Bugs is now deprecated, please use .Notes instead
+	Sometimes bugs have multiple paragraphs.
+	
+	Like this one.
+
+
+BUGS
+BUG(rsc)	Sometimes bugs have multiple paragraphs.
+	
+	Like this one.
+
diff --git a/src/go/doc/testdata/bugpara.2.golden b/src/go/doc/testdata/bugpara.2.golden
new file mode 100644
index 000000000..580485950
--- /dev/null
+++ b/src/go/doc/testdata/bugpara.2.golden
@@ -0,0 +1,20 @@
+// 
+PACKAGE bugpara
+
+IMPORTPATH
+	testdata/bugpara
+
+FILENAMES
+	testdata/bugpara.go
+
+BUGS .Bugs is now deprecated, please use .Notes instead
+	Sometimes bugs have multiple paragraphs.
+	
+	Like this one.
+
+
+BUGS
+BUG(rsc)	Sometimes bugs have multiple paragraphs.
+	
+	Like this one.
+
diff --git a/src/go/doc/testdata/bugpara.go b/src/go/doc/testdata/bugpara.go
new file mode 100644
index 000000000..f5345a797
--- /dev/null
+++ b/src/go/doc/testdata/bugpara.go
@@ -0,0 +1,5 @@
+package bugpara
+
+// BUG(rsc): Sometimes bugs have multiple paragraphs.
+//
+// Like this one.
diff --git a/src/go/doc/testdata/c.0.golden b/src/go/doc/testdata/c.0.golden
new file mode 100644
index 000000000..e21959b19
--- /dev/null
+++ b/src/go/doc/testdata/c.0.golden
@@ -0,0 +1,48 @@
+// 
+PACKAGE c
+
+IMPORTPATH
+	testdata/c
+
+IMPORTS
+	a
+
+FILENAMES
+	testdata/c.go
+
+TYPES
+	// A (should see this) 
+	type A struct{}
+
+	// B (should see this) 
+	type B struct{}
+
+	// C (should see this) 
+	type C struct{}
+
+	// D (should see this) 
+	type D struct{}
+
+	// E1 (should see this) 
+	type E1 struct{}
+
+	// E (should see this for E2 and E3) 
+	type E2 struct{}
+
+	// E (should see this for E2 and E3) 
+	type E3 struct{}
+
+	// E4 (should see this) 
+	type E4 struct{}
+
+	// 
+	type T1 struct{}
+
+	// 
+	func (t1 *T1) M()
+
+	// T2 must not show methods of local T1 
+	type T2 struct {
+		a.T1	// not the same as locally declared T1
+	}
+
diff --git a/src/go/doc/testdata/c.1.golden b/src/go/doc/testdata/c.1.golden
new file mode 100644
index 000000000..e21959b19
--- /dev/null
+++ b/src/go/doc/testdata/c.1.golden
@@ -0,0 +1,48 @@
+// 
+PACKAGE c
+
+IMPORTPATH
+	testdata/c
+
+IMPORTS
+	a
+
+FILENAMES
+	testdata/c.go
+
+TYPES
+	// A (should see this) 
+	type A struct{}
+
+	// B (should see this) 
+	type B struct{}
+
+	// C (should see this) 
+	type C struct{}
+
+	// D (should see this) 
+	type D struct{}
+
+	// E1 (should see this) 
+	type E1 struct{}
+
+	// E (should see this for E2 and E3) 
+	type E2 struct{}
+
+	// E (should see this for E2 and E3) 
+	type E3 struct{}
+
+	// E4 (should see this) 
+	type E4 struct{}
+
+	// 
+	type T1 struct{}
+
+	// 
+	func (t1 *T1) M()
+
+	// T2 must not show methods of local T1 
+	type T2 struct {
+		a.T1	// not the same as locally declared T1
+	}
+
diff --git a/src/go/doc/testdata/c.2.golden b/src/go/doc/testdata/c.2.golden
new file mode 100644
index 000000000..e21959b19
--- /dev/null
+++ b/src/go/doc/testdata/c.2.golden
@@ -0,0 +1,48 @@
+// 
+PACKAGE c
+
+IMPORTPATH
+	testdata/c
+
+IMPORTS
+	a
+
+FILENAMES
+	testdata/c.go
+
+TYPES
+	// A (should see this) 
+	type A struct{}
+
+	// B (should see this) 
+	type B struct{}
+
+	// C (should see this) 
+	type C struct{}
+
+	// D (should see this) 
+	type D struct{}
+
+	// E1 (should see this) 
+	type E1 struct{}
+
+	// E (should see this for E2 and E3) 
+	type E2 struct{}
+
+	// E (should see this for E2 and E3) 
+	type E3 struct{}
+
+	// E4 (should see this) 
+	type E4 struct{}
+
+	// 
+	type T1 struct{}
+
+	// 
+	func (t1 *T1) M()
+
+	// T2 must not show methods of local T1 
+	type T2 struct {
+		a.T1	// not the same as locally declared T1
+	}
+
diff --git a/src/go/doc/testdata/c.go b/src/go/doc/testdata/c.go
new file mode 100644
index 000000000..e0f39196d
--- /dev/null
+++ b/src/go/doc/testdata/c.go
@@ -0,0 +1,62 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package c
+
+import "a"
+
+// ----------------------------------------------------------------------------
+// Test that empty declarations don't cause problems
+
+const ()
+
+type ()
+
+var ()
+
+// ----------------------------------------------------------------------------
+// Test that types with documentation on both, the Decl and the Spec node
+// are handled correctly.
+
+// A (should see this)
+type A struct{}
+
+// B (should see this)
+type (
+	B struct{}
+)
+
+type (
+	// C (should see this)
+	C struct{}
+)
+
+// D (should not see this)
+type (
+	// D (should see this)
+	D struct{}
+)
+
+// E (should see this for E2 and E3)
+type (
+	// E1 (should see this)
+	E1 struct{}
+	E2 struct{}
+	E3 struct{}
+	// E4 (should see this)
+	E4 struct{}
+)
+
+// ----------------------------------------------------------------------------
+// Test that local and imported types are different when
+// handling anonymous fields.
+
+type T1 struct{}
+
+func (t1 *T1) M() {}
+
+// T2 must not show methods of local T1
+type T2 struct {
+	a.T1 // not the same as locally declared T1
+}
diff --git a/src/go/doc/testdata/d.0.golden b/src/go/doc/testdata/d.0.golden
new file mode 100644
index 000000000..c00519953
--- /dev/null
+++ b/src/go/doc/testdata/d.0.golden
@@ -0,0 +1,104 @@
+// 
+PACKAGE d
+
+IMPORTPATH
+	testdata/d
+
+FILENAMES
+	testdata/d1.go
+	testdata/d2.go
+
+CONSTANTS
+	// CBx constants should appear before CAx constants. 
+	const (
+		CB2	= iota	// before CB1
+		CB1		// before CB0
+		CB0		// at end
+	)
+
+	// CAx constants should appear after CBx constants. 
+	const (
+		CA2	= iota	// before CA1
+		CA1		// before CA0
+		CA0		// at end
+	)
+
+	// C0 should be first. 
+	const C0 = 0
+
+	// C1 should be second. 
+	const C1 = 1
+
+	// C2 should be third. 
+	const C2 = 2
+
+	// 
+	const (
+		// Single const declarations inside ()'s are considered ungrouped
+		// and show up in sorted order.
+		Cungrouped = 0
+	)
+
+
+VARIABLES
+	// VBx variables should appear before VAx variables. 
+	var (
+		VB2	int	// before VB1
+		VB1	int	// before VB0
+		VB0	int	// at end
+	)
+
+	// VAx variables should appear after VBx variables. 
+	var (
+		VA2	int	// before VA1
+		VA1	int	// before VA0
+		VA0	int	// at end
+	)
+
+	// V0 should be first. 
+	var V0 uintptr
+
+	// V1 should be second. 
+	var V1 uint
+
+	// V2 should be third. 
+	var V2 int
+
+	// 
+	var (
+		// Single var declarations inside ()'s are considered ungrouped
+		// and show up in sorted order.
+		Vungrouped = 0
+	)
+
+
+FUNCTIONS
+	// F0 should be first. 
+	func F0()
+
+	// F1 should be second. 
+	func F1()
+
+	// F2 should be third. 
+	func F2()
+
+
+TYPES
+	// T0 should be first. 
+	type T0 struct{}
+
+	// T1 should be second. 
+	type T1 struct{}
+
+	// T2 should be third. 
+	type T2 struct{}
+
+	// TG0 should be first. 
+	type TG0 struct{}
+
+	// TG1 should be second. 
+	type TG1 struct{}
+
+	// TG2 should be third. 
+	type TG2 struct{}
+
diff --git a/src/go/doc/testdata/d.1.golden b/src/go/doc/testdata/d.1.golden
new file mode 100644
index 000000000..c00519953
--- /dev/null
+++ b/src/go/doc/testdata/d.1.golden
@@ -0,0 +1,104 @@
+// 
+PACKAGE d
+
+IMPORTPATH
+	testdata/d
+
+FILENAMES
+	testdata/d1.go
+	testdata/d2.go
+
+CONSTANTS
+	// CBx constants should appear before CAx constants. 
+	const (
+		CB2	= iota	// before CB1
+		CB1		// before CB0
+		CB0		// at end
+	)
+
+	// CAx constants should appear after CBx constants. 
+	const (
+		CA2	= iota	// before CA1
+		CA1		// before CA0
+		CA0		// at end
+	)
+
+	// C0 should be first. 
+	const C0 = 0
+
+	// C1 should be second. 
+	const C1 = 1
+
+	// C2 should be third. 
+	const C2 = 2
+
+	// 
+	const (
+		// Single const declarations inside ()'s are considered ungrouped
+		// and show up in sorted order.
+		Cungrouped = 0
+	)
+
+
+VARIABLES
+	// VBx variables should appear before VAx variables. 
+	var (
+		VB2	int	// before VB1
+		VB1	int	// before VB0
+		VB0	int	// at end
+	)
+
+	// VAx variables should appear after VBx variables. 
+	var (
+		VA2	int	// before VA1
+		VA1	int	// before VA0
+		VA0	int	// at end
+	)
+
+	// V0 should be first. 
+	var V0 uintptr
+
+	// V1 should be second. 
+	var V1 uint
+
+	// V2 should be third. 
+	var V2 int
+
+	// 
+	var (
+		// Single var declarations inside ()'s are considered ungrouped
+		// and show up in sorted order.
+		Vungrouped = 0
+	)
+
+
+FUNCTIONS
+	// F0 should be first. 
+	func F0()
+
+	// F1 should be second. 
+	func F1()
+
+	// F2 should be third. 
+	func F2()
+
+
+TYPES
+	// T0 should be first. 
+	type T0 struct{}
+
+	// T1 should be second. 
+	type T1 struct{}
+
+	// T2 should be third. 
+	type T2 struct{}
+
+	// TG0 should be first. 
+	type TG0 struct{}
+
+	// TG1 should be second. 
+	type TG1 struct{}
+
+	// TG2 should be third. 
+	type TG2 struct{}
+
diff --git a/src/go/doc/testdata/d.2.golden b/src/go/doc/testdata/d.2.golden
new file mode 100644
index 000000000..c00519953
--- /dev/null
+++ b/src/go/doc/testdata/d.2.golden
@@ -0,0 +1,104 @@
+// 
+PACKAGE d
+
+IMPORTPATH
+	testdata/d
+
+FILENAMES
+	testdata/d1.go
+	testdata/d2.go
+
+CONSTANTS
+	// CBx constants should appear before CAx constants. 
+	const (
+		CB2	= iota	// before CB1
+		CB1		// before CB0
+		CB0		// at end
+	)
+
+	// CAx constants should appear after CBx constants. 
+	const (
+		CA2	= iota	// before CA1
+		CA1		// before CA0
+		CA0		// at end
+	)
+
+	// C0 should be first. 
+	const C0 = 0
+
+	// C1 should be second. 
+	const C1 = 1
+
+	// C2 should be third. 
+	const C2 = 2
+
+	// 
+	const (
+		// Single const declarations inside ()'s are considered ungrouped
+		// and show up in sorted order.
+		Cungrouped = 0
+	)
+
+
+VARIABLES
+	// VBx variables should appear before VAx variables. 
+	var (
+		VB2	int	// before VB1
+		VB1	int	// before VB0
+		VB0	int	// at end
+	)
+
+	// VAx variables should appear after VBx variables. 
+	var (
+		VA2	int	// before VA1
+		VA1	int	// before VA0
+		VA0	int	// at end
+	)
+
+	// V0 should be first. 
+	var V0 uintptr
+
+	// V1 should be second. 
+	var V1 uint
+
+	// V2 should be third. 
+	var V2 int
+
+	// 
+	var (
+		// Single var declarations inside ()'s are considered ungrouped
+		// and show up in sorted order.
+		Vungrouped = 0
+	)
+
+
+FUNCTIONS
+	// F0 should be first. 
+	func F0()
+
+	// F1 should be second. 
+	func F1()
+
+	// F2 should be third. 
+	func F2()
+
+
+TYPES
+	// T0 should be first. 
+	type T0 struct{}
+
+	// T1 should be second. 
+	type T1 struct{}
+
+	// T2 should be third. 
+	type T2 struct{}
+
+	// TG0 should be first. 
+	type TG0 struct{}
+
+	// TG1 should be second. 
+	type TG1 struct{}
+
+	// TG2 should be third. 
+	type TG2 struct{}
+
diff --git a/src/go/doc/testdata/d1.go b/src/go/doc/testdata/d1.go
new file mode 100644
index 000000000..ebd694195
--- /dev/null
+++ b/src/go/doc/testdata/d1.go
@@ -0,0 +1,57 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test cases for sort order of declarations.
+
+package d
+
+// C2 should be third.
+const C2 = 2
+
+// V2 should be third.
+var V2 int
+
+// CBx constants should appear before CAx constants.
+const (
+	CB2 = iota // before CB1
+	CB1        // before CB0
+	CB0        // at end
+)
+
+// VBx variables should appear before VAx variables.
+var (
+	VB2 int // before VB1
+	VB1 int // before VB0
+	VB0 int // at end
+)
+
+const (
+	// Single const declarations inside ()'s are considered ungrouped
+	// and show up in sorted order.
+	Cungrouped = 0
+)
+
+var (
+	// Single var declarations inside ()'s are considered ungrouped
+	// and show up in sorted order.
+	Vungrouped = 0
+)
+
+// T2 should be third.
+type T2 struct{}
+
+// Grouped types are sorted nevertheless.
+type (
+	// TG2 should be third.
+	TG2 struct{}
+
+	// TG1 should be second.
+	TG1 struct{}
+
+	// TG0 should be first.
+	TG0 struct{}
+)
+
+// F2 should be third.
+func F2() {}
diff --git a/src/go/doc/testdata/d2.go b/src/go/doc/testdata/d2.go
new file mode 100644
index 000000000..2f56f4fa4
--- /dev/null
+++ b/src/go/doc/testdata/d2.go
@@ -0,0 +1,45 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test cases for sort order of declarations.
+
+package d
+
+// C1 should be second.
+const C1 = 1
+
+// C0 should be first.
+const C0 = 0
+
+// V1 should be second.
+var V1 uint
+
+// V0 should be first.
+var V0 uintptr
+
+// CAx constants should appear after CBx constants.
+const (
+	CA2 = iota // before CA1
+	CA1        // before CA0
+	CA0        // at end
+)
+
+// VAx variables should appear after VBx variables.
+var (
+	VA2 int // before VA1
+	VA1 int // before VA0
+	VA0 int // at end
+)
+
+// T1 should be second.
+type T1 struct{}
+
+// T0 should be first.
+type T0 struct{}
+
+// F1 should be second.
+func F1() {}
+
+// F0 should be first.
+func F0() {}
diff --git a/src/go/doc/testdata/e.0.golden b/src/go/doc/testdata/e.0.golden
new file mode 100644
index 000000000..6987e5867
--- /dev/null
+++ b/src/go/doc/testdata/e.0.golden
@@ -0,0 +1,109 @@
+// The package e is a go/doc test for embedded methods. 
+PACKAGE e
+
+IMPORTPATH
+	testdata/e
+
+FILENAMES
+	testdata/e.go
+
+TYPES
+	// T1 has no embedded (level 1) M method due to conflict. 
+	type T1 struct {
+		// contains filtered or unexported fields
+	}
+
+	// T2 has only M as top-level method. 
+	type T2 struct {
+		// contains filtered or unexported fields
+	}
+
+	// T2.M should appear as method of T2. 
+	func (T2) M()
+
+	// T3 has only M as top-level method. 
+	type T3 struct {
+		// contains filtered or unexported fields
+	}
+
+	// T3.M should appear as method of T3. 
+	func (T3) M()
+
+	// 
+	type T4 struct{}
+
+	// T4.M should appear as method of T5 only if AllMethods is set. 
+	func (*T4) M()
+
+	// 
+	type T5 struct {
+		T4
+	}
+
+	// 
+	type U1 struct {
+		*U1
+	}
+
+	// U1.M should appear as method of U1. 
+	func (*U1) M()
+
+	// 
+	type U2 struct {
+		*U3
+	}
+
+	// U2.M should appear as method of U2 and as method of U3 only if ...
+	func (*U2) M()
+
+	// 
+	type U3 struct {
+		*U2
+	}
+
+	// U3.N should appear as method of U3 and as method of U2 only if ...
+	func (*U3) N()
+
+	// 
+	type U4 struct {
+		// contains filtered or unexported fields
+	}
+
+	// U4.M should appear as method of U4. 
+	func (*U4) M()
+
+	// 
+	type V1 struct {
+		*V2
+		*V5
+	}
+
+	// 
+	type V2 struct {
+		*V3
+	}
+
+	// 
+	type V3 struct {
+		*V4
+	}
+
+	// 
+	type V4 struct {
+		*V5
+	}
+
+	// V4.M should appear as method of V2 and V3 if AllMethods is set. 
+	func (*V4) M()
+
+	// 
+	type V5 struct {
+		*V6
+	}
+
+	// 
+	type V6 struct{}
+
+	// V6.M should appear as method of V1 and V5 if AllMethods is set. 
+	func (*V6) M()
+
diff --git a/src/go/doc/testdata/e.1.golden b/src/go/doc/testdata/e.1.golden
new file mode 100644
index 000000000..cbe22e0bf
--- /dev/null
+++ b/src/go/doc/testdata/e.1.golden
@@ -0,0 +1,144 @@
+// The package e is a go/doc test for embedded methods. 
+PACKAGE e
+
+IMPORTPATH
+	testdata/e
+
+FILENAMES
+	testdata/e.go
+
+TYPES
+	// T1 has no embedded (level 1) M method due to conflict. 
+	type T1 struct {
+		t1
+		t2
+	}
+
+	// T2 has only M as top-level method. 
+	type T2 struct {
+		t1
+	}
+
+	// T2.M should appear as method of T2. 
+	func (T2) M()
+
+	// T3 has only M as top-level method. 
+	type T3 struct {
+		t1e
+		t2e
+	}
+
+	// T3.M should appear as method of T3. 
+	func (T3) M()
+
+	// 
+	type T4 struct{}
+
+	// T4.M should appear as method of T5 only if AllMethods is set. 
+	func (*T4) M()
+
+	// 
+	type T5 struct {
+		T4
+	}
+
+	// 
+	type U1 struct {
+		*U1
+	}
+
+	// U1.M should appear as method of U1. 
+	func (*U1) M()
+
+	// 
+	type U2 struct {
+		*U3
+	}
+
+	// U2.M should appear as method of U2 and as method of U3 only if ...
+	func (*U2) M()
+
+	// 
+	type U3 struct {
+		*U2
+	}
+
+	// U3.N should appear as method of U3 and as method of U2 only if ...
+	func (*U3) N()
+
+	// 
+	type U4 struct {
+		*u5
+	}
+
+	// U4.M should appear as method of U4. 
+	func (*U4) M()
+
+	// 
+	type V1 struct {
+		*V2
+		*V5
+	}
+
+	// 
+	type V2 struct {
+		*V3
+	}
+
+	// 
+	type V3 struct {
+		*V4
+	}
+
+	// 
+	type V4 struct {
+		*V5
+	}
+
+	// V4.M should appear as method of V2 and V3 if AllMethods is set. 
+	func (*V4) M()
+
+	// 
+	type V5 struct {
+		*V6
+	}
+
+	// 
+	type V6 struct{}
+
+	// V6.M should appear as method of V1 and V5 if AllMethods is set. 
+	func (*V6) M()
+
+	// 
+	type t1 struct{}
+
+	// t1.M should not appear as method in a Tx type. 
+	func (t1) M()
+
+	// 
+	type t1e struct {
+		t1
+	}
+
+	// t1.M should not appear as method in a Tx type. 
+	func (t1e) M()
+
+	// 
+	type t2 struct{}
+
+	// t2.M should not appear as method in a Tx type. 
+	func (t2) M()
+
+	// 
+	type t2e struct {
+		t2
+	}
+
+	// t2.M should not appear as method in a Tx type. 
+	func (t2e) M()
+
+	// 
+	type u5 struct {
+		*U4
+	}
+
diff --git a/src/go/doc/testdata/e.2.golden b/src/go/doc/testdata/e.2.golden
new file mode 100644
index 000000000..e7b05e80f
--- /dev/null
+++ b/src/go/doc/testdata/e.2.golden
@@ -0,0 +1,130 @@
+// The package e is a go/doc test for embedded methods. 
+PACKAGE e
+
+IMPORTPATH
+	testdata/e
+
+FILENAMES
+	testdata/e.go
+
+TYPES
+	// T1 has no embedded (level 1) M method due to conflict. 
+	type T1 struct {
+		// contains filtered or unexported fields
+	}
+
+	// T2 has only M as top-level method. 
+	type T2 struct {
+		// contains filtered or unexported fields
+	}
+
+	// T2.M should appear as method of T2. 
+	func (T2) M()
+
+	// T3 has only M as top-level method. 
+	type T3 struct {
+		// contains filtered or unexported fields
+	}
+
+	// T3.M should appear as method of T3. 
+	func (T3) M()
+
+	// 
+	type T4 struct{}
+
+	// T4.M should appear as method of T5 only if AllMethods is set. 
+	func (*T4) M()
+
+	// 
+	type T5 struct {
+		T4
+	}
+
+	// T4.M should appear as method of T5 only if AllMethods is set. 
+	func (*T5) M()
+
+	// 
+	type U1 struct {
+		*U1
+	}
+
+	// U1.M should appear as method of U1. 
+	func (*U1) M()
+
+	// 
+	type U2 struct {
+		*U3
+	}
+
+	// U2.M should appear as method of U2 and as method of U3 only if ...
+	func (*U2) M()
+
+	// U3.N should appear as method of U3 and as method of U2 only if ...
+	func (U2) N()
+
+	// 
+	type U3 struct {
+		*U2
+	}
+
+	// U2.M should appear as method of U2 and as method of U3 only if ...
+	func (U3) M()
+
+	// U3.N should appear as method of U3 and as method of U2 only if ...
+	func (*U3) N()
+
+	// 
+	type U4 struct {
+		// contains filtered or unexported fields
+	}
+
+	// U4.M should appear as method of U4. 
+	func (*U4) M()
+
+	// 
+	type V1 struct {
+		*V2
+		*V5
+	}
+
+	// V6.M should appear as method of V1 and V5 if AllMethods is set. 
+	func (V1) M()
+
+	// 
+	type V2 struct {
+		*V3
+	}
+
+	// V4.M should appear as method of V2 and V3 if AllMethods is set. 
+	func (V2) M()
+
+	// 
+	type V3 struct {
+		*V4
+	}
+
+	// V4.M should appear as method of V2 and V3 if AllMethods is set. 
+	func (V3) M()
+
+	// 
+	type V4 struct {
+		*V5
+	}
+
+	// V4.M should appear as method of V2 and V3 if AllMethods is set. 
+	func (*V4) M()
+
+	// 
+	type V5 struct {
+		*V6
+	}
+
+	// V6.M should appear as method of V1 and V5 if AllMethods is set. 
+	func (V5) M()
+
+	// 
+	type V6 struct{}
+
+	// V6.M should appear as method of V1 and V5 if AllMethods is set. 
+	func (*V6) M()
+
diff --git a/src/go/doc/testdata/e.go b/src/go/doc/testdata/e.go
new file mode 100644
index 000000000..ec432e3e5
--- /dev/null
+++ b/src/go/doc/testdata/e.go
@@ -0,0 +1,147 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The package e is a go/doc test for embedded methods.
+package e
+
+// ----------------------------------------------------------------------------
+// Conflicting methods M must not show up.
+
+type t1 struct{}
+
+// t1.M should not appear as method in a Tx type.
+func (t1) M() {}
+
+type t2 struct{}
+
+// t2.M should not appear as method in a Tx type.
+func (t2) M() {}
+
+// T1 has no embedded (level 1) M method due to conflict.
+type T1 struct {
+	t1
+	t2
+}
+
+// ----------------------------------------------------------------------------
+// Higher-level method M wins over lower-level method M.
+
+// T2 has only M as top-level method.
+type T2 struct {
+	t1
+}
+
+// T2.M should appear as method of T2.
+func (T2) M() {}
+
+// ----------------------------------------------------------------------------
+// Higher-level method M wins over lower-level conflicting methods M.
+
+type t1e struct {
+	t1
+}
+
+type t2e struct {
+	t2
+}
+
+// T3 has only M as top-level method.
+type T3 struct {
+	t1e
+	t2e
+}
+
+// T3.M should appear as method of T3.
+func (T3) M() {}
+
+// ----------------------------------------------------------------------------
+// Don't show conflicting methods M embedded via an exported and non-exported
+// type.
+
+// T1 has no embedded (level 1) M method due to conflict.
+type T4 struct {
+	t2
+	T2
+}
+
+// ----------------------------------------------------------------------------
+// Don't show embedded methods of exported anonymous fields unless AllMethods
+// is set.
+
+type T4 struct{}
+
+// T4.M should appear as method of T5 only if AllMethods is set.
+func (*T4) M() {}
+
+type T5 struct {
+	T4
+}
+
+// ----------------------------------------------------------------------------
+// Recursive type declarations must not lead to endless recursion.
+
+type U1 struct {
+	*U1
+}
+
+// U1.M should appear as method of U1.
+func (*U1) M() {}
+
+type U2 struct {
+	*U3
+}
+
+// U2.M should appear as method of U2 and as method of U3 only if AllMethods is set.
+func (*U2) M() {}
+
+type U3 struct {
+	*U2
+}
+
+// U3.N should appear as method of U3 and as method of U2 only if AllMethods is set.
+func (*U3) N() {}
+
+type U4 struct {
+	*u5
+}
+
+// U4.M should appear as method of U4.
+func (*U4) M() {}
+
+type u5 struct {
+	*U4
+}
+
+// ----------------------------------------------------------------------------
+// A higher-level embedded type (and its methods) wins over the same type (and
+// its methods) embedded at a lower level.
+
+type V1 struct {
+	*V2
+	*V5
+}
+
+type V2 struct {
+	*V3
+}
+
+type V3 struct {
+	*V4
+}
+
+type V4 struct {
+	*V5
+}
+
+type V5 struct {
+	*V6
+}
+
+type V6 struct{}
+
+// V4.M should appear as method of V2 and V3 if AllMethods is set.
+func (*V4) M() {}
+
+// V6.M should appear as method of V1 and V5 if AllMethods is set.
+func (*V6) M() {}
diff --git a/src/go/doc/testdata/error1.0.golden b/src/go/doc/testdata/error1.0.golden
new file mode 100644
index 000000000..6c6fe5d49
--- /dev/null
+++ b/src/go/doc/testdata/error1.0.golden
@@ -0,0 +1,30 @@
+// 
+PACKAGE error1
+
+IMPORTPATH
+	testdata/error1
+
+FILENAMES
+	testdata/error1.go
+
+TYPES
+	// 
+	type I0 interface {
+		// When embedded, the predeclared error interface
+		// must remain visible in interface types.
+		error
+	}
+
+	// 
+	type S0 struct {
+		// contains filtered or unexported fields
+	}
+
+	// 
+	type T0 struct {
+		ExportedField interface {
+			// error should be visible
+			error
+		}
+	}
+
diff --git a/src/go/doc/testdata/error1.1.golden b/src/go/doc/testdata/error1.1.golden
new file mode 100644
index 000000000..a8dc2e71d
--- /dev/null
+++ b/src/go/doc/testdata/error1.1.golden
@@ -0,0 +1,32 @@
+// 
+PACKAGE error1
+
+IMPORTPATH
+	testdata/error1
+
+FILENAMES
+	testdata/error1.go
+
+TYPES
+	// 
+	type I0 interface {
+		// When embedded, the predeclared error interface
+		// must remain visible in interface types.
+		error
+	}
+
+	// 
+	type S0 struct {
+		// In struct types, an embedded error must only be visible
+		// if AllDecls is set.
+		error
+	}
+
+	// 
+	type T0 struct {
+		ExportedField interface {
+			// error should be visible
+			error
+		}
+	}
+
diff --git a/src/go/doc/testdata/error1.2.golden b/src/go/doc/testdata/error1.2.golden
new file mode 100644
index 000000000..6c6fe5d49
--- /dev/null
+++ b/src/go/doc/testdata/error1.2.golden
@@ -0,0 +1,30 @@
+// 
+PACKAGE error1
+
+IMPORTPATH
+	testdata/error1
+
+FILENAMES
+	testdata/error1.go
+
+TYPES
+	// 
+	type I0 interface {
+		// When embedded, the predeclared error interface
+		// must remain visible in interface types.
+		error
+	}
+
+	// 
+	type S0 struct {
+		// contains filtered or unexported fields
+	}
+
+	// 
+	type T0 struct {
+		ExportedField interface {
+			// error should be visible
+			error
+		}
+	}
+
diff --git a/src/go/doc/testdata/error1.go b/src/go/doc/testdata/error1.go
new file mode 100644
index 000000000..3c777a780
--- /dev/null
+++ b/src/go/doc/testdata/error1.go
@@ -0,0 +1,24 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package error1
+
+type I0 interface {
+	// When embedded, the predeclared error interface
+	// must remain visible in interface types.
+	error
+}
+
+type T0 struct {
+	ExportedField interface {
+		// error should be visible
+		error
+	}
+}
+
+type S0 struct {
+	// In struct types, an embedded error must only be visible
+	// if AllDecls is set.
+	error
+}
diff --git a/src/go/doc/testdata/error2.0.golden b/src/go/doc/testdata/error2.0.golden
new file mode 100644
index 000000000..dedfe412a
--- /dev/null
+++ b/src/go/doc/testdata/error2.0.golden
@@ -0,0 +1,27 @@
+// 
+PACKAGE error2
+
+IMPORTPATH
+	testdata/error2
+
+FILENAMES
+	testdata/error2.go
+
+TYPES
+	// 
+	type I0 interface {
+		// contains filtered or unexported methods
+	}
+
+	// 
+	type S0 struct {
+		// contains filtered or unexported fields
+	}
+
+	// 
+	type T0 struct {
+		ExportedField interface {
+			// contains filtered or unexported methods
+		}
+	}
+
diff --git a/src/go/doc/testdata/error2.1.golden b/src/go/doc/testdata/error2.1.golden
new file mode 100644
index 000000000..dbcc1b03e
--- /dev/null
+++ b/src/go/doc/testdata/error2.1.golden
@@ -0,0 +1,37 @@
+// 
+PACKAGE error2
+
+IMPORTPATH
+	testdata/error2
+
+FILENAMES
+	testdata/error2.go
+
+TYPES
+	// 
+	type I0 interface {
+		// When embedded, the locally-declared error interface
+		// is only visible if all declarations are shown.
+		error
+	}
+
+	// 
+	type S0 struct {
+		// In struct types, an embedded error must only be visible
+		// if AllDecls is set.
+		error
+	}
+
+	// 
+	type T0 struct {
+		ExportedField interface {
+			// error should not be visible
+			error
+		}
+	}
+
+	// This error declaration shadows the predeclared error type. 
+	type error interface {
+		Error() string
+	}
+
diff --git a/src/go/doc/testdata/error2.2.golden b/src/go/doc/testdata/error2.2.golden
new file mode 100644
index 000000000..dedfe412a
--- /dev/null
+++ b/src/go/doc/testdata/error2.2.golden
@@ -0,0 +1,27 @@
+// 
+PACKAGE error2
+
+IMPORTPATH
+	testdata/error2
+
+FILENAMES
+	testdata/error2.go
+
+TYPES
+	// 
+	type I0 interface {
+		// contains filtered or unexported methods
+	}
+
+	// 
+	type S0 struct {
+		// contains filtered or unexported fields
+	}
+
+	// 
+	type T0 struct {
+		ExportedField interface {
+			// contains filtered or unexported methods
+		}
+	}
+
diff --git a/src/go/doc/testdata/error2.go b/src/go/doc/testdata/error2.go
new file mode 100644
index 000000000..6ee96c245
--- /dev/null
+++ b/src/go/doc/testdata/error2.go
@@ -0,0 +1,29 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package error2
+
+type I0 interface {
+	// When embedded, the locally-declared error interface
+	// is only visible if all declarations are shown.
+	error
+}
+
+type T0 struct {
+	ExportedField interface {
+		// error should not be visible
+		error
+	}
+}
+
+type S0 struct {
+	// In struct types, an embedded error must only be visible
+	// if AllDecls is set.
+	error
+}
+
+// This error declaration shadows the predeclared error type.
+type error interface {
+	Error() string
+}
diff --git a/src/go/doc/testdata/example.go b/src/go/doc/testdata/example.go
new file mode 100644
index 000000000..fdeda137e
--- /dev/null
+++ b/src/go/doc/testdata/example.go
@@ -0,0 +1,81 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+	"time"
+)
+
+type InternalExample struct {
+	Name   string
+	F      func()
+	Output string
+}
+
+func RunExamples(examples []InternalExample) (ok bool) {
+	ok = true
+
+	var eg InternalExample
+
+	stdout, stderr := os.Stdout, os.Stderr
+	defer func() {
+		os.Stdout, os.Stderr = stdout, stderr
+		if e := recover(); e != nil {
+			fmt.Printf("--- FAIL: %s\npanic: %v\n", eg.Name, e)
+			os.Exit(1)
+		}
+	}()
+
+	for _, eg = range examples {
+		if *chatty {
+			fmt.Printf("=== RUN: %s\n", eg.Name)
+		}
+
+		// capture stdout and stderr
+		r, w, err := os.Pipe()
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(1)
+		}
+		os.Stdout, os.Stderr = w, w
+		outC := make(chan string)
+		go func() {
+			buf := new(bytes.Buffer)
+			_, err := io.Copy(buf, r)
+			if err != nil {
+				fmt.Fprintf(stderr, "testing: copying pipe: %v\n", err)
+				os.Exit(1)
+			}
+			outC <- buf.String()
+		}()
+
+		// run example
+		t0 := time.Now()
+		eg.F()
+		dt := time.Now().Sub(t0)
+
+		// close pipe, restore stdout/stderr, get output
+		w.Close()
+		os.Stdout, os.Stderr = stdout, stderr
+		out := <-outC
+
+		// report any errors
+		tstr := fmt.Sprintf("(%.2f seconds)", dt.Seconds())
+		if g, e := strings.TrimSpace(out), strings.TrimSpace(eg.Output); g != e {
+			fmt.Printf("--- FAIL: %s %s\ngot:\n%s\nwant:\n%s\n",
+				eg.Name, tstr, g, e)
+			ok = false
+		} else if *chatty {
+			fmt.Printf("--- PASS: %s %s\n", eg.Name, tstr)
+		}
+	}
+
+	return
+}
diff --git a/src/go/doc/testdata/f.0.golden b/src/go/doc/testdata/f.0.golden
new file mode 100644
index 000000000..817590186
--- /dev/null
+++ b/src/go/doc/testdata/f.0.golden
@@ -0,0 +1,13 @@
+// The package f is a go/doc test for functions and factory ...
+PACKAGE f
+
+IMPORTPATH
+	testdata/f
+
+FILENAMES
+	testdata/f.go
+
+FUNCTIONS
+	// Exported must always be visible. Was issue 2824. 
+	func Exported() private
+
diff --git a/src/go/doc/testdata/f.1.golden b/src/go/doc/testdata/f.1.golden
new file mode 100644
index 000000000..ba68e884c
--- /dev/null
+++ b/src/go/doc/testdata/f.1.golden
@@ -0,0 +1,16 @@
+// The package f is a go/doc test for functions and factory ...
+PACKAGE f
+
+IMPORTPATH
+	testdata/f
+
+FILENAMES
+	testdata/f.go
+
+TYPES
+	// 
+	type private struct{}
+
+	// Exported must always be visible. Was issue 2824. 
+	func Exported() private
+
diff --git a/src/go/doc/testdata/f.2.golden b/src/go/doc/testdata/f.2.golden
new file mode 100644
index 000000000..817590186
--- /dev/null
+++ b/src/go/doc/testdata/f.2.golden
@@ -0,0 +1,13 @@
+// The package f is a go/doc test for functions and factory ...
+PACKAGE f
+
+IMPORTPATH
+	testdata/f
+
+FILENAMES
+	testdata/f.go
+
+FUNCTIONS
+	// Exported must always be visible. Was issue 2824. 
+	func Exported() private
+
diff --git a/src/go/doc/testdata/f.go b/src/go/doc/testdata/f.go
new file mode 100644
index 000000000..7e9add907
--- /dev/null
+++ b/src/go/doc/testdata/f.go
@@ -0,0 +1,14 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The package f is a go/doc test for functions and factory methods.
+package f
+
+// ----------------------------------------------------------------------------
+// Factory functions for non-exported types must not get lost.
+
+type private struct{}
+
+// Exported must always be visible. Was issue 2824.
+func Exported() private {}
diff --git a/src/go/doc/testdata/template.txt b/src/go/doc/testdata/template.txt
new file mode 100644
index 000000000..1b0738261
--- /dev/null
+++ b/src/go/doc/testdata/template.txt
@@ -0,0 +1,68 @@
+{{synopsis .Doc}}
+PACKAGE {{.Name}}
+
+IMPORTPATH
+	{{.ImportPath}}
+
+{{with .Imports}}IMPORTS
+{{range .}}	{{.}}
+{{end}}
+{{end}}{{/*
+
+*/}}FILENAMES
+{{range .Filenames}}	{{.}}
+{{end}}{{/*
+
+*/}}{{with .Consts}}
+CONSTANTS
+{{range .}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{end}}{{/*
+
+*/}}{{with .Vars}}
+VARIABLES
+{{range .}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{end}}{{/*
+
+*/}}{{with .Funcs}}
+FUNCTIONS
+{{range .}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{end}}{{/*
+
+*/}}{{with .Types}}
+TYPES
+{{range .}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{range .Consts}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{/*
+
+*/}}{{range .Vars}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{/*
+
+*/}}{{range .Funcs}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{/*
+
+*/}}{{range .Methods}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{end}}{{end}}{{/*
+
+*/}}{{with .Bugs}}
+BUGS .Bugs is now deprecated, please use .Notes instead
+{{range .}}{{indent "\t" .}}
+{{end}}{{end}}{{with .Notes}}{{range $marker, $content := .}}
+{{$marker}}S
+{{range $content}}{{$marker}}({{.UID}}){{indent "\t" .Body}}
+{{end}}{{end}}{{end}}
\ No newline at end of file
diff --git a/src/go/doc/testdata/testing.0.golden b/src/go/doc/testdata/testing.0.golden
new file mode 100644
index 000000000..f8348f1ac
--- /dev/null
+++ b/src/go/doc/testdata/testing.0.golden
@@ -0,0 +1,156 @@
+// Package testing provides support for automated testing of Go ...
+PACKAGE testing
+
+IMPORTPATH
+	testdata/testing
+
+IMPORTS
+	bytes
+	flag
+	fmt
+	io
+	os
+	runtime
+	runtime/pprof
+	strconv
+	strings
+	time
+
+FILENAMES
+	testdata/benchmark.go
+	testdata/example.go
+	testdata/testing.go
+
+FUNCTIONS
+	// An internal function but exported because it is cross-package; ...
+	func Main(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample)
+
+	// An internal function but exported because it is cross-package; ...
+	func RunBenchmarks(matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark)
+
+	// 
+	func RunExamples(examples []InternalExample) (ok bool)
+
+	// 
+	func RunTests(matchString func(pat, str string) (bool, error), tests []InternalTest) (ok bool)
+
+	// Short reports whether the -test.short flag is set. 
+	func Short() bool
+
+
+TYPES
+	// B is a type passed to Benchmark functions to manage benchmark ...
+	type B struct {
+		N int
+		// contains filtered or unexported fields
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *B) Error(args ...interface{})
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *B) Errorf(format string, args ...interface{})
+
+	// Fail marks the function as having failed but continues ...
+	func (c *B) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *B) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *B) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *B) Fatal(args ...interface{})
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *B) Fatalf(format string, args ...interface{})
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *B) Log(args ...interface{})
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *B) Logf(format string, args ...interface{})
+
+	// ResetTimer sets the elapsed benchmark time to zero. It does not ...
+	func (b *B) ResetTimer()
+
+	// SetBytes records the number of bytes processed in a single ...
+	func (b *B) SetBytes(n int64)
+
+	// StartTimer starts timing a test.  This function is called ...
+	func (b *B) StartTimer()
+
+	// StopTimer stops timing a test.  This can be used to pause the ...
+	func (b *B) StopTimer()
+
+	// The results of a benchmark run. 
+	type BenchmarkResult struct {
+		N	int		// The number of iterations.
+		T	time.Duration	// The total time taken.
+		Bytes	int64		// Bytes processed in one iteration.
+	}
+
+	// Benchmark benchmarks a single function. Useful for creating ...
+	func Benchmark(f func(b *B)) BenchmarkResult
+
+	// 
+	func (r BenchmarkResult) NsPerOp() int64
+
+	// 
+	func (r BenchmarkResult) String() string
+
+	// An internal type but exported because it is cross-package; part ...
+	type InternalBenchmark struct {
+		Name	string
+		F	func(b *B)
+	}
+
+	// 
+	type InternalExample struct {
+		Name	string
+		F	func()
+		Output	string
+	}
+
+	// An internal type but exported because it is cross-package; part ...
+	type InternalTest struct {
+		Name	string
+		F	func(*T)
+	}
+
+	// T is a type passed to Test functions to manage test state and ...
+	type T struct {
+		// contains filtered or unexported fields
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *T) Error(args ...interface{})
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *T) Errorf(format string, args ...interface{})
+
+	// Fail marks the function as having failed but continues ...
+	func (c *T) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *T) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *T) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *T) Fatal(args ...interface{})
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *T) Fatalf(format string, args ...interface{})
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *T) Log(args ...interface{})
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *T) Logf(format string, args ...interface{})
+
+	// Parallel signals that this test is to be run in parallel with ...
+	func (t *T) Parallel()
+
diff --git a/src/go/doc/testdata/testing.1.golden b/src/go/doc/testdata/testing.1.golden
new file mode 100644
index 000000000..282bb1015
--- /dev/null
+++ b/src/go/doc/testdata/testing.1.golden
@@ -0,0 +1,298 @@
+// Package testing provides support for automated testing of Go ...
+PACKAGE testing
+
+IMPORTPATH
+	testdata/testing
+
+IMPORTS
+	bytes
+	flag
+	fmt
+	io
+	os
+	runtime
+	runtime/pprof
+	strconv
+	strings
+	time
+
+FILENAMES
+	testdata/benchmark.go
+	testdata/example.go
+	testdata/testing.go
+
+VARIABLES
+	// 
+	var (
+		// The short flag requests that tests run more quickly, but its functionality
+		// is provided by test writers themselves.  The testing package is just its
+		// home.  The all.bash installation script sets it to make installation more
+		// efficient, but by default the flag is off so a plain "go test" will do a
+		// full test of the package.
+		short	= flag.Bool("test.short", false, "run smaller test suite to save time")
+	
+		// Report as tests are run; default is silent for success.
+		chatty		= flag.Bool("test.v", false, "verbose: print additional output")
+		match		= flag.String("test.run", "", "regular expression to select tests to run")
+		memProfile	= flag.String("test.memprofile", "", "write a memory profile to the named file after execution")
+		memProfileRate	= flag.Int("test.memprofilerate", 0, "if >=0, sets runtime.MemProfileRate")
+		cpuProfile	= flag.String("test.cpuprofile", "", "write a cpu profile to the named file during execution")
+		timeout		= flag.Duration("test.timeout", 0, "if positive, sets an aggregate time limit for all tests")
+		cpuListStr	= flag.String("test.cpu", "", "comma-separated list of number of CPUs to use for each test")
+		parallel	= flag.Int("test.parallel", runtime.GOMAXPROCS(0), "maximum test parallelism")
+	
+		cpuList	[]int
+	)
+
+	// 
+	var benchTime = flag.Duration("test.benchtime", 1*time.Second, "approximate run time for each benchmark")
+
+	// 
+	var matchBenchmarks = flag.String("test.bench", "", "regular expression to select benchmarks to run")
+
+	// 
+	var timer *time.Timer
+
+
+FUNCTIONS
+	// An internal function but exported because it is cross-package; ...
+	func Main(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample)
+
+	// An internal function but exported because it is cross-package; ...
+	func RunBenchmarks(matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark)
+
+	// 
+	func RunExamples(examples []InternalExample) (ok bool)
+
+	// 
+	func RunTests(matchString func(pat, str string) (bool, error), tests []InternalTest) (ok bool)
+
+	// Short reports whether the -test.short flag is set. 
+	func Short() bool
+
+	// after runs after all testing. 
+	func after()
+
+	// alarm is called if the timeout expires. 
+	func alarm()
+
+	// before runs before all testing. 
+	func before()
+
+	// decorate inserts the final newline if needed and indentation ...
+	func decorate(s string, addFileLine bool) string
+
+	// 
+	func max(x, y int) int
+
+	// 
+	func min(x, y int) int
+
+	// 
+	func parseCpuList()
+
+	// roundDown10 rounds a number down to the nearest power of 10. 
+	func roundDown10(n int) int
+
+	// roundUp rounds x up to a number of the form [1eX, 2eX, 5eX]. 
+	func roundUp(n int) int
+
+	// startAlarm starts an alarm if requested. 
+	func startAlarm()
+
+	// stopAlarm turns off the alarm. 
+	func stopAlarm()
+
+	// 
+	func tRunner(t *T, test *InternalTest)
+
+
+TYPES
+	// B is a type passed to Benchmark functions to manage benchmark ...
+	type B struct {
+		common
+		N		int
+		benchmark	InternalBenchmark
+		bytes		int64
+		timerOn		bool
+		result		BenchmarkResult
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *B) Error(args ...interface{})
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *B) Errorf(format string, args ...interface{})
+
+	// Fail marks the function as having failed but continues ...
+	func (c *B) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *B) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *B) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *B) Fatal(args ...interface{})
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *B) Fatalf(format string, args ...interface{})
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *B) Log(args ...interface{})
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *B) Logf(format string, args ...interface{})
+
+	// ResetTimer sets the elapsed benchmark time to zero. It does not ...
+	func (b *B) ResetTimer()
+
+	// SetBytes records the number of bytes processed in a single ...
+	func (b *B) SetBytes(n int64)
+
+	// StartTimer starts timing a test.  This function is called ...
+	func (b *B) StartTimer()
+
+	// StopTimer stops timing a test.  This can be used to pause the ...
+	func (b *B) StopTimer()
+
+	// launch launches the benchmark function.  It gradually increases ...
+	func (b *B) launch()
+
+	// log generates the output. It's always at the same stack depth. 
+	func (c *B) log(s string)
+
+	// 
+	func (b *B) nsPerOp() int64
+
+	// run times the benchmark function in a separate goroutine. 
+	func (b *B) run() BenchmarkResult
+
+	// runN runs a single benchmark for the specified number of ...
+	func (b *B) runN(n int)
+
+	// trimOutput shortens the output from a benchmark, which can be ...
+	func (b *B) trimOutput()
+
+	// The results of a benchmark run. 
+	type BenchmarkResult struct {
+		N	int		// The number of iterations.
+		T	time.Duration	// The total time taken.
+		Bytes	int64		// Bytes processed in one iteration.
+	}
+
+	// Benchmark benchmarks a single function. Useful for creating ...
+	func Benchmark(f func(b *B)) BenchmarkResult
+
+	// 
+	func (r BenchmarkResult) NsPerOp() int64
+
+	// 
+	func (r BenchmarkResult) String() string
+
+	// 
+	func (r BenchmarkResult) mbPerSec() float64
+
+	// An internal type but exported because it is cross-package; part ...
+	type InternalBenchmark struct {
+		Name	string
+		F	func(b *B)
+	}
+
+	// 
+	type InternalExample struct {
+		Name	string
+		F	func()
+		Output	string
+	}
+
+	// An internal type but exported because it is cross-package; part ...
+	type InternalTest struct {
+		Name	string
+		F	func(*T)
+	}
+
+	// T is a type passed to Test functions to manage test state and ...
+	type T struct {
+		common
+		name		string		// Name of test.
+		startParallel	chan bool	// Parallel tests will wait on this.
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *T) Error(args ...interface{})
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *T) Errorf(format string, args ...interface{})
+
+	// Fail marks the function as having failed but continues ...
+	func (c *T) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *T) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *T) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *T) Fatal(args ...interface{})
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *T) Fatalf(format string, args ...interface{})
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *T) Log(args ...interface{})
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *T) Logf(format string, args ...interface{})
+
+	// Parallel signals that this test is to be run in parallel with ...
+	func (t *T) Parallel()
+
+	// log generates the output. It's always at the same stack depth. 
+	func (c *T) log(s string)
+
+	// 
+	func (t *T) report()
+
+	// common holds the elements common between T and B and captures ...
+	type common struct {
+		output		[]byte		// Output generated by test or benchmark.
+		failed		bool		// Test or benchmark has failed.
+		start		time.Time	// Time test or benchmark started
+		duration	time.Duration
+		self		interface{}		// To be sent on signal channel when done.
+		signal		chan interface{}	// Output for serial tests.
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *common) Error(args ...interface{})
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *common) Errorf(format string, args ...interface{})
+
+	// Fail marks the function as having failed but continues ...
+	func (c *common) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *common) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *common) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *common) Fatal(args ...interface{})
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *common) Fatalf(format string, args ...interface{})
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *common) Log(args ...interface{})
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *common) Logf(format string, args ...interface{})
+
+	// log generates the output. It's always at the same stack depth. 
+	func (c *common) log(s string)
+
diff --git a/src/go/doc/testdata/testing.2.golden b/src/go/doc/testdata/testing.2.golden
new file mode 100644
index 000000000..f8348f1ac
--- /dev/null
+++ b/src/go/doc/testdata/testing.2.golden
@@ -0,0 +1,156 @@
+// Package testing provides support for automated testing of Go ...
+PACKAGE testing
+
+IMPORTPATH
+	testdata/testing
+
+IMPORTS
+	bytes
+	flag
+	fmt
+	io
+	os
+	runtime
+	runtime/pprof
+	strconv
+	strings
+	time
+
+FILENAMES
+	testdata/benchmark.go
+	testdata/example.go
+	testdata/testing.go
+
+FUNCTIONS
+	// An internal function but exported because it is cross-package; ...
+	func Main(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample)
+
+	// An internal function but exported because it is cross-package; ...
+	func RunBenchmarks(matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark)
+
+	// 
+	func RunExamples(examples []InternalExample) (ok bool)
+
+	// 
+	func RunTests(matchString func(pat, str string) (bool, error), tests []InternalTest) (ok bool)
+
+	// Short reports whether the -test.short flag is set. 
+	func Short() bool
+
+
+TYPES
+	// B is a type passed to Benchmark functions to manage benchmark ...
+	type B struct {
+		N int
+		// contains filtered or unexported fields
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *B) Error(args ...interface{})
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *B) Errorf(format string, args ...interface{})
+
+	// Fail marks the function as having failed but continues ...
+	func (c *B) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *B) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *B) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *B) Fatal(args ...interface{})
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *B) Fatalf(format string, args ...interface{})
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *B) Log(args ...interface{})
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *B) Logf(format string, args ...interface{})
+
+	// ResetTimer sets the elapsed benchmark time to zero. It does not ...
+	func (b *B) ResetTimer()
+
+	// SetBytes records the number of bytes processed in a single ...
+	func (b *B) SetBytes(n int64)
+
+	// StartTimer starts timing a test.  This function is called ...
+	func (b *B) StartTimer()
+
+	// StopTimer stops timing a test.  This can be used to pause the ...
+	func (b *B) StopTimer()
+
+	// The results of a benchmark run. 
+	type BenchmarkResult struct {
+		N	int		// The number of iterations.
+		T	time.Duration	// The total time taken.
+		Bytes	int64		// Bytes processed in one iteration.
+	}
+
+	// Benchmark benchmarks a single function. Useful for creating ...
+	func Benchmark(f func(b *B)) BenchmarkResult
+
+	// 
+	func (r BenchmarkResult) NsPerOp() int64
+
+	// 
+	func (r BenchmarkResult) String() string
+
+	// An internal type but exported because it is cross-package; part ...
+	type InternalBenchmark struct {
+		Name	string
+		F	func(b *B)
+	}
+
+	// 
+	type InternalExample struct {
+		Name	string
+		F	func()
+		Output	string
+	}
+
+	// An internal type but exported because it is cross-package; part ...
+	type InternalTest struct {
+		Name	string
+		F	func(*T)
+	}
+
+	// T is a type passed to Test functions to manage test state and ...
+	type T struct {
+		// contains filtered or unexported fields
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *T) Error(args ...interface{})
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *T) Errorf(format string, args ...interface{})
+
+	// Fail marks the function as having failed but continues ...
+	func (c *T) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *T) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *T) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *T) Fatal(args ...interface{})
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *T) Fatalf(format string, args ...interface{})
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *T) Log(args ...interface{})
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *T) Logf(format string, args ...interface{})
+
+	// Parallel signals that this test is to be run in parallel with ...
+	func (t *T) Parallel()
+
diff --git a/src/go/doc/testdata/testing.go b/src/go/doc/testdata/testing.go
new file mode 100644
index 000000000..93ed494c3
--- /dev/null
+++ b/src/go/doc/testdata/testing.go
@@ -0,0 +1,404 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package testing provides support for automated testing of Go packages.
+// It is intended to be used in concert with the ``go test'' utility, which automates
+// execution of any function of the form
+//     func TestXxx(*testing.T)
+// where Xxx can be any alphanumeric string (but the first letter must not be in
+// [a-z]) and serves to identify the test routine.
+// These TestXxx routines should be declared within the package they are testing.
+//
+// Functions of the form
+//     func BenchmarkXxx(*testing.B)
+// are considered benchmarks, and are executed by go test when the -test.bench
+// flag is provided.
+//
+// A sample benchmark function looks like this:
+//     func BenchmarkHello(b *testing.B) {
+//         for i := 0; i < b.N; i++ {
+//             fmt.Sprintf("hello")
+//         }
+//     }
+// The benchmark package will vary b.N until the benchmark function lasts
+// long enough to be timed reliably.  The output
+//     testing.BenchmarkHello    10000000    282 ns/op
+// means that the loop ran 10000000 times at a speed of 282 ns per loop.
+//
+// If a benchmark needs some expensive setup before running, the timer
+// may be stopped:
+//     func BenchmarkBigLen(b *testing.B) {
+//         b.StopTimer()
+//         big := NewBig()
+//         b.StartTimer()
+//         for i := 0; i < b.N; i++ {
+//             big.Len()
+//         }
+//     }
+package testing
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"runtime"
+	"runtime/pprof"
+	"strconv"
+	"strings"
+	"time"
+)
+
+var (
+	// The short flag requests that tests run more quickly, but its functionality
+	// is provided by test writers themselves.  The testing package is just its
+	// home.  The all.bash installation script sets it to make installation more
+	// efficient, but by default the flag is off so a plain "go test" will do a
+	// full test of the package.
+	short = flag.Bool("test.short", false, "run smaller test suite to save time")
+
+	// Report as tests are run; default is silent for success.
+	chatty         = flag.Bool("test.v", false, "verbose: print additional output")
+	match          = flag.String("test.run", "", "regular expression to select tests to run")
+	memProfile     = flag.String("test.memprofile", "", "write a memory profile to the named file after execution")
+	memProfileRate = flag.Int("test.memprofilerate", 0, "if >=0, sets runtime.MemProfileRate")
+	cpuProfile     = flag.String("test.cpuprofile", "", "write a cpu profile to the named file during execution")
+	timeout        = flag.Duration("test.timeout", 0, "if positive, sets an aggregate time limit for all tests")
+	cpuListStr     = flag.String("test.cpu", "", "comma-separated list of number of CPUs to use for each test")
+	parallel       = flag.Int("test.parallel", runtime.GOMAXPROCS(0), "maximum test parallelism")
+
+	cpuList []int
+)
+
+// common holds the elements common between T and B and
+// captures common methods such as Errorf.
+type common struct {
+	output   []byte    // Output generated by test or benchmark.
+	failed   bool      // Test or benchmark has failed.
+	start    time.Time // Time test or benchmark started
+	duration time.Duration
+	self     interface{}      // To be sent on signal channel when done.
+	signal   chan interface{} // Output for serial tests.
+}
+
+// Short reports whether the -test.short flag is set.
+func Short() bool {
+	return *short
+}
+
+// decorate inserts the final newline if needed and indentation tabs for formatting.
+// If addFileLine is true, it also prefixes the string with the file and line of the call site.
+func decorate(s string, addFileLine bool) string {
+	if addFileLine {
+		_, file, line, ok := runtime.Caller(3) // decorate + log + public function.
+		if ok {
+			// Truncate file name at last file name separator.
+			if index := strings.LastIndex(file, "/"); index >= 0 {
+				file = file[index+1:]
+			} else if index = strings.LastIndex(file, "\\"); index >= 0 {
+				file = file[index+1:]
+			}
+		} else {
+			file = "???"
+			line = 1
+		}
+		s = fmt.Sprintf("%s:%d: %s", file, line, s)
+	}
+	s = "\t" + s // Every line is indented at least one tab.
+	n := len(s)
+	if n > 0 && s[n-1] != '\n' {
+		s += "\n"
+		n++
+	}
+	for i := 0; i < n-1; i++ { // -1 to avoid final newline
+		if s[i] == '\n' {
+			// Second and subsequent lines are indented an extra tab.
+			return s[0:i+1] + "\t" + decorate(s[i+1:n], false)
+		}
+	}
+	return s
+}
+
+// T is a type passed to Test functions to manage test state and support formatted test logs.
+// Logs are accumulated during execution and dumped to standard error when done.
+type T struct {
+	common
+	name          string    // Name of test.
+	startParallel chan bool // Parallel tests will wait on this.
+}
+
+// Fail marks the function as having failed but continues execution.
+func (c *common) Fail() { c.failed = true }
+
+// Failed reports whether the function has failed.
+func (c *common) Failed() bool { return c.failed }
+
+// FailNow marks the function as having failed and stops its execution.
+// Execution will continue at the next Test.
+func (c *common) FailNow() {
+	c.Fail()
+
+	// Calling runtime.Goexit will exit the goroutine, which
+	// will run the deferred functions in this goroutine,
+	// which will eventually run the deferred lines in tRunner,
+	// which will signal to the test loop that this test is done.
+	//
+	// A previous version of this code said:
+	//
+	//	c.duration = ...
+	//	c.signal <- c.self
+	//	runtime.Goexit()
+	//
+	// This previous version duplicated code (those lines are in
+	// tRunner no matter what), but worse the goroutine teardown
+	// implicit in runtime.Goexit was not guaranteed to complete
+	// before the test exited.  If a test deferred an important cleanup
+	// function (like removing temporary files), there was no guarantee
+	// it would run on a test failure.  Because we send on c.signal during
+	// a top-of-stack deferred function now, we know that the send
+	// only happens after any other stacked defers have completed.
+	runtime.Goexit()
+}
+
+// log generates the output. It's always at the same stack depth.
+func (c *common) log(s string) {
+	c.output = append(c.output, decorate(s, true)...)
+}
+
+// Log formats its arguments using default formatting, analogous to Println(),
+// and records the text in the error log.
+func (c *common) Log(args ...interface{}) { c.log(fmt.Sprintln(args...)) }
+
+// Logf formats its arguments according to the format, analogous to Printf(),
+// and records the text in the error log.
+func (c *common) Logf(format string, args ...interface{}) { c.log(fmt.Sprintf(format, args...)) }
+
+// Error is equivalent to Log() followed by Fail().
+func (c *common) Error(args ...interface{}) {
+	c.log(fmt.Sprintln(args...))
+	c.Fail()
+}
+
+// Errorf is equivalent to Logf() followed by Fail().
+func (c *common) Errorf(format string, args ...interface{}) {
+	c.log(fmt.Sprintf(format, args...))
+	c.Fail()
+}
+
+// Fatal is equivalent to Log() followed by FailNow().
+func (c *common) Fatal(args ...interface{}) {
+	c.log(fmt.Sprintln(args...))
+	c.FailNow()
+}
+
+// Fatalf is equivalent to Logf() followed by FailNow().
+func (c *common) Fatalf(format string, args ...interface{}) {
+	c.log(fmt.Sprintf(format, args...))
+	c.FailNow()
+}
+
+// Parallel signals that this test is to be run in parallel with (and only with)
+// other parallel tests in this CPU group.
+func (t *T) Parallel() {
+	t.signal <- (*T)(nil) // Release main testing loop
+	<-t.startParallel     // Wait for serial tests to finish
+}
+
+// An internal type but exported because it is cross-package; part of the implementation
+// of go test.
+type InternalTest struct {
+	Name string
+	F    func(*T)
+}
+
+func tRunner(t *T, test *InternalTest) {
+	t.start = time.Now()
+
+	// When this goroutine is done, either because test.F(t)
+	// returned normally or because a test failure triggered
+	// a call to runtime.Goexit, record the duration and send
+	// a signal saying that the test is done.
+	defer func() {
+		t.duration = time.Now().Sub(t.start)
+		t.signal <- t
+	}()
+
+	test.F(t)
+}
+
+// An internal function but exported because it is cross-package; part of the implementation
+// of go test.
+func Main(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample) {
+	flag.Parse()
+	parseCpuList()
+
+	before()
+	startAlarm()
+	testOk := RunTests(matchString, tests)
+	exampleOk := RunExamples(examples)
+	if !testOk || !exampleOk {
+		fmt.Println("FAIL")
+		os.Exit(1)
+	}
+	fmt.Println("PASS")
+	stopAlarm()
+	RunBenchmarks(matchString, benchmarks)
+	after()
+}
+
+func (t *T) report() {
+	tstr := fmt.Sprintf("(%.2f seconds)", t.duration.Seconds())
+	format := "--- %s: %s %s\n%s"
+	if t.failed {
+		fmt.Printf(format, "FAIL", t.name, tstr, t.output)
+	} else if *chatty {
+		fmt.Printf(format, "PASS", t.name, tstr, t.output)
+	}
+}
+
+func RunTests(matchString func(pat, str string) (bool, error), tests []InternalTest) (ok bool) {
+	ok = true
+	if len(tests) == 0 {
+		fmt.Fprintln(os.Stderr, "testing: warning: no tests to run")
+		return
+	}
+	for _, procs := range cpuList {
+		runtime.GOMAXPROCS(procs)
+		// We build a new channel tree for each run of the loop.
+		// collector merges in one channel all the upstream signals from parallel tests.
+		// If all tests pump to the same channel, a bug can occur where a test
+		// kicks off a goroutine that Fails, yet the test still delivers a completion signal,
+		// which skews the counting.
+		var collector = make(chan interface{})
+
+		numParallel := 0
+		startParallel := make(chan bool)
+
+		for i := 0; i < len(tests); i++ {
+			matched, err := matchString(*match, tests[i].Name)
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "testing: invalid regexp for -test.run: %s\n", err)
+				os.Exit(1)
+			}
+			if !matched {
+				continue
+			}
+			testName := tests[i].Name
+			if procs != 1 {
+				testName = fmt.Sprintf("%s-%d", tests[i].Name, procs)
+			}
+			t := &T{
+				common: common{
+					signal: make(chan interface{}),
+				},
+				name:          testName,
+				startParallel: startParallel,
+			}
+			t.self = t
+			if *chatty {
+				fmt.Printf("=== RUN %s\n", t.name)
+			}
+			go tRunner(t, &tests[i])
+			out := (<-t.signal).(*T)
+			if out == nil { // Parallel run.
+				go func() {
+					collector <- <-t.signal
+				}()
+				numParallel++
+				continue
+			}
+			t.report()
+			ok = ok && !out.failed
+		}
+
+		running := 0
+		for numParallel+running > 0 {
+			if running < *parallel && numParallel > 0 {
+				startParallel <- true
+				running++
+				numParallel--
+				continue
+			}
+			t := (<-collector).(*T)
+			t.report()
+			ok = ok && !t.failed
+			running--
+		}
+	}
+	return
+}
+
+// before runs before all testing.
+func before() {
+	if *memProfileRate > 0 {
+		runtime.MemProfileRate = *memProfileRate
+	}
+	if *cpuProfile != "" {
+		f, err := os.Create(*cpuProfile)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "testing: %s", err)
+			return
+		}
+		if err := pprof.StartCPUProfile(f); err != nil {
+			fmt.Fprintf(os.Stderr, "testing: can't start cpu profile: %s", err)
+			f.Close()
+			return
+		}
+		// Could save f so after can call f.Close; not worth the effort.
+	}
+
+}
+
+// after runs after all testing.
+func after() {
+	if *cpuProfile != "" {
+		pprof.StopCPUProfile() // flushes profile to disk
+	}
+	if *memProfile != "" {
+		f, err := os.Create(*memProfile)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "testing: %s", err)
+			return
+		}
+		if err = pprof.WriteHeapProfile(f); err != nil {
+			fmt.Fprintf(os.Stderr, "testing: can't write %s: %s", *memProfile, err)
+		}
+		f.Close()
+	}
+}
+
+var timer *time.Timer
+
+// startAlarm starts an alarm if requested.
+func startAlarm() {
+	if *timeout > 0 {
+		timer = time.AfterFunc(*timeout, alarm)
+	}
+}
+
+// stopAlarm turns off the alarm.
+func stopAlarm() {
+	if *timeout > 0 {
+		timer.Stop()
+	}
+}
+
+// alarm is called if the timeout expires.
+func alarm() {
+	panic("test timed out")
+}
+
+func parseCpuList() {
+	if len(*cpuListStr) == 0 {
+		cpuList = append(cpuList, runtime.GOMAXPROCS(-1))
+	} else {
+		for _, val := range strings.Split(*cpuListStr, ",") {
+			cpu, err := strconv.Atoi(val)
+			if err != nil || cpu <= 0 {
+				fmt.Fprintf(os.Stderr, "testing: invalid value %q for -test.cpu", val)
+				os.Exit(1)
+			}
+			cpuList = append(cpuList, cpu)
+		}
+	}
+}
diff --git a/src/go/format/format.go b/src/go/format/format.go
new file mode 100644
index 000000000..3d00a645d
--- /dev/null
+++ b/src/go/format/format.go
@@ -0,0 +1,199 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package format implements standard formatting of Go source.
+package format
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/printer"
+	"go/token"
+	"io"
+	"strings"
+)
+
+var config = printer.Config{Mode: printer.UseSpaces | printer.TabIndent, Tabwidth: 8}
+
+// Node formats node in canonical gofmt style and writes the result to dst.
+//
+// The node type must be *ast.File, *printer.CommentedNode, []ast.Decl,
+// []ast.Stmt, or assignment-compatible to ast.Expr, ast.Decl, ast.Spec,
+// or ast.Stmt. Node does not modify node. Imports are not sorted for
+// nodes representing partial source files (i.e., if the node is not an
+// *ast.File or a *printer.CommentedNode not wrapping an *ast.File).
+//
+// The function may return early (before the entire result is written)
+// and return a formatting error, for instance due to an incorrect AST.
+//
+func Node(dst io.Writer, fset *token.FileSet, node interface{}) error {
+	// Determine if we have a complete source file (file != nil).
+	var file *ast.File
+	var cnode *printer.CommentedNode
+	switch n := node.(type) {
+	case *ast.File:
+		file = n
+	case *printer.CommentedNode:
+		if f, ok := n.Node.(*ast.File); ok {
+			file = f
+			cnode = n
+		}
+	}
+
+	// Sort imports if necessary.
+	if file != nil && hasUnsortedImports(file) {
+		// Make a copy of the AST because ast.SortImports is destructive.
+		// TODO(gri) Do this more efficiently.
+		var buf bytes.Buffer
+		err := config.Fprint(&buf, fset, file)
+		if err != nil {
+			return err
+		}
+		file, err = parser.ParseFile(fset, "", buf.Bytes(), parser.ParseComments)
+		if err != nil {
+			// We should never get here. If we do, provide good diagnostic.
+			return fmt.Errorf("format.Node internal error (%s)", err)
+		}
+		ast.SortImports(fset, file)
+
+		// Use new file with sorted imports.
+		node = file
+		if cnode != nil {
+			node = &printer.CommentedNode{Node: file, Comments: cnode.Comments}
+		}
+	}
+
+	return config.Fprint(dst, fset, node)
+}
+
+// Source formats src in canonical gofmt style and returns the result
+// or an (I/O or syntax) error. src is expected to be a syntactically
+// correct Go source file, or a list of Go declarations or statements.
+//
+// If src is a partial source file, the leading and trailing space of src
+// is applied to the result (such that it has the same leading and trailing
+// space as src), and the result is indented by the same amount as the first
+// line of src containing code. Imports are not sorted for partial source files.
+//
+func Source(src []byte) ([]byte, error) {
+	fset := token.NewFileSet()
+	node, err := parse(fset, src)
+	if err != nil {
+		return nil, err
+	}
+
+	var buf bytes.Buffer
+	if file, ok := node.(*ast.File); ok {
+		// Complete source file.
+		ast.SortImports(fset, file)
+		err := config.Fprint(&buf, fset, file)
+		if err != nil {
+			return nil, err
+		}
+
+	} else {
+		// Partial source file.
+		// Determine and prepend leading space.
+		i, j := 0, 0
+		for j < len(src) && isSpace(src[j]) {
+			if src[j] == '\n' {
+				i = j + 1 // index of last line in leading space
+			}
+			j++
+		}
+		buf.Write(src[:i])
+
+		// Determine indentation of first code line.
+		// Spaces are ignored unless there are no tabs,
+		// in which case spaces count as one tab.
+		indent := 0
+		hasSpace := false
+		for _, b := range src[i:j] {
+			switch b {
+			case ' ':
+				hasSpace = true
+			case '\t':
+				indent++
+			}
+		}
+		if indent == 0 && hasSpace {
+			indent = 1
+		}
+
+		// Format the source.
+		cfg := config
+		cfg.Indent = indent
+		err := cfg.Fprint(&buf, fset, node)
+		if err != nil {
+			return nil, err
+		}
+
+		// Determine and append trailing space.
+		i = len(src)
+		for i > 0 && isSpace(src[i-1]) {
+			i--
+		}
+		buf.Write(src[i:])
+	}
+
+	return buf.Bytes(), nil
+}
+
+func hasUnsortedImports(file *ast.File) bool {
+	for _, d := range file.Decls {
+		d, ok := d.(*ast.GenDecl)
+		if !ok || d.Tok != token.IMPORT {
+			// Not an import declaration, so we're done.
+			// Imports are always first.
+			return false
+		}
+		if d.Lparen.IsValid() {
+			// For now assume all grouped imports are unsorted.
+			// TODO(gri) Should check if they are sorted already.
+			return true
+		}
+		// Ungrouped imports are sorted by default.
+	}
+	return false
+}
+
+func isSpace(b byte) bool {
+	return b == ' ' || b == '\t' || b == '\n' || b == '\r'
+}
+
+func parse(fset *token.FileSet, src []byte) (interface{}, error) {
+	// Try as a complete source file.
+	file, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err == nil {
+		return file, nil
+	}
+	// If the source is missing a package clause, try as a source fragment; otherwise fail.
+	if !strings.Contains(err.Error(), "expected 'package'") {
+		return nil, err
+	}
+
+	// Try as a declaration list by prepending a package clause in front of src.
+	// Use ';' not '\n' to keep line numbers intact.
+	psrc := append([]byte("package p;"), src...)
+	file, err = parser.ParseFile(fset, "", psrc, parser.ParseComments)
+	if err == nil {
+		return file.Decls, nil
+	}
+	// If the source is missing a declaration, try as a statement list; otherwise fail.
+	if !strings.Contains(err.Error(), "expected declaration") {
+		return nil, err
+	}
+
+	// Try as statement list by wrapping a function around src.
+	fsrc := append(append([]byte("package p; func _() {"), src...), '}')
+	file, err = parser.ParseFile(fset, "", fsrc, parser.ParseComments)
+	if err == nil {
+		return file.Decls[0].(*ast.FuncDecl).Body.List, nil
+	}
+
+	// Failed, and out of options.
+	return nil, err
+}
diff --git a/src/go/format/format_test.go b/src/go/format/format_test.go
new file mode 100644
index 000000000..93f099247
--- /dev/null
+++ b/src/go/format/format_test.go
@@ -0,0 +1,124 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package format
+
+import (
+	"bytes"
+	"go/parser"
+	"go/token"
+	"io/ioutil"
+	"strings"
+	"testing"
+)
+
+const testfile = "format_test.go"
+
+func diff(t *testing.T, dst, src []byte) {
+	line := 1
+	offs := 0 // line offset
+	for i := 0; i < len(dst) && i < len(src); i++ {
+		d := dst[i]
+		s := src[i]
+		if d != s {
+			t.Errorf("dst:%d: %s\n", line, dst[offs:i+1])
+			t.Errorf("src:%d: %s\n", line, src[offs:i+1])
+			return
+		}
+		if s == '\n' {
+			line++
+			offs = i + 1
+		}
+	}
+	if len(dst) != len(src) {
+		t.Errorf("len(dst) = %d, len(src) = %d\nsrc = %q", len(dst), len(src), src)
+	}
+}
+
+func TestNode(t *testing.T) {
+	src, err := ioutil.ReadFile(testfile)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, testfile, src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var buf bytes.Buffer
+
+	if err = Node(&buf, fset, file); err != nil {
+		t.Fatal("Node failed:", err)
+	}
+
+	diff(t, buf.Bytes(), src)
+}
+
+func TestSource(t *testing.T) {
+	src, err := ioutil.ReadFile(testfile)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	res, err := Source(src)
+	if err != nil {
+		t.Fatal("Source failed:", err)
+	}
+
+	diff(t, res, src)
+}
+
+// Test cases that are expected to fail are marked by the prefix "ERROR".
+var tests = []string{
+	// declaration lists
+	`import "go/format"`,
+	"var x int",
+	"var x int\n\ntype T struct{}",
+
+	// statement lists
+	"x := 0",
+	"f(a, b, c)\nvar x int = f(1, 2, 3)",
+
+	// indentation, leading and trailing space
+	"\tx := 0\n\tgo f()",
+	"\tx := 0\n\tgo f()\n\n\n",
+	"\n\t\t\n\n\tx := 0\n\tgo f()\n\n\n",
+	"\n\t\t\n\n\t\t\tx := 0\n\t\t\tgo f()\n\n\n",
+	"\n\t\t\n\n\t\t\tx := 0\n\t\t\tconst s = `\nfoo\n`\n\n\n", // no indentation inside raw strings
+
+	// erroneous programs
+	"ERROR1 + 2 +",
+	"ERRORx :=  0",
+}
+
+func String(s string) (string, error) {
+	res, err := Source([]byte(s))
+	if err != nil {
+		return "", err
+	}
+	return string(res), nil
+}
+
+func TestPartial(t *testing.T) {
+	for _, src := range tests {
+		if strings.HasPrefix(src, "ERROR") {
+			// test expected to fail
+			src = src[5:] // remove ERROR prefix
+			res, err := String(src)
+			if err == nil && res == src {
+				t.Errorf("formatting succeeded but was expected to fail:\n%q", src)
+			}
+		} else {
+			// test expected to succeed
+			res, err := String(src)
+			if err != nil {
+				t.Errorf("formatting failed (%s):\n%q", err, src)
+			} else if res != src {
+				t.Errorf("formatting incorrect:\nsource: %q\nresult: %q", src, res)
+			}
+		}
+	}
+}
diff --git a/src/go/parser/error_test.go b/src/go/parser/error_test.go
new file mode 100644
index 000000000..48fb53e5b
--- /dev/null
+++ b/src/go/parser/error_test.go
@@ -0,0 +1,182 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements a parser test harness. The files in the testdata
+// directory are parsed and the errors reported are compared against the
+// error messages expected in the test files. The test files must end in
+// .src rather than .go so that they are not disturbed by gofmt runs.
+//
+// Expected errors are indicated in the test files by putting a comment
+// of the form /* ERROR "rx" */ immediately following an offending token.
+// The harness will verify that an error matching the regular expression
+// rx is reported at that source position.
+//
+// For instance, the following test file indicates that a "not declared"
+// error should be reported for the undeclared variable x:
+//
+//	package p
+//	func f() {
+//		_ = x /* ERROR "not declared" */ + 1
+//	}
+
+package parser
+
+import (
+	"go/scanner"
+	"go/token"
+	"io/ioutil"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+const testdata = "testdata"
+
+var fsetErrs = token.NewFileSet()
+
+// getFile assumes that each filename occurs at most once
+func getFile(filename string) (file *token.File) {
+	fsetErrs.Iterate(func(f *token.File) bool {
+		if f.Name() == filename {
+			if file != nil {
+				panic(filename + " used multiple times")
+			}
+			file = f
+		}
+		return true
+	})
+	return file
+}
+
+func getPos(filename string, offset int) token.Pos {
+	if f := getFile(filename); f != nil {
+		return f.Pos(offset)
+	}
+	return token.NoPos
+}
+
+// ERROR comments must be of the form /* ERROR "rx" */ and rx is
+// a regular expression that matches the expected error message.
+// The special form /* ERROR HERE "rx" */ must be used for error
+// messages that appear immediately after a token, rather than at
+// a token's position.
+//
+var errRx = regexp.MustCompile(`^/\* *ERROR *(HERE)? *"([^"]*)" *\*/$`)
+
+// expectedErrors collects the regular expressions of ERROR comments found
+// in files and returns them as a map of error positions to error messages.
+//
+func expectedErrors(t *testing.T, filename string, src []byte) map[token.Pos]string {
+	errors := make(map[token.Pos]string)
+
+	var s scanner.Scanner
+	// file was parsed already - do not add it again to the file
+	// set otherwise the position information returned here will
+	// not match the position information collected by the parser
+	s.Init(getFile(filename), src, nil, scanner.ScanComments)
+	var prev token.Pos // position of last non-comment, non-semicolon token
+	var here token.Pos // position immediately after the token at position prev
+
+	for {
+		pos, tok, lit := s.Scan()
+		switch tok {
+		case token.EOF:
+			return errors
+		case token.COMMENT:
+			s := errRx.FindStringSubmatch(lit)
+			if len(s) == 3 {
+				pos := prev
+				if s[1] == "HERE" {
+					pos = here
+				}
+				errors[pos] = string(s[2])
+			}
+		default:
+			prev = pos
+			var l int // token length
+			if tok.IsLiteral() {
+				l = len(lit)
+			} else {
+				l = len(tok.String())
+			}
+			here = prev + token.Pos(l)
+		}
+	}
+}
+
+// compareErrors compares the map of expected error messages with the list
+// of found errors and reports discrepancies.
+//
+func compareErrors(t *testing.T, expected map[token.Pos]string, found scanner.ErrorList) {
+	for _, error := range found {
+		// error.Pos is a token.Position, but we want
+		// a token.Pos so we can do a map lookup
+		pos := getPos(error.Pos.Filename, error.Pos.Offset)
+		if msg, found := expected[pos]; found {
+			// we expect a message at pos; check if it matches
+			rx, err := regexp.Compile(msg)
+			if err != nil {
+				t.Errorf("%s: %v", error.Pos, err)
+				continue
+			}
+			if match := rx.MatchString(error.Msg); !match {
+				t.Errorf("%s: %q does not match %q", error.Pos, error.Msg, msg)
+				continue
+			}
+			// we have a match - eliminate this error
+			delete(expected, pos)
+		} else {
+			// To keep in mind when analyzing failed test output:
+			// If the same error position occurs multiple times in errors,
+			// this message will be triggered (because the first error at
+			// the position removes this position from the expected errors).
+			t.Errorf("%s: unexpected error: %s", error.Pos, error.Msg)
+		}
+	}
+
+	// there should be no expected errors left
+	if len(expected) > 0 {
+		t.Errorf("%d errors not reported:", len(expected))
+		for pos, msg := range expected {
+			t.Errorf("%s: %s\n", fsetErrs.Position(pos), msg)
+		}
+	}
+}
+
+func checkErrors(t *testing.T, filename string, input interface{}) {
+	src, err := readSource(filename, input)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	_, err = ParseFile(fsetErrs, filename, src, DeclarationErrors|AllErrors)
+	found, ok := err.(scanner.ErrorList)
+	if err != nil && !ok {
+		t.Error(err)
+		return
+	}
+	found.RemoveMultiples()
+
+	// we are expecting the following errors
+	// (collect these after parsing a file so that it is found in the file set)
+	expected := expectedErrors(t, filename, src)
+
+	// verify errors returned by the parser
+	compareErrors(t, expected, found)
+}
+
+func TestErrors(t *testing.T) {
+	list, err := ioutil.ReadDir(testdata)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, fi := range list {
+		name := fi.Name()
+		if !fi.IsDir() && !strings.HasPrefix(name, ".") && strings.HasSuffix(name, ".src") {
+			checkErrors(t, filepath.Join(testdata, name), nil)
+		}
+	}
+}
diff --git a/src/go/parser/example_test.go b/src/go/parser/example_test.go
new file mode 100644
index 000000000..3c58e63a9
--- /dev/null
+++ b/src/go/parser/example_test.go
@@ -0,0 +1,34 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package parser_test
+
+import (
+	"fmt"
+	"go/parser"
+	"go/token"
+)
+
+func ExampleParseFile() {
+	fset := token.NewFileSet() // positions are relative to fset
+
+	// Parse the file containing this very example
+	// but stop after processing the imports.
+	f, err := parser.ParseFile(fset, "example_test.go", nil, parser.ImportsOnly)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+
+	// Print the imports from the file's AST.
+	for _, s := range f.Imports {
+		fmt.Println(s.Path.Value)
+	}
+
+	// output:
+	//
+	// "fmt"
+	// "go/parser"
+	// "go/token"
+}
diff --git a/src/go/parser/interface.go b/src/go/parser/interface.go
new file mode 100644
index 000000000..49103058b
--- /dev/null
+++ b/src/go/parser/interface.go
@@ -0,0 +1,198 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains the exported entry points for invoking the parser.
+
+package parser
+
+import (
+	"bytes"
+	"errors"
+	"go/ast"
+	"go/token"
+	"io"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// If src != nil, readSource converts src to a []byte if possible;
+// otherwise it returns an error. If src == nil, readSource returns
+// the result of reading the file specified by filename.
+//
+func readSource(filename string, src interface{}) ([]byte, error) {
+	if src != nil {
+		switch s := src.(type) {
+		case string:
+			return []byte(s), nil
+		case []byte:
+			return s, nil
+		case *bytes.Buffer:
+			// is io.Reader, but src is already available in []byte form
+			if s != nil {
+				return s.Bytes(), nil
+			}
+		case io.Reader:
+			var buf bytes.Buffer
+			if _, err := io.Copy(&buf, s); err != nil {
+				return nil, err
+			}
+			return buf.Bytes(), nil
+		}
+		return nil, errors.New("invalid source")
+	}
+	return ioutil.ReadFile(filename)
+}
+
+// A Mode value is a set of flags (or 0).
+// They control the amount of source code parsed and other optional
+// parser functionality.
+//
+type Mode uint
+
+const (
+	PackageClauseOnly Mode             = 1 << iota // stop parsing after package clause
+	ImportsOnly                                    // stop parsing after import declarations
+	ParseComments                                  // parse comments and add them to AST
+	Trace                                          // print a trace of parsed productions
+	DeclarationErrors                              // report declaration errors
+	SpuriousErrors                                 // same as AllErrors, for backward-compatibility
+	AllErrors         = SpuriousErrors             // report all errors (not just the first 10 on different lines)
+)
+
+// ParseFile parses the source code of a single Go source file and returns
+// the corresponding ast.File node. The source code may be provided via
+// the filename of the source file, or via the src parameter.
+//
+// If src != nil, ParseFile parses the source from src and the filename is
+// only used when recording position information. The type of the argument
+// for the src parameter must be string, []byte, or io.Reader.
+// If src == nil, ParseFile parses the file specified by filename.
+//
+// The mode parameter controls the amount of source text parsed and other
+// optional parser functionality. Position information is recorded in the
+// file set fset.
+//
+// If the source couldn't be read, the returned AST is nil and the error
+// indicates the specific failure. If the source was read but syntax
+// errors were found, the result is a partial AST (with ast.Bad* nodes
+// representing the fragments of erroneous source code). Multiple errors
+// are returned via a scanner.ErrorList which is sorted by file position.
+//
+func ParseFile(fset *token.FileSet, filename string, src interface{}, mode Mode) (f *ast.File, err error) {
+	// get source
+	text, err := readSource(filename, src)
+	if err != nil {
+		return nil, err
+	}
+
+	var p parser
+	defer func() {
+		if e := recover(); e != nil {
+			_ = e.(bailout) // re-panics if it's not a bailout
+		}
+
+		// set result values
+		if f == nil {
+			// source is not a valid Go source file - satisfy
+			// ParseFile API and return a valid (but) empty
+			// *ast.File
+			f = &ast.File{
+				Name:  new(ast.Ident),
+				Scope: ast.NewScope(nil),
+			}
+		}
+
+		p.errors.Sort()
+		err = p.errors.Err()
+	}()
+
+	// parse source
+	p.init(fset, filename, text, mode)
+	f = p.parseFile()
+
+	return
+}
+
+// ParseDir calls ParseFile for all files with names ending in ".go" in the
+// directory specified by path and returns a map of package name -> package
+// AST with all the packages found.
+//
+// If filter != nil, only the files with os.FileInfo entries passing through
+// the filter (and ending in ".go") are considered. The mode bits are passed
+// to ParseFile unchanged. Position information is recorded in fset.
+//
+// If the directory couldn't be read, a nil map and the respective error are
+// returned. If a parse error occurred, a non-nil but incomplete map and the
+// first error encountered are returned.
+//
+func ParseDir(fset *token.FileSet, path string, filter func(os.FileInfo) bool, mode Mode) (pkgs map[string]*ast.Package, first error) {
+	fd, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer fd.Close()
+
+	list, err := fd.Readdir(-1)
+	if err != nil {
+		return nil, err
+	}
+
+	pkgs = make(map[string]*ast.Package)
+	for _, d := range list {
+		if strings.HasSuffix(d.Name(), ".go") && (filter == nil || filter(d)) {
+			filename := filepath.Join(path, d.Name())
+			if src, err := ParseFile(fset, filename, nil, mode); err == nil {
+				name := src.Name.Name
+				pkg, found := pkgs[name]
+				if !found {
+					pkg = &ast.Package{
+						Name:  name,
+						Files: make(map[string]*ast.File),
+					}
+					pkgs[name] = pkg
+				}
+				pkg.Files[filename] = src
+			} else if first == nil {
+				first = err
+			}
+		}
+	}
+
+	return
+}
+
+// ParseExpr is a convenience function for obtaining the AST of an expression x.
+// The position information recorded in the AST is undefined. The filename used
+// in error messages is the empty string.
+//
+func ParseExpr(x string) (ast.Expr, error) {
+	var p parser
+	p.init(token.NewFileSet(), "", []byte(x), 0)
+
+	// Set up pkg-level scopes to avoid nil-pointer errors.
+	// This is not needed for a correct expression x as the
+	// parser will be ok with a nil topScope, but be cautious
+	// in case of an erroneous x.
+	p.openScope()
+	p.pkgScope = p.topScope
+	e := p.parseRhsOrType()
+	p.closeScope()
+	assert(p.topScope == nil, "unbalanced scopes")
+
+	// If a semicolon was inserted, consume it;
+	// report an error if there's more tokens.
+	if p.tok == token.SEMICOLON && p.lit == "\n" {
+		p.next()
+	}
+	p.expect(token.EOF)
+
+	if p.errors.Len() > 0 {
+		p.errors.Sort()
+		return nil, p.errors.Err()
+	}
+
+	return e, nil
+}
diff --git a/src/go/parser/parser.go b/src/go/parser/parser.go
new file mode 100644
index 000000000..9c62076f2
--- /dev/null
+++ b/src/go/parser/parser.go
@@ -0,0 +1,2460 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package parser implements a parser for Go source files. Input may be
+// provided in a variety of forms (see the various Parse* functions); the
+// output is an abstract syntax tree (AST) representing the Go source. The
+// parser is invoked through one of the Parse* functions.
+//
+package parser
+
+import (
+	"fmt"
+	"go/ast"
+	"go/scanner"
+	"go/token"
+	"strconv"
+	"strings"
+	"unicode"
+)
+
+// The parser structure holds the parser's internal state.
+type parser struct {
+	file    *token.File
+	errors  scanner.ErrorList
+	scanner scanner.Scanner
+
+	// Tracing/debugging
+	mode   Mode // parsing mode
+	trace  bool // == (mode & Trace != 0)
+	indent int  // indentation used for tracing output
+
+	// Comments
+	comments    []*ast.CommentGroup
+	leadComment *ast.CommentGroup // last lead comment
+	lineComment *ast.CommentGroup // last line comment
+
+	// Next token
+	pos token.Pos   // token position
+	tok token.Token // one token look-ahead
+	lit string      // token literal
+
+	// Error recovery
+	// (used to limit the number of calls to syncXXX functions
+	// w/o making scanning progress - avoids potential endless
+	// loops across multiple parser functions during error recovery)
+	syncPos token.Pos // last synchronization position
+	syncCnt int       // number of calls to syncXXX without progress
+
+	// Non-syntactic parser control
+	exprLev int  // < 0: in control clause, >= 0: in expression
+	inRhs   bool // if set, the parser is parsing a rhs expression
+
+	// Ordinary identifier scopes
+	pkgScope   *ast.Scope        // pkgScope.Outer == nil
+	topScope   *ast.Scope        // top-most scope; may be pkgScope
+	unresolved []*ast.Ident      // unresolved identifiers
+	imports    []*ast.ImportSpec // list of imports
+
+	// Label scopes
+	// (maintained by open/close LabelScope)
+	labelScope  *ast.Scope     // label scope for current function
+	targetStack [][]*ast.Ident // stack of unresolved labels
+}
+
+func (p *parser) init(fset *token.FileSet, filename string, src []byte, mode Mode) {
+	p.file = fset.AddFile(filename, -1, len(src))
+	var m scanner.Mode
+	if mode&ParseComments != 0 {
+		m = scanner.ScanComments
+	}
+	eh := func(pos token.Position, msg string) { p.errors.Add(pos, msg) }
+	p.scanner.Init(p.file, src, eh, m)
+
+	p.mode = mode
+	p.trace = mode&Trace != 0 // for convenience (p.trace is used frequently)
+
+	p.next()
+}
+
+// ----------------------------------------------------------------------------
+// Scoping support
+
+func (p *parser) openScope() {
+	p.topScope = ast.NewScope(p.topScope)
+}
+
+func (p *parser) closeScope() {
+	p.topScope = p.topScope.Outer
+}
+
+func (p *parser) openLabelScope() {
+	p.labelScope = ast.NewScope(p.labelScope)
+	p.targetStack = append(p.targetStack, nil)
+}
+
+func (p *parser) closeLabelScope() {
+	// resolve labels
+	n := len(p.targetStack) - 1
+	scope := p.labelScope
+	for _, ident := range p.targetStack[n] {
+		ident.Obj = scope.Lookup(ident.Name)
+		if ident.Obj == nil && p.mode&DeclarationErrors != 0 {
+			p.error(ident.Pos(), fmt.Sprintf("label %s undefined", ident.Name))
+		}
+	}
+	// pop label scope
+	p.targetStack = p.targetStack[0:n]
+	p.labelScope = p.labelScope.Outer
+}
+
+func (p *parser) declare(decl, data interface{}, scope *ast.Scope, kind ast.ObjKind, idents ...*ast.Ident) {
+	for _, ident := range idents {
+		assert(ident.Obj == nil, "identifier already declared or resolved")
+		obj := ast.NewObj(kind, ident.Name)
+		// remember the corresponding declaration for redeclaration
+		// errors and global variable resolution/typechecking phase
+		obj.Decl = decl
+		obj.Data = data
+		ident.Obj = obj
+		if ident.Name != "_" {
+			if alt := scope.Insert(obj); alt != nil && p.mode&DeclarationErrors != 0 {
+				prevDecl := ""
+				if pos := alt.Pos(); pos.IsValid() {
+					prevDecl = fmt.Sprintf("\n\tprevious declaration at %s", p.file.Position(pos))
+				}
+				p.error(ident.Pos(), fmt.Sprintf("%s redeclared in this block%s", ident.Name, prevDecl))
+			}
+		}
+	}
+}
+
+func (p *parser) shortVarDecl(decl *ast.AssignStmt, list []ast.Expr) {
+	// Go spec: A short variable declaration may redeclare variables
+	// provided they were originally declared in the same block with
+	// the same type, and at least one of the non-blank variables is new.
+	n := 0 // number of new variables
+	for _, x := range list {
+		if ident, isIdent := x.(*ast.Ident); isIdent {
+			assert(ident.Obj == nil, "identifier already declared or resolved")
+			obj := ast.NewObj(ast.Var, ident.Name)
+			// remember corresponding assignment for other tools
+			obj.Decl = decl
+			ident.Obj = obj
+			if ident.Name != "_" {
+				if alt := p.topScope.Insert(obj); alt != nil {
+					ident.Obj = alt // redeclaration
+				} else {
+					n++ // new declaration
+				}
+			}
+		} else {
+			p.errorExpected(x.Pos(), "identifier on left side of :=")
+		}
+	}
+	if n == 0 && p.mode&DeclarationErrors != 0 {
+		p.error(list[0].Pos(), "no new variables on left side of :=")
+	}
+}
+
+// The unresolved object is a sentinel to mark identifiers that have been added
+// to the list of unresolved identifiers. The sentinel is only used for verifying
+// internal consistency.
+var unresolved = new(ast.Object)
+
+// If x is an identifier, tryResolve attempts to resolve x by looking up
+// the object it denotes. If no object is found and collectUnresolved is
+// set, x is marked as unresolved and collected in the list of unresolved
+// identifiers.
+//
+func (p *parser) tryResolve(x ast.Expr, collectUnresolved bool) {
+	// nothing to do if x is not an identifier or the blank identifier
+	ident, _ := x.(*ast.Ident)
+	if ident == nil {
+		return
+	}
+	assert(ident.Obj == nil, "identifier already declared or resolved")
+	if ident.Name == "_" {
+		return
+	}
+	// try to resolve the identifier
+	for s := p.topScope; s != nil; s = s.Outer {
+		if obj := s.Lookup(ident.Name); obj != nil {
+			ident.Obj = obj
+			return
+		}
+	}
+	// all local scopes are known, so any unresolved identifier
+	// must be found either in the file scope, package scope
+	// (perhaps in another file), or universe scope --- collect
+	// them so that they can be resolved later
+	if collectUnresolved {
+		ident.Obj = unresolved
+		p.unresolved = append(p.unresolved, ident)
+	}
+}
+
+func (p *parser) resolve(x ast.Expr) {
+	p.tryResolve(x, true)
+}
+
+// ----------------------------------------------------------------------------
+// Parsing support
+
+func (p *parser) printTrace(a ...interface{}) {
+	const dots = ". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "
+	const n = len(dots)
+	pos := p.file.Position(p.pos)
+	fmt.Printf("%5d:%3d: ", pos.Line, pos.Column)
+	i := 2 * p.indent
+	for i > n {
+		fmt.Print(dots)
+		i -= n
+	}
+	// i <= n
+	fmt.Print(dots[0:i])
+	fmt.Println(a...)
+}
+
+func trace(p *parser, msg string) *parser {
+	p.printTrace(msg, "(")
+	p.indent++
+	return p
+}
+
+// Usage pattern: defer un(trace(p, "..."))
+func un(p *parser) {
+	p.indent--
+	p.printTrace(")")
+}
+
+// Advance to the next token.
+func (p *parser) next0() {
+	// Because of one-token look-ahead, print the previous token
+	// when tracing as it provides a more readable output. The
+	// very first token (!p.pos.IsValid()) is not initialized
+	// (it is token.ILLEGAL), so don't print it .
+	if p.trace && p.pos.IsValid() {
+		s := p.tok.String()
+		switch {
+		case p.tok.IsLiteral():
+			p.printTrace(s, p.lit)
+		case p.tok.IsOperator(), p.tok.IsKeyword():
+			p.printTrace("\"" + s + "\"")
+		default:
+			p.printTrace(s)
+		}
+	}
+
+	p.pos, p.tok, p.lit = p.scanner.Scan()
+}
+
+// Consume a comment and return it and the line on which it ends.
+func (p *parser) consumeComment() (comment *ast.Comment, endline int) {
+	// /*-style comments may end on a different line than where they start.
+	// Scan the comment for '\n' chars and adjust endline accordingly.
+	endline = p.file.Line(p.pos)
+	if p.lit[1] == '*' {
+		// don't use range here - no need to decode Unicode code points
+		for i := 0; i < len(p.lit); i++ {
+			if p.lit[i] == '\n' {
+				endline++
+			}
+		}
+	}
+
+	comment = &ast.Comment{Slash: p.pos, Text: p.lit}
+	p.next0()
+
+	return
+}
+
+// Consume a group of adjacent comments, add it to the parser's
+// comments list, and return it together with the line at which
+// the last comment in the group ends. A non-comment token or n
+// empty lines terminate a comment group.
+//
+func (p *parser) consumeCommentGroup(n int) (comments *ast.CommentGroup, endline int) {
+	var list []*ast.Comment
+	endline = p.file.Line(p.pos)
+	for p.tok == token.COMMENT && p.file.Line(p.pos) <= endline+n {
+		var comment *ast.Comment
+		comment, endline = p.consumeComment()
+		list = append(list, comment)
+	}
+
+	// add comment group to the comments list
+	comments = &ast.CommentGroup{List: list}
+	p.comments = append(p.comments, comments)
+
+	return
+}
+
+// Advance to the next non-comment token. In the process, collect
+// any comment groups encountered, and remember the last lead and
+// and line comments.
+//
+// A lead comment is a comment group that starts and ends in a
+// line without any other tokens and that is followed by a non-comment
+// token on the line immediately after the comment group.
+//
+// A line comment is a comment group that follows a non-comment
+// token on the same line, and that has no tokens after it on the line
+// where it ends.
+//
+// Lead and line comments may be considered documentation that is
+// stored in the AST.
+//
+func (p *parser) next() {
+	p.leadComment = nil
+	p.lineComment = nil
+	prev := p.pos
+	p.next0()
+
+	if p.tok == token.COMMENT {
+		var comment *ast.CommentGroup
+		var endline int
+
+		if p.file.Line(p.pos) == p.file.Line(prev) {
+			// The comment is on same line as the previous token; it
+			// cannot be a lead comment but may be a line comment.
+			comment, endline = p.consumeCommentGroup(0)
+			if p.file.Line(p.pos) != endline {
+				// The next token is on a different line, thus
+				// the last comment group is a line comment.
+				p.lineComment = comment
+			}
+		}
+
+		// consume successor comments, if any
+		endline = -1
+		for p.tok == token.COMMENT {
+			comment, endline = p.consumeCommentGroup(1)
+		}
+
+		if endline+1 == p.file.Line(p.pos) {
+			// The next token is following on the line immediately after the
+			// comment group, thus the last comment group is a lead comment.
+			p.leadComment = comment
+		}
+	}
+}
+
+// A bailout panic is raised to indicate early termination.
+type bailout struct{}
+
+func (p *parser) error(pos token.Pos, msg string) {
+	epos := p.file.Position(pos)
+
+	// If AllErrors is not set, discard errors reported on the same line
+	// as the last recorded error and stop parsing if there are more than
+	// 10 errors.
+	if p.mode&AllErrors == 0 {
+		n := len(p.errors)
+		if n > 0 && p.errors[n-1].Pos.Line == epos.Line {
+			return // discard - likely a spurious error
+		}
+		if n > 10 {
+			panic(bailout{})
+		}
+	}
+
+	p.errors.Add(epos, msg)
+}
+
+func (p *parser) errorExpected(pos token.Pos, msg string) {
+	msg = "expected " + msg
+	if pos == p.pos {
+		// the error happened at the current position;
+		// make the error message more specific
+		if p.tok == token.SEMICOLON && p.lit == "\n" {
+			msg += ", found newline"
+		} else {
+			msg += ", found '" + p.tok.String() + "'"
+			if p.tok.IsLiteral() {
+				msg += " " + p.lit
+			}
+		}
+	}
+	p.error(pos, msg)
+}
+
+func (p *parser) expect(tok token.Token) token.Pos {
+	pos := p.pos
+	if p.tok != tok {
+		p.errorExpected(pos, "'"+tok.String()+"'")
+	}
+	p.next() // make progress
+	return pos
+}
+
+// expectClosing is like expect but provides a better error message
+// for the common case of a missing comma before a newline.
+//
+func (p *parser) expectClosing(tok token.Token, context string) token.Pos {
+	if p.tok != tok && p.tok == token.SEMICOLON && p.lit == "\n" {
+		p.error(p.pos, "missing ',' before newline in "+context)
+		p.next()
+	}
+	return p.expect(tok)
+}
+
+func (p *parser) expectSemi() {
+	// semicolon is optional before a closing ')' or '}'
+	if p.tok != token.RPAREN && p.tok != token.RBRACE {
+		if p.tok == token.SEMICOLON {
+			p.next()
+		} else {
+			p.errorExpected(p.pos, "';'")
+			syncStmt(p)
+		}
+	}
+}
+
+func (p *parser) atComma(context string) bool {
+	if p.tok == token.COMMA {
+		return true
+	}
+	if p.tok == token.SEMICOLON && p.lit == "\n" {
+		p.error(p.pos, "missing ',' before newline in "+context)
+		return true // "insert" the comma and continue
+
+	}
+	return false
+}
+
+func assert(cond bool, msg string) {
+	if !cond {
+		panic("go/parser internal error: " + msg)
+	}
+}
+
+// syncStmt advances to the next statement.
+// Used for synchronization after an error.
+//
+func syncStmt(p *parser) {
+	for {
+		switch p.tok {
+		case token.BREAK, token.CONST, token.CONTINUE, token.DEFER,
+			token.FALLTHROUGH, token.FOR, token.GO, token.GOTO,
+			token.IF, token.RETURN, token.SELECT, token.SWITCH,
+			token.TYPE, token.VAR:
+			// Return only if parser made some progress since last
+			// sync or if it has not reached 10 sync calls without
+			// progress. Otherwise consume at least one token to
+			// avoid an endless parser loop (it is possible that
+			// both parseOperand and parseStmt call syncStmt and
+			// correctly do not advance, thus the need for the
+			// invocation limit p.syncCnt).
+			if p.pos == p.syncPos && p.syncCnt < 10 {
+				p.syncCnt++
+				return
+			}
+			if p.pos > p.syncPos {
+				p.syncPos = p.pos
+				p.syncCnt = 0
+				return
+			}
+			// Reaching here indicates a parser bug, likely an
+			// incorrect token list in this function, but it only
+			// leads to skipping of possibly correct code if a
+			// previous error is present, and thus is preferred
+			// over a non-terminating parse.
+		case token.EOF:
+			return
+		}
+		p.next()
+	}
+}
+
+// syncDecl advances to the next declaration.
+// Used for synchronization after an error.
+//
+func syncDecl(p *parser) {
+	for {
+		switch p.tok {
+		case token.CONST, token.TYPE, token.VAR:
+			// see comments in syncStmt
+			if p.pos == p.syncPos && p.syncCnt < 10 {
+				p.syncCnt++
+				return
+			}
+			if p.pos > p.syncPos {
+				p.syncPos = p.pos
+				p.syncCnt = 0
+				return
+			}
+		case token.EOF:
+			return
+		}
+		p.next()
+	}
+}
+
+// safePos returns a valid file position for a given position: If pos
+// is valid to begin with, safePos returns pos. If pos is out-of-range,
+// safePos returns the EOF position.
+//
+// This is hack to work around "artificial" end positions in the AST which
+// are computed by adding 1 to (presumably valid) token positions. If the
+// token positions are invalid due to parse errors, the resulting end position
+// may be past the file's EOF position, which would lead to panics if used
+// later on.
+//
+func (p *parser) safePos(pos token.Pos) (res token.Pos) {
+	defer func() {
+		if recover() != nil {
+			res = token.Pos(p.file.Base() + p.file.Size()) // EOF position
+		}
+	}()
+	_ = p.file.Offset(pos) // trigger a panic if position is out-of-range
+	return pos
+}
+
+// ----------------------------------------------------------------------------
+// Identifiers
+
+func (p *parser) parseIdent() *ast.Ident {
+	pos := p.pos
+	name := "_"
+	if p.tok == token.IDENT {
+		name = p.lit
+		p.next()
+	} else {
+		p.expect(token.IDENT) // use expect() error handling
+	}
+	return &ast.Ident{NamePos: pos, Name: name}
+}
+
+func (p *parser) parseIdentList() (list []*ast.Ident) {
+	if p.trace {
+		defer un(trace(p, "IdentList"))
+	}
+
+	list = append(list, p.parseIdent())
+	for p.tok == token.COMMA {
+		p.next()
+		list = append(list, p.parseIdent())
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Common productions
+
+// If lhs is set, result list elements which are identifiers are not resolved.
+func (p *parser) parseExprList(lhs bool) (list []ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "ExpressionList"))
+	}
+
+	list = append(list, p.checkExpr(p.parseExpr(lhs)))
+	for p.tok == token.COMMA {
+		p.next()
+		list = append(list, p.checkExpr(p.parseExpr(lhs)))
+	}
+
+	return
+}
+
+func (p *parser) parseLhsList() []ast.Expr {
+	old := p.inRhs
+	p.inRhs = false
+	list := p.parseExprList(true)
+	switch p.tok {
+	case token.DEFINE:
+		// lhs of a short variable declaration
+		// but doesn't enter scope until later:
+		// caller must call p.shortVarDecl(p.makeIdentList(list))
+		// at appropriate time.
+	case token.COLON:
+		// lhs of a label declaration or a communication clause of a select
+		// statement (parseLhsList is not called when parsing the case clause
+		// of a switch statement):
+		// - labels are declared by the caller of parseLhsList
+		// - for communication clauses, if there is a stand-alone identifier
+		//   followed by a colon, we have a syntax error; there is no need
+		//   to resolve the identifier in that case
+	default:
+		// identifiers must be declared elsewhere
+		for _, x := range list {
+			p.resolve(x)
+		}
+	}
+	p.inRhs = old
+	return list
+}
+
+func (p *parser) parseRhsList() []ast.Expr {
+	old := p.inRhs
+	p.inRhs = true
+	list := p.parseExprList(false)
+	p.inRhs = old
+	return list
+}
+
+// ----------------------------------------------------------------------------
+// Types
+
+func (p *parser) parseType() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Type"))
+	}
+
+	typ := p.tryType()
+
+	if typ == nil {
+		pos := p.pos
+		p.errorExpected(pos, "type")
+		p.next() // make progress
+		return &ast.BadExpr{From: pos, To: p.pos}
+	}
+
+	return typ
+}
+
+// If the result is an identifier, it is not resolved.
+func (p *parser) parseTypeName() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "TypeName"))
+	}
+
+	ident := p.parseIdent()
+	// don't resolve ident yet - it may be a parameter or field name
+
+	if p.tok == token.PERIOD {
+		// ident is a package name
+		p.next()
+		p.resolve(ident)
+		sel := p.parseIdent()
+		return &ast.SelectorExpr{X: ident, Sel: sel}
+	}
+
+	return ident
+}
+
+func (p *parser) parseArrayType() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "ArrayType"))
+	}
+
+	lbrack := p.expect(token.LBRACK)
+	var len ast.Expr
+	// always permit ellipsis for more fault-tolerant parsing
+	if p.tok == token.ELLIPSIS {
+		len = &ast.Ellipsis{Ellipsis: p.pos}
+		p.next()
+	} else if p.tok != token.RBRACK {
+		len = p.parseRhs()
+	}
+	p.expect(token.RBRACK)
+	elt := p.parseType()
+
+	return &ast.ArrayType{Lbrack: lbrack, Len: len, Elt: elt}
+}
+
+func (p *parser) makeIdentList(list []ast.Expr) []*ast.Ident {
+	idents := make([]*ast.Ident, len(list))
+	for i, x := range list {
+		ident, isIdent := x.(*ast.Ident)
+		if !isIdent {
+			if _, isBad := x.(*ast.BadExpr); !isBad {
+				// only report error if it's a new one
+				p.errorExpected(x.Pos(), "identifier")
+			}
+			ident = &ast.Ident{NamePos: x.Pos(), Name: "_"}
+		}
+		idents[i] = ident
+	}
+	return idents
+}
+
+func (p *parser) parseFieldDecl(scope *ast.Scope) *ast.Field {
+	if p.trace {
+		defer un(trace(p, "FieldDecl"))
+	}
+
+	doc := p.leadComment
+
+	// FieldDecl
+	list, typ := p.parseVarList(false)
+
+	// Tag
+	var tag *ast.BasicLit
+	if p.tok == token.STRING {
+		tag = &ast.BasicLit{ValuePos: p.pos, Kind: p.tok, Value: p.lit}
+		p.next()
+	}
+
+	// analyze case
+	var idents []*ast.Ident
+	if typ != nil {
+		// IdentifierList Type
+		idents = p.makeIdentList(list)
+	} else {
+		// ["*"] TypeName (AnonymousField)
+		typ = list[0] // we always have at least one element
+		if n := len(list); n > 1 || !isTypeName(deref(typ)) {
+			pos := typ.Pos()
+			p.errorExpected(pos, "anonymous field")
+			typ = &ast.BadExpr{From: pos, To: p.safePos(list[n-1].End())}
+		}
+	}
+
+	p.expectSemi() // call before accessing p.linecomment
+
+	field := &ast.Field{Doc: doc, Names: idents, Type: typ, Tag: tag, Comment: p.lineComment}
+	p.declare(field, nil, scope, ast.Var, idents...)
+	p.resolve(typ)
+
+	return field
+}
+
+func (p *parser) parseStructType() *ast.StructType {
+	if p.trace {
+		defer un(trace(p, "StructType"))
+	}
+
+	pos := p.expect(token.STRUCT)
+	lbrace := p.expect(token.LBRACE)
+	scope := ast.NewScope(nil) // struct scope
+	var list []*ast.Field
+	for p.tok == token.IDENT || p.tok == token.MUL || p.tok == token.LPAREN {
+		// a field declaration cannot start with a '(' but we accept
+		// it here for more robust parsing and better error messages
+		// (parseFieldDecl will check and complain if necessary)
+		list = append(list, p.parseFieldDecl(scope))
+	}
+	rbrace := p.expect(token.RBRACE)
+
+	return &ast.StructType{
+		Struct: pos,
+		Fields: &ast.FieldList{
+			Opening: lbrace,
+			List:    list,
+			Closing: rbrace,
+		},
+	}
+}
+
+func (p *parser) parsePointerType() *ast.StarExpr {
+	if p.trace {
+		defer un(trace(p, "PointerType"))
+	}
+
+	star := p.expect(token.MUL)
+	base := p.parseType()
+
+	return &ast.StarExpr{Star: star, X: base}
+}
+
+// If the result is an identifier, it is not resolved.
+func (p *parser) tryVarType(isParam bool) ast.Expr {
+	if isParam && p.tok == token.ELLIPSIS {
+		pos := p.pos
+		p.next()
+		typ := p.tryIdentOrType() // don't use parseType so we can provide better error message
+		if typ != nil {
+			p.resolve(typ)
+		} else {
+			p.error(pos, "'...' parameter is missing type")
+			typ = &ast.BadExpr{From: pos, To: p.pos}
+		}
+		return &ast.Ellipsis{Ellipsis: pos, Elt: typ}
+	}
+	return p.tryIdentOrType()
+}
+
+// If the result is an identifier, it is not resolved.
+func (p *parser) parseVarType(isParam bool) ast.Expr {
+	typ := p.tryVarType(isParam)
+	if typ == nil {
+		pos := p.pos
+		p.errorExpected(pos, "type")
+		p.next() // make progress
+		typ = &ast.BadExpr{From: pos, To: p.pos}
+	}
+	return typ
+}
+
+// If any of the results are identifiers, they are not resolved.
+func (p *parser) parseVarList(isParam bool) (list []ast.Expr, typ ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "VarList"))
+	}
+
+	// a list of identifiers looks like a list of type names
+	//
+	// parse/tryVarType accepts any type (including parenthesized
+	// ones) even though the syntax does not permit them here: we
+	// accept them all for more robust parsing and complain later
+	for typ := p.parseVarType(isParam); typ != nil; {
+		list = append(list, typ)
+		if p.tok != token.COMMA {
+			break
+		}
+		p.next()
+		typ = p.tryVarType(isParam) // maybe nil as in: func f(int,) {}
+	}
+
+	// if we had a list of identifiers, it must be followed by a type
+	typ = p.tryVarType(isParam)
+
+	return
+}
+
+func (p *parser) parseParameterList(scope *ast.Scope, ellipsisOk bool) (params []*ast.Field) {
+	if p.trace {
+		defer un(trace(p, "ParameterList"))
+	}
+
+	// ParameterDecl
+	list, typ := p.parseVarList(ellipsisOk)
+
+	// analyze case
+	if typ != nil {
+		// IdentifierList Type
+		idents := p.makeIdentList(list)
+		field := &ast.Field{Names: idents, Type: typ}
+		params = append(params, field)
+		// Go spec: The scope of an identifier denoting a function
+		// parameter or result variable is the function body.
+		p.declare(field, nil, scope, ast.Var, idents...)
+		p.resolve(typ)
+		if !p.atComma("parameter list") {
+			return
+		}
+		p.next()
+		for p.tok != token.RPAREN && p.tok != token.EOF {
+			idents := p.parseIdentList()
+			typ := p.parseVarType(ellipsisOk)
+			field := &ast.Field{Names: idents, Type: typ}
+			params = append(params, field)
+			// Go spec: The scope of an identifier denoting a function
+			// parameter or result variable is the function body.
+			p.declare(field, nil, scope, ast.Var, idents...)
+			p.resolve(typ)
+			if !p.atComma("parameter list") {
+				break
+			}
+			p.next()
+		}
+		return
+	}
+
+	// Type { "," Type } (anonymous parameters)
+	params = make([]*ast.Field, len(list))
+	for i, typ := range list {
+		p.resolve(typ)
+		params[i] = &ast.Field{Type: typ}
+	}
+	return
+}
+
+func (p *parser) parseParameters(scope *ast.Scope, ellipsisOk bool) *ast.FieldList {
+	if p.trace {
+		defer un(trace(p, "Parameters"))
+	}
+
+	var params []*ast.Field
+	lparen := p.expect(token.LPAREN)
+	if p.tok != token.RPAREN {
+		params = p.parseParameterList(scope, ellipsisOk)
+	}
+	rparen := p.expect(token.RPAREN)
+
+	return &ast.FieldList{Opening: lparen, List: params, Closing: rparen}
+}
+
+func (p *parser) parseResult(scope *ast.Scope) *ast.FieldList {
+	if p.trace {
+		defer un(trace(p, "Result"))
+	}
+
+	if p.tok == token.LPAREN {
+		return p.parseParameters(scope, false)
+	}
+
+	typ := p.tryType()
+	if typ != nil {
+		list := make([]*ast.Field, 1)
+		list[0] = &ast.Field{Type: typ}
+		return &ast.FieldList{List: list}
+	}
+
+	return nil
+}
+
+func (p *parser) parseSignature(scope *ast.Scope) (params, results *ast.FieldList) {
+	if p.trace {
+		defer un(trace(p, "Signature"))
+	}
+
+	params = p.parseParameters(scope, true)
+	results = p.parseResult(scope)
+
+	return
+}
+
+func (p *parser) parseFuncType() (*ast.FuncType, *ast.Scope) {
+	if p.trace {
+		defer un(trace(p, "FuncType"))
+	}
+
+	pos := p.expect(token.FUNC)
+	scope := ast.NewScope(p.topScope) // function scope
+	params, results := p.parseSignature(scope)
+
+	return &ast.FuncType{Func: pos, Params: params, Results: results}, scope
+}
+
+func (p *parser) parseMethodSpec(scope *ast.Scope) *ast.Field {
+	if p.trace {
+		defer un(trace(p, "MethodSpec"))
+	}
+
+	doc := p.leadComment
+	var idents []*ast.Ident
+	var typ ast.Expr
+	x := p.parseTypeName()
+	if ident, isIdent := x.(*ast.Ident); isIdent && p.tok == token.LPAREN {
+		// method
+		idents = []*ast.Ident{ident}
+		scope := ast.NewScope(nil) // method scope
+		params, results := p.parseSignature(scope)
+		typ = &ast.FuncType{Func: token.NoPos, Params: params, Results: results}
+	} else {
+		// embedded interface
+		typ = x
+		p.resolve(typ)
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	spec := &ast.Field{Doc: doc, Names: idents, Type: typ, Comment: p.lineComment}
+	p.declare(spec, nil, scope, ast.Fun, idents...)
+
+	return spec
+}
+
+func (p *parser) parseInterfaceType() *ast.InterfaceType {
+	if p.trace {
+		defer un(trace(p, "InterfaceType"))
+	}
+
+	pos := p.expect(token.INTERFACE)
+	lbrace := p.expect(token.LBRACE)
+	scope := ast.NewScope(nil) // interface scope
+	var list []*ast.Field
+	for p.tok == token.IDENT {
+		list = append(list, p.parseMethodSpec(scope))
+	}
+	rbrace := p.expect(token.RBRACE)
+
+	return &ast.InterfaceType{
+		Interface: pos,
+		Methods: &ast.FieldList{
+			Opening: lbrace,
+			List:    list,
+			Closing: rbrace,
+		},
+	}
+}
+
+func (p *parser) parseMapType() *ast.MapType {
+	if p.trace {
+		defer un(trace(p, "MapType"))
+	}
+
+	pos := p.expect(token.MAP)
+	p.expect(token.LBRACK)
+	key := p.parseType()
+	p.expect(token.RBRACK)
+	value := p.parseType()
+
+	return &ast.MapType{Map: pos, Key: key, Value: value}
+}
+
+func (p *parser) parseChanType() *ast.ChanType {
+	if p.trace {
+		defer un(trace(p, "ChanType"))
+	}
+
+	pos := p.pos
+	dir := ast.SEND | ast.RECV
+	var arrow token.Pos
+	if p.tok == token.CHAN {
+		p.next()
+		if p.tok == token.ARROW {
+			arrow = p.pos
+			p.next()
+			dir = ast.SEND
+		}
+	} else {
+		arrow = p.expect(token.ARROW)
+		p.expect(token.CHAN)
+		dir = ast.RECV
+	}
+	value := p.parseType()
+
+	return &ast.ChanType{Begin: pos, Arrow: arrow, Dir: dir, Value: value}
+}
+
+// If the result is an identifier, it is not resolved.
+func (p *parser) tryIdentOrType() ast.Expr {
+	switch p.tok {
+	case token.IDENT:
+		return p.parseTypeName()
+	case token.LBRACK:
+		return p.parseArrayType()
+	case token.STRUCT:
+		return p.parseStructType()
+	case token.MUL:
+		return p.parsePointerType()
+	case token.FUNC:
+		typ, _ := p.parseFuncType()
+		return typ
+	case token.INTERFACE:
+		return p.parseInterfaceType()
+	case token.MAP:
+		return p.parseMapType()
+	case token.CHAN, token.ARROW:
+		return p.parseChanType()
+	case token.LPAREN:
+		lparen := p.pos
+		p.next()
+		typ := p.parseType()
+		rparen := p.expect(token.RPAREN)
+		return &ast.ParenExpr{Lparen: lparen, X: typ, Rparen: rparen}
+	}
+
+	// no type found
+	return nil
+}
+
+func (p *parser) tryType() ast.Expr {
+	typ := p.tryIdentOrType()
+	if typ != nil {
+		p.resolve(typ)
+	}
+	return typ
+}
+
+// ----------------------------------------------------------------------------
+// Blocks
+
+func (p *parser) parseStmtList() (list []ast.Stmt) {
+	if p.trace {
+		defer un(trace(p, "StatementList"))
+	}
+
+	for p.tok != token.CASE && p.tok != token.DEFAULT && p.tok != token.RBRACE && p.tok != token.EOF {
+		list = append(list, p.parseStmt())
+	}
+
+	return
+}
+
+func (p *parser) parseBody(scope *ast.Scope) *ast.BlockStmt {
+	if p.trace {
+		defer un(trace(p, "Body"))
+	}
+
+	lbrace := p.expect(token.LBRACE)
+	p.topScope = scope // open function scope
+	p.openLabelScope()
+	list := p.parseStmtList()
+	p.closeLabelScope()
+	p.closeScope()
+	rbrace := p.expect(token.RBRACE)
+
+	return &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
+}
+
+func (p *parser) parseBlockStmt() *ast.BlockStmt {
+	if p.trace {
+		defer un(trace(p, "BlockStmt"))
+	}
+
+	lbrace := p.expect(token.LBRACE)
+	p.openScope()
+	list := p.parseStmtList()
+	p.closeScope()
+	rbrace := p.expect(token.RBRACE)
+
+	return &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
+}
+
+// ----------------------------------------------------------------------------
+// Expressions
+
+func (p *parser) parseFuncTypeOrLit() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "FuncTypeOrLit"))
+	}
+
+	typ, scope := p.parseFuncType()
+	if p.tok != token.LBRACE {
+		// function type only
+		return typ
+	}
+
+	p.exprLev++
+	body := p.parseBody(scope)
+	p.exprLev--
+
+	return &ast.FuncLit{Type: typ, Body: body}
+}
+
+// parseOperand may return an expression or a raw type (incl. array
+// types of the form [...]T. Callers must verify the result.
+// If lhs is set and the result is an identifier, it is not resolved.
+//
+func (p *parser) parseOperand(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Operand"))
+	}
+
+	switch p.tok {
+	case token.IDENT:
+		x := p.parseIdent()
+		if !lhs {
+			p.resolve(x)
+		}
+		return x
+
+	case token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING:
+		x := &ast.BasicLit{ValuePos: p.pos, Kind: p.tok, Value: p.lit}
+		p.next()
+		return x
+
+	case token.LPAREN:
+		lparen := p.pos
+		p.next()
+		p.exprLev++
+		x := p.parseRhsOrType() // types may be parenthesized: (some type)
+		p.exprLev--
+		rparen := p.expect(token.RPAREN)
+		return &ast.ParenExpr{Lparen: lparen, X: x, Rparen: rparen}
+
+	case token.FUNC:
+		return p.parseFuncTypeOrLit()
+	}
+
+	if typ := p.tryIdentOrType(); typ != nil {
+		// could be type for composite literal or conversion
+		_, isIdent := typ.(*ast.Ident)
+		assert(!isIdent, "type cannot be identifier")
+		return typ
+	}
+
+	// we have an error
+	pos := p.pos
+	p.errorExpected(pos, "operand")
+	syncStmt(p)
+	return &ast.BadExpr{From: pos, To: p.pos}
+}
+
+func (p *parser) parseSelector(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Selector"))
+	}
+
+	sel := p.parseIdent()
+
+	return &ast.SelectorExpr{X: x, Sel: sel}
+}
+
+func (p *parser) parseTypeAssertion(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "TypeAssertion"))
+	}
+
+	lparen := p.expect(token.LPAREN)
+	var typ ast.Expr
+	if p.tok == token.TYPE {
+		// type switch: typ == nil
+		p.next()
+	} else {
+		typ = p.parseType()
+	}
+	rparen := p.expect(token.RPAREN)
+
+	return &ast.TypeAssertExpr{X: x, Type: typ, Lparen: lparen, Rparen: rparen}
+}
+
+func (p *parser) parseIndexOrSlice(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "IndexOrSlice"))
+	}
+
+	const N = 3 // change the 3 to 2 to disable 3-index slices
+	lbrack := p.expect(token.LBRACK)
+	p.exprLev++
+	var index [N]ast.Expr
+	var colons [N - 1]token.Pos
+	if p.tok != token.COLON {
+		index[0] = p.parseRhs()
+	}
+	ncolons := 0
+	for p.tok == token.COLON && ncolons < len(colons) {
+		colons[ncolons] = p.pos
+		ncolons++
+		p.next()
+		if p.tok != token.COLON && p.tok != token.RBRACK && p.tok != token.EOF {
+			index[ncolons] = p.parseRhs()
+		}
+	}
+	p.exprLev--
+	rbrack := p.expect(token.RBRACK)
+
+	if ncolons > 0 {
+		// slice expression
+		slice3 := false
+		if ncolons == 2 {
+			slice3 = true
+			// Check presence of 2nd and 3rd index here rather than during type-checking
+			// to prevent erroneous programs from passing through gofmt (was issue 7305).
+			if index[1] == nil {
+				p.error(colons[0], "2nd index required in 3-index slice")
+				index[1] = &ast.BadExpr{From: colons[0] + 1, To: colons[1]}
+			}
+			if index[2] == nil {
+				p.error(colons[1], "3rd index required in 3-index slice")
+				index[2] = &ast.BadExpr{From: colons[1] + 1, To: rbrack}
+			}
+		}
+		return &ast.SliceExpr{X: x, Lbrack: lbrack, Low: index[0], High: index[1], Max: index[2], Slice3: slice3, Rbrack: rbrack}
+	}
+
+	return &ast.IndexExpr{X: x, Lbrack: lbrack, Index: index[0], Rbrack: rbrack}
+}
+
+func (p *parser) parseCallOrConversion(fun ast.Expr) *ast.CallExpr {
+	if p.trace {
+		defer un(trace(p, "CallOrConversion"))
+	}
+
+	lparen := p.expect(token.LPAREN)
+	p.exprLev++
+	var list []ast.Expr
+	var ellipsis token.Pos
+	for p.tok != token.RPAREN && p.tok != token.EOF && !ellipsis.IsValid() {
+		list = append(list, p.parseRhsOrType()) // builtins may expect a type: make(some type, ...)
+		if p.tok == token.ELLIPSIS {
+			ellipsis = p.pos
+			p.next()
+		}
+		if !p.atComma("argument list") {
+			break
+		}
+		p.next()
+	}
+	p.exprLev--
+	rparen := p.expectClosing(token.RPAREN, "argument list")
+
+	return &ast.CallExpr{Fun: fun, Lparen: lparen, Args: list, Ellipsis: ellipsis, Rparen: rparen}
+}
+
+func (p *parser) parseElement(keyOk bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Element"))
+	}
+
+	if p.tok == token.LBRACE {
+		return p.parseLiteralValue(nil)
+	}
+
+	// Because the parser doesn't know the composite literal type, it cannot
+	// know if a key that's an identifier is a struct field name or a name
+	// denoting a value. The former is not resolved by the parser or the
+	// resolver.
+	//
+	// Instead, _try_ to resolve such a key if possible. If it resolves,
+	// it a) has correctly resolved, or b) incorrectly resolved because
+	// the key is a struct field with a name matching another identifier.
+	// In the former case we are done, and in the latter case we don't
+	// care because the type checker will do a separate field lookup.
+	//
+	// If the key does not resolve, it a) must be defined at the top
+	// level in another file of the same package, the universe scope, or be
+	// undeclared; or b) it is a struct field. In the former case, the type
+	// checker can do a top-level lookup, and in the latter case it will do
+	// a separate field lookup.
+	x := p.checkExpr(p.parseExpr(keyOk))
+	if keyOk {
+		if p.tok == token.COLON {
+			colon := p.pos
+			p.next()
+			// Try to resolve the key but don't collect it
+			// as unresolved identifier if it fails so that
+			// we don't get (possibly false) errors about
+			// undeclared names.
+			p.tryResolve(x, false)
+			return &ast.KeyValueExpr{Key: x, Colon: colon, Value: p.parseElement(false)}
+		}
+		p.resolve(x) // not a key
+	}
+
+	return x
+}
+
+func (p *parser) parseElementList() (list []ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "ElementList"))
+	}
+
+	for p.tok != token.RBRACE && p.tok != token.EOF {
+		list = append(list, p.parseElement(true))
+		if !p.atComma("composite literal") {
+			break
+		}
+		p.next()
+	}
+
+	return
+}
+
+func (p *parser) parseLiteralValue(typ ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "LiteralValue"))
+	}
+
+	lbrace := p.expect(token.LBRACE)
+	var elts []ast.Expr
+	p.exprLev++
+	if p.tok != token.RBRACE {
+		elts = p.parseElementList()
+	}
+	p.exprLev--
+	rbrace := p.expectClosing(token.RBRACE, "composite literal")
+	return &ast.CompositeLit{Type: typ, Lbrace: lbrace, Elts: elts, Rbrace: rbrace}
+}
+
+// checkExpr checks that x is an expression (and not a type).
+func (p *parser) checkExpr(x ast.Expr) ast.Expr {
+	switch unparen(x).(type) {
+	case *ast.BadExpr:
+	case *ast.Ident:
+	case *ast.BasicLit:
+	case *ast.FuncLit:
+	case *ast.CompositeLit:
+	case *ast.ParenExpr:
+		panic("unreachable")
+	case *ast.SelectorExpr:
+	case *ast.IndexExpr:
+	case *ast.SliceExpr:
+	case *ast.TypeAssertExpr:
+		// If t.Type == nil we have a type assertion of the form
+		// y.(type), which is only allowed in type switch expressions.
+		// It's hard to exclude those but for the case where we are in
+		// a type switch. Instead be lenient and test this in the type
+		// checker.
+	case *ast.CallExpr:
+	case *ast.StarExpr:
+	case *ast.UnaryExpr:
+	case *ast.BinaryExpr:
+	default:
+		// all other nodes are not proper expressions
+		p.errorExpected(x.Pos(), "expression")
+		x = &ast.BadExpr{From: x.Pos(), To: p.safePos(x.End())}
+	}
+	return x
+}
+
+// isTypeName returns true iff x is a (qualified) TypeName.
+func isTypeName(x ast.Expr) bool {
+	switch t := x.(type) {
+	case *ast.BadExpr:
+	case *ast.Ident:
+	case *ast.SelectorExpr:
+		_, isIdent := t.X.(*ast.Ident)
+		return isIdent
+	default:
+		return false // all other nodes are not type names
+	}
+	return true
+}
+
+// isLiteralType returns true iff x is a legal composite literal type.
+func isLiteralType(x ast.Expr) bool {
+	switch t := x.(type) {
+	case *ast.BadExpr:
+	case *ast.Ident:
+	case *ast.SelectorExpr:
+		_, isIdent := t.X.(*ast.Ident)
+		return isIdent
+	case *ast.ArrayType:
+	case *ast.StructType:
+	case *ast.MapType:
+	default:
+		return false // all other nodes are not legal composite literal types
+	}
+	return true
+}
+
+// If x is of the form *T, deref returns T, otherwise it returns x.
+func deref(x ast.Expr) ast.Expr {
+	if p, isPtr := x.(*ast.StarExpr); isPtr {
+		x = p.X
+	}
+	return x
+}
+
+// If x is of the form (T), unparen returns unparen(T), otherwise it returns x.
+func unparen(x ast.Expr) ast.Expr {
+	if p, isParen := x.(*ast.ParenExpr); isParen {
+		x = unparen(p.X)
+	}
+	return x
+}
+
+// checkExprOrType checks that x is an expression or a type
+// (and not a raw type such as [...]T).
+//
+func (p *parser) checkExprOrType(x ast.Expr) ast.Expr {
+	switch t := unparen(x).(type) {
+	case *ast.ParenExpr:
+		panic("unreachable")
+	case *ast.UnaryExpr:
+	case *ast.ArrayType:
+		if len, isEllipsis := t.Len.(*ast.Ellipsis); isEllipsis {
+			p.error(len.Pos(), "expected array length, found '...'")
+			x = &ast.BadExpr{From: x.Pos(), To: p.safePos(x.End())}
+		}
+	}
+
+	// all other nodes are expressions or types
+	return x
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+func (p *parser) parsePrimaryExpr(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "PrimaryExpr"))
+	}
+
+	x := p.parseOperand(lhs)
+L:
+	for {
+		switch p.tok {
+		case token.PERIOD:
+			p.next()
+			if lhs {
+				p.resolve(x)
+			}
+			switch p.tok {
+			case token.IDENT:
+				x = p.parseSelector(p.checkExprOrType(x))
+			case token.LPAREN:
+				x = p.parseTypeAssertion(p.checkExpr(x))
+			default:
+				pos := p.pos
+				p.errorExpected(pos, "selector or type assertion")
+				p.next() // make progress
+				x = &ast.BadExpr{From: pos, To: p.pos}
+			}
+		case token.LBRACK:
+			if lhs {
+				p.resolve(x)
+			}
+			x = p.parseIndexOrSlice(p.checkExpr(x))
+		case token.LPAREN:
+			if lhs {
+				p.resolve(x)
+			}
+			x = p.parseCallOrConversion(p.checkExprOrType(x))
+		case token.LBRACE:
+			if isLiteralType(x) && (p.exprLev >= 0 || !isTypeName(x)) {
+				if lhs {
+					p.resolve(x)
+				}
+				x = p.parseLiteralValue(x)
+			} else {
+				break L
+			}
+		default:
+			break L
+		}
+		lhs = false // no need to try to resolve again
+	}
+
+	return x
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+func (p *parser) parseUnaryExpr(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "UnaryExpr"))
+	}
+
+	switch p.tok {
+	case token.ADD, token.SUB, token.NOT, token.XOR, token.AND:
+		pos, op := p.pos, p.tok
+		p.next()
+		x := p.parseUnaryExpr(false)
+		return &ast.UnaryExpr{OpPos: pos, Op: op, X: p.checkExpr(x)}
+
+	case token.ARROW:
+		// channel type or receive expression
+		arrow := p.pos
+		p.next()
+
+		// If the next token is token.CHAN we still don't know if it
+		// is a channel type or a receive operation - we only know
+		// once we have found the end of the unary expression. There
+		// are two cases:
+		//
+		//   <- type  => (<-type) must be channel type
+		//   <- expr  => <-(expr) is a receive from an expression
+		//
+		// In the first case, the arrow must be re-associated with
+		// the channel type parsed already:
+		//
+		//   <- (chan type)    =>  (<-chan type)
+		//   <- (chan<- type)  =>  (<-chan (<-type))
+
+		x := p.parseUnaryExpr(false)
+
+		// determine which case we have
+		if typ, ok := x.(*ast.ChanType); ok {
+			// (<-type)
+
+			// re-associate position info and <-
+			dir := ast.SEND
+			for ok && dir == ast.SEND {
+				if typ.Dir == ast.RECV {
+					// error: (<-type) is (<-(<-chan T))
+					p.errorExpected(typ.Arrow, "'chan'")
+				}
+				arrow, typ.Begin, typ.Arrow = typ.Arrow, arrow, arrow
+				dir, typ.Dir = typ.Dir, ast.RECV
+				typ, ok = typ.Value.(*ast.ChanType)
+			}
+			if dir == ast.SEND {
+				p.errorExpected(arrow, "channel type")
+			}
+
+			return x
+		}
+
+		// <-(expr)
+		return &ast.UnaryExpr{OpPos: arrow, Op: token.ARROW, X: p.checkExpr(x)}
+
+	case token.MUL:
+		// pointer type or unary "*" expression
+		pos := p.pos
+		p.next()
+		x := p.parseUnaryExpr(false)
+		return &ast.StarExpr{Star: pos, X: p.checkExprOrType(x)}
+	}
+
+	return p.parsePrimaryExpr(lhs)
+}
+
+func (p *parser) tokPrec() (token.Token, int) {
+	tok := p.tok
+	if p.inRhs && tok == token.ASSIGN {
+		tok = token.EQL
+	}
+	return tok, tok.Precedence()
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+func (p *parser) parseBinaryExpr(lhs bool, prec1 int) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "BinaryExpr"))
+	}
+
+	x := p.parseUnaryExpr(lhs)
+	for _, prec := p.tokPrec(); prec >= prec1; prec-- {
+		for {
+			op, oprec := p.tokPrec()
+			if oprec != prec {
+				break
+			}
+			pos := p.expect(op)
+			if lhs {
+				p.resolve(x)
+				lhs = false
+			}
+			y := p.parseBinaryExpr(false, prec+1)
+			x = &ast.BinaryExpr{X: p.checkExpr(x), OpPos: pos, Op: op, Y: p.checkExpr(y)}
+		}
+	}
+
+	return x
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+// The result may be a type or even a raw type ([...]int). Callers must
+// check the result (using checkExpr or checkExprOrType), depending on
+// context.
+func (p *parser) parseExpr(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Expression"))
+	}
+
+	return p.parseBinaryExpr(lhs, token.LowestPrec+1)
+}
+
+func (p *parser) parseRhs() ast.Expr {
+	old := p.inRhs
+	p.inRhs = true
+	x := p.checkExpr(p.parseExpr(false))
+	p.inRhs = old
+	return x
+}
+
+func (p *parser) parseRhsOrType() ast.Expr {
+	old := p.inRhs
+	p.inRhs = true
+	x := p.checkExprOrType(p.parseExpr(false))
+	p.inRhs = old
+	return x
+}
+
+// ----------------------------------------------------------------------------
+// Statements
+
+// Parsing modes for parseSimpleStmt.
+const (
+	basic = iota
+	labelOk
+	rangeOk
+)
+
+// parseSimpleStmt returns true as 2nd result if it parsed the assignment
+// of a range clause (with mode == rangeOk). The returned statement is an
+// assignment with a right-hand side that is a single unary expression of
+// the form "range x". No guarantees are given for the left-hand side.
+func (p *parser) parseSimpleStmt(mode int) (ast.Stmt, bool) {
+	if p.trace {
+		defer un(trace(p, "SimpleStmt"))
+	}
+
+	x := p.parseLhsList()
+
+	switch p.tok {
+	case
+		token.DEFINE, token.ASSIGN, token.ADD_ASSIGN,
+		token.SUB_ASSIGN, token.MUL_ASSIGN, token.QUO_ASSIGN,
+		token.REM_ASSIGN, token.AND_ASSIGN, token.OR_ASSIGN,
+		token.XOR_ASSIGN, token.SHL_ASSIGN, token.SHR_ASSIGN, token.AND_NOT_ASSIGN:
+		// assignment statement, possibly part of a range clause
+		pos, tok := p.pos, p.tok
+		p.next()
+		var y []ast.Expr
+		isRange := false
+		if mode == rangeOk && p.tok == token.RANGE && (tok == token.DEFINE || tok == token.ASSIGN) {
+			pos := p.pos
+			p.next()
+			y = []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
+			isRange = true
+		} else {
+			y = p.parseRhsList()
+		}
+		as := &ast.AssignStmt{Lhs: x, TokPos: pos, Tok: tok, Rhs: y}
+		if tok == token.DEFINE {
+			p.shortVarDecl(as, x)
+		}
+		return as, isRange
+	}
+
+	if len(x) > 1 {
+		p.errorExpected(x[0].Pos(), "1 expression")
+		// continue with first expression
+	}
+
+	switch p.tok {
+	case token.COLON:
+		// labeled statement
+		colon := p.pos
+		p.next()
+		if label, isIdent := x[0].(*ast.Ident); mode == labelOk && isIdent {
+			// Go spec: The scope of a label is the body of the function
+			// in which it is declared and excludes the body of any nested
+			// function.
+			stmt := &ast.LabeledStmt{Label: label, Colon: colon, Stmt: p.parseStmt()}
+			p.declare(stmt, nil, p.labelScope, ast.Lbl, label)
+			return stmt, false
+		}
+		// The label declaration typically starts at x[0].Pos(), but the label
+		// declaration may be erroneous due to a token after that position (and
+		// before the ':'). If SpuriousErrors is not set, the (only) error re-
+		// ported for the line is the illegal label error instead of the token
+		// before the ':' that caused the problem. Thus, use the (latest) colon
+		// position for error reporting.
+		p.error(colon, "illegal label declaration")
+		return &ast.BadStmt{From: x[0].Pos(), To: colon + 1}, false
+
+	case token.ARROW:
+		// send statement
+		arrow := p.pos
+		p.next()
+		y := p.parseRhs()
+		return &ast.SendStmt{Chan: x[0], Arrow: arrow, Value: y}, false
+
+	case token.INC, token.DEC:
+		// increment or decrement
+		s := &ast.IncDecStmt{X: x[0], TokPos: p.pos, Tok: p.tok}
+		p.next()
+		return s, false
+	}
+
+	// expression
+	return &ast.ExprStmt{X: x[0]}, false
+}
+
+func (p *parser) parseCallExpr(callType string) *ast.CallExpr {
+	x := p.parseRhsOrType() // could be a conversion: (some type)(x)
+	if call, isCall := x.(*ast.CallExpr); isCall {
+		return call
+	}
+	if _, isBad := x.(*ast.BadExpr); !isBad {
+		// only report error if it's a new one
+		p.error(p.safePos(x.End()), fmt.Sprintf("function must be invoked in %s statement", callType))
+	}
+	return nil
+}
+
+func (p *parser) parseGoStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "GoStmt"))
+	}
+
+	pos := p.expect(token.GO)
+	call := p.parseCallExpr("go")
+	p.expectSemi()
+	if call == nil {
+		return &ast.BadStmt{From: pos, To: pos + 2} // len("go")
+	}
+
+	return &ast.GoStmt{Go: pos, Call: call}
+}
+
+func (p *parser) parseDeferStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "DeferStmt"))
+	}
+
+	pos := p.expect(token.DEFER)
+	call := p.parseCallExpr("defer")
+	p.expectSemi()
+	if call == nil {
+		return &ast.BadStmt{From: pos, To: pos + 5} // len("defer")
+	}
+
+	return &ast.DeferStmt{Defer: pos, Call: call}
+}
+
+func (p *parser) parseReturnStmt() *ast.ReturnStmt {
+	if p.trace {
+		defer un(trace(p, "ReturnStmt"))
+	}
+
+	pos := p.pos
+	p.expect(token.RETURN)
+	var x []ast.Expr
+	if p.tok != token.SEMICOLON && p.tok != token.RBRACE {
+		x = p.parseRhsList()
+	}
+	p.expectSemi()
+
+	return &ast.ReturnStmt{Return: pos, Results: x}
+}
+
+func (p *parser) parseBranchStmt(tok token.Token) *ast.BranchStmt {
+	if p.trace {
+		defer un(trace(p, "BranchStmt"))
+	}
+
+	pos := p.expect(tok)
+	var label *ast.Ident
+	if tok != token.FALLTHROUGH && p.tok == token.IDENT {
+		label = p.parseIdent()
+		// add to list of unresolved targets
+		n := len(p.targetStack) - 1
+		p.targetStack[n] = append(p.targetStack[n], label)
+	}
+	p.expectSemi()
+
+	return &ast.BranchStmt{TokPos: pos, Tok: tok, Label: label}
+}
+
+func (p *parser) makeExpr(s ast.Stmt, kind string) ast.Expr {
+	if s == nil {
+		return nil
+	}
+	if es, isExpr := s.(*ast.ExprStmt); isExpr {
+		return p.checkExpr(es.X)
+	}
+	p.error(s.Pos(), fmt.Sprintf("expected %s, found simple statement (missing parentheses around composite literal?)", kind))
+	return &ast.BadExpr{From: s.Pos(), To: p.safePos(s.End())}
+}
+
+func (p *parser) parseIfStmt() *ast.IfStmt {
+	if p.trace {
+		defer un(trace(p, "IfStmt"))
+	}
+
+	pos := p.expect(token.IF)
+	p.openScope()
+	defer p.closeScope()
+
+	var s ast.Stmt
+	var x ast.Expr
+	{
+		prevLev := p.exprLev
+		p.exprLev = -1
+		if p.tok == token.SEMICOLON {
+			p.next()
+			x = p.parseRhs()
+		} else {
+			s, _ = p.parseSimpleStmt(basic)
+			if p.tok == token.SEMICOLON {
+				p.next()
+				x = p.parseRhs()
+			} else {
+				x = p.makeExpr(s, "boolean expression")
+				s = nil
+			}
+		}
+		p.exprLev = prevLev
+	}
+
+	body := p.parseBlockStmt()
+	var else_ ast.Stmt
+	if p.tok == token.ELSE {
+		p.next()
+		else_ = p.parseStmt()
+	} else {
+		p.expectSemi()
+	}
+
+	return &ast.IfStmt{If: pos, Init: s, Cond: x, Body: body, Else: else_}
+}
+
+func (p *parser) parseTypeList() (list []ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "TypeList"))
+	}
+
+	list = append(list, p.parseType())
+	for p.tok == token.COMMA {
+		p.next()
+		list = append(list, p.parseType())
+	}
+
+	return
+}
+
+func (p *parser) parseCaseClause(typeSwitch bool) *ast.CaseClause {
+	if p.trace {
+		defer un(trace(p, "CaseClause"))
+	}
+
+	pos := p.pos
+	var list []ast.Expr
+	if p.tok == token.CASE {
+		p.next()
+		if typeSwitch {
+			list = p.parseTypeList()
+		} else {
+			list = p.parseRhsList()
+		}
+	} else {
+		p.expect(token.DEFAULT)
+	}
+
+	colon := p.expect(token.COLON)
+	p.openScope()
+	body := p.parseStmtList()
+	p.closeScope()
+
+	return &ast.CaseClause{Case: pos, List: list, Colon: colon, Body: body}
+}
+
+func isTypeSwitchAssert(x ast.Expr) bool {
+	a, ok := x.(*ast.TypeAssertExpr)
+	return ok && a.Type == nil
+}
+
+func isTypeSwitchGuard(s ast.Stmt) bool {
+	switch t := s.(type) {
+	case *ast.ExprStmt:
+		// x.(nil)
+		return isTypeSwitchAssert(t.X)
+	case *ast.AssignStmt:
+		// v := x.(nil)
+		return len(t.Lhs) == 1 && t.Tok == token.DEFINE && len(t.Rhs) == 1 && isTypeSwitchAssert(t.Rhs[0])
+	}
+	return false
+}
+
+func (p *parser) parseSwitchStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "SwitchStmt"))
+	}
+
+	pos := p.expect(token.SWITCH)
+	p.openScope()
+	defer p.closeScope()
+
+	var s1, s2 ast.Stmt
+	if p.tok != token.LBRACE {
+		prevLev := p.exprLev
+		p.exprLev = -1
+		if p.tok != token.SEMICOLON {
+			s2, _ = p.parseSimpleStmt(basic)
+		}
+		if p.tok == token.SEMICOLON {
+			p.next()
+			s1 = s2
+			s2 = nil
+			if p.tok != token.LBRACE {
+				// A TypeSwitchGuard may declare a variable in addition
+				// to the variable declared in the initial SimpleStmt.
+				// Introduce extra scope to avoid redeclaration errors:
+				//
+				//	switch t := 0; t := x.(T) { ... }
+				//
+				// (this code is not valid Go because the first t
+				// cannot be accessed and thus is never used, the extra
+				// scope is needed for the correct error message).
+				//
+				// If we don't have a type switch, s2 must be an expression.
+				// Having the extra nested but empty scope won't affect it.
+				p.openScope()
+				defer p.closeScope()
+				s2, _ = p.parseSimpleStmt(basic)
+			}
+		}
+		p.exprLev = prevLev
+	}
+
+	typeSwitch := isTypeSwitchGuard(s2)
+	lbrace := p.expect(token.LBRACE)
+	var list []ast.Stmt
+	for p.tok == token.CASE || p.tok == token.DEFAULT {
+		list = append(list, p.parseCaseClause(typeSwitch))
+	}
+	rbrace := p.expect(token.RBRACE)
+	p.expectSemi()
+	body := &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
+
+	if typeSwitch {
+		return &ast.TypeSwitchStmt{Switch: pos, Init: s1, Assign: s2, Body: body}
+	}
+
+	return &ast.SwitchStmt{Switch: pos, Init: s1, Tag: p.makeExpr(s2, "switch expression"), Body: body}
+}
+
+func (p *parser) parseCommClause() *ast.CommClause {
+	if p.trace {
+		defer un(trace(p, "CommClause"))
+	}
+
+	p.openScope()
+	pos := p.pos
+	var comm ast.Stmt
+	if p.tok == token.CASE {
+		p.next()
+		lhs := p.parseLhsList()
+		if p.tok == token.ARROW {
+			// SendStmt
+			if len(lhs) > 1 {
+				p.errorExpected(lhs[0].Pos(), "1 expression")
+				// continue with first expression
+			}
+			arrow := p.pos
+			p.next()
+			rhs := p.parseRhs()
+			comm = &ast.SendStmt{Chan: lhs[0], Arrow: arrow, Value: rhs}
+		} else {
+			// RecvStmt
+			if tok := p.tok; tok == token.ASSIGN || tok == token.DEFINE {
+				// RecvStmt with assignment
+				if len(lhs) > 2 {
+					p.errorExpected(lhs[0].Pos(), "1 or 2 expressions")
+					// continue with first two expressions
+					lhs = lhs[0:2]
+				}
+				pos := p.pos
+				p.next()
+				rhs := p.parseRhs()
+				as := &ast.AssignStmt{Lhs: lhs, TokPos: pos, Tok: tok, Rhs: []ast.Expr{rhs}}
+				if tok == token.DEFINE {
+					p.shortVarDecl(as, lhs)
+				}
+				comm = as
+			} else {
+				// lhs must be single receive operation
+				if len(lhs) > 1 {
+					p.errorExpected(lhs[0].Pos(), "1 expression")
+					// continue with first expression
+				}
+				comm = &ast.ExprStmt{X: lhs[0]}
+			}
+		}
+	} else {
+		p.expect(token.DEFAULT)
+	}
+
+	colon := p.expect(token.COLON)
+	body := p.parseStmtList()
+	p.closeScope()
+
+	return &ast.CommClause{Case: pos, Comm: comm, Colon: colon, Body: body}
+}
+
+func (p *parser) parseSelectStmt() *ast.SelectStmt {
+	if p.trace {
+		defer un(trace(p, "SelectStmt"))
+	}
+
+	pos := p.expect(token.SELECT)
+	lbrace := p.expect(token.LBRACE)
+	var list []ast.Stmt
+	for p.tok == token.CASE || p.tok == token.DEFAULT {
+		list = append(list, p.parseCommClause())
+	}
+	rbrace := p.expect(token.RBRACE)
+	p.expectSemi()
+	body := &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
+
+	return &ast.SelectStmt{Select: pos, Body: body}
+}
+
+func (p *parser) parseForStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "ForStmt"))
+	}
+
+	pos := p.expect(token.FOR)
+	p.openScope()
+	defer p.closeScope()
+
+	var s1, s2, s3 ast.Stmt
+	var isRange bool
+	if p.tok != token.LBRACE {
+		prevLev := p.exprLev
+		p.exprLev = -1
+		if p.tok != token.SEMICOLON {
+			if p.tok == token.RANGE {
+				// "for range x" (nil lhs in assignment)
+				pos := p.pos
+				p.next()
+				y := []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
+				s2 = &ast.AssignStmt{Rhs: y}
+				isRange = true
+			} else {
+				s2, isRange = p.parseSimpleStmt(rangeOk)
+			}
+		}
+		if !isRange && p.tok == token.SEMICOLON {
+			p.next()
+			s1 = s2
+			s2 = nil
+			if p.tok != token.SEMICOLON {
+				s2, _ = p.parseSimpleStmt(basic)
+			}
+			p.expectSemi()
+			if p.tok != token.LBRACE {
+				s3, _ = p.parseSimpleStmt(basic)
+			}
+		}
+		p.exprLev = prevLev
+	}
+
+	body := p.parseBlockStmt()
+	p.expectSemi()
+
+	if isRange {
+		as := s2.(*ast.AssignStmt)
+		// check lhs
+		var key, value ast.Expr
+		switch len(as.Lhs) {
+		case 0:
+			// nothing to do
+		case 1:
+			key = as.Lhs[0]
+		case 2:
+			key, value = as.Lhs[0], as.Lhs[1]
+		default:
+			p.errorExpected(as.Lhs[len(as.Lhs)-1].Pos(), "at most 2 expressions")
+			return &ast.BadStmt{From: pos, To: p.safePos(body.End())}
+		}
+		// parseSimpleStmt returned a right-hand side that
+		// is a single unary expression of the form "range x"
+		x := as.Rhs[0].(*ast.UnaryExpr).X
+		return &ast.RangeStmt{
+			For:    pos,
+			Key:    key,
+			Value:  value,
+			TokPos: as.TokPos,
+			Tok:    as.Tok,
+			X:      x,
+			Body:   body,
+		}
+	}
+
+	// regular for statement
+	return &ast.ForStmt{
+		For:  pos,
+		Init: s1,
+		Cond: p.makeExpr(s2, "boolean or range expression"),
+		Post: s3,
+		Body: body,
+	}
+}
+
+func (p *parser) parseStmt() (s ast.Stmt) {
+	if p.trace {
+		defer un(trace(p, "Statement"))
+	}
+
+	switch p.tok {
+	case token.CONST, token.TYPE, token.VAR:
+		s = &ast.DeclStmt{Decl: p.parseDecl(syncStmt)}
+	case
+		// tokens that may start an expression
+		token.IDENT, token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING, token.FUNC, token.LPAREN, // operands
+		token.LBRACK, token.STRUCT, // composite types
+		token.ADD, token.SUB, token.MUL, token.AND, token.XOR, token.ARROW, token.NOT: // unary operators
+		s, _ = p.parseSimpleStmt(labelOk)
+		// because of the required look-ahead, labeled statements are
+		// parsed by parseSimpleStmt - don't expect a semicolon after
+		// them
+		if _, isLabeledStmt := s.(*ast.LabeledStmt); !isLabeledStmt {
+			p.expectSemi()
+		}
+	case token.GO:
+		s = p.parseGoStmt()
+	case token.DEFER:
+		s = p.parseDeferStmt()
+	case token.RETURN:
+		s = p.parseReturnStmt()
+	case token.BREAK, token.CONTINUE, token.GOTO, token.FALLTHROUGH:
+		s = p.parseBranchStmt(p.tok)
+	case token.LBRACE:
+		s = p.parseBlockStmt()
+		p.expectSemi()
+	case token.IF:
+		s = p.parseIfStmt()
+	case token.SWITCH:
+		s = p.parseSwitchStmt()
+	case token.SELECT:
+		s = p.parseSelectStmt()
+	case token.FOR:
+		s = p.parseForStmt()
+	case token.SEMICOLON:
+		s = &ast.EmptyStmt{Semicolon: p.pos}
+		p.next()
+	case token.RBRACE:
+		// a semicolon may be omitted before a closing "}"
+		s = &ast.EmptyStmt{Semicolon: p.pos}
+	default:
+		// no statement found
+		pos := p.pos
+		p.errorExpected(pos, "statement")
+		syncStmt(p)
+		s = &ast.BadStmt{From: pos, To: p.pos}
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Declarations
+
+type parseSpecFunction func(doc *ast.CommentGroup, keyword token.Token, iota int) ast.Spec
+
+func isValidImport(lit string) bool {
+	const illegalChars = `!"#$%&'()*,:;<=>?[\]^{|}` + "`\uFFFD"
+	s, _ := strconv.Unquote(lit) // go/scanner returns a legal string literal
+	for _, r := range s {
+		if !unicode.IsGraphic(r) || unicode.IsSpace(r) || strings.ContainsRune(illegalChars, r) {
+			return false
+		}
+	}
+	return s != ""
+}
+
+func (p *parser) parseImportSpec(doc *ast.CommentGroup, _ token.Token, _ int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, "ImportSpec"))
+	}
+
+	var ident *ast.Ident
+	switch p.tok {
+	case token.PERIOD:
+		ident = &ast.Ident{NamePos: p.pos, Name: "."}
+		p.next()
+	case token.IDENT:
+		ident = p.parseIdent()
+	}
+
+	pos := p.pos
+	var path string
+	if p.tok == token.STRING {
+		path = p.lit
+		if !isValidImport(path) {
+			p.error(pos, "invalid import path: "+path)
+		}
+		p.next()
+	} else {
+		p.expect(token.STRING) // use expect() error handling
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	// collect imports
+	spec := &ast.ImportSpec{
+		Doc:     doc,
+		Name:    ident,
+		Path:    &ast.BasicLit{ValuePos: pos, Kind: token.STRING, Value: path},
+		Comment: p.lineComment,
+	}
+	p.imports = append(p.imports, spec)
+
+	return spec
+}
+
+func (p *parser) parseValueSpec(doc *ast.CommentGroup, keyword token.Token, iota int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, keyword.String()+"Spec"))
+	}
+
+	idents := p.parseIdentList()
+	typ := p.tryType()
+	var values []ast.Expr
+	// always permit optional initialization for more tolerant parsing
+	if p.tok == token.ASSIGN {
+		p.next()
+		values = p.parseRhsList()
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	// Go spec: The scope of a constant or variable identifier declared inside
+	// a function begins at the end of the ConstSpec or VarSpec and ends at
+	// the end of the innermost containing block.
+	// (Global identifiers are resolved in a separate phase after parsing.)
+	spec := &ast.ValueSpec{
+		Doc:     doc,
+		Names:   idents,
+		Type:    typ,
+		Values:  values,
+		Comment: p.lineComment,
+	}
+	kind := ast.Con
+	if keyword == token.VAR {
+		kind = ast.Var
+	}
+	p.declare(spec, iota, p.topScope, kind, idents...)
+
+	return spec
+}
+
+func (p *parser) parseTypeSpec(doc *ast.CommentGroup, _ token.Token, _ int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, "TypeSpec"))
+	}
+
+	ident := p.parseIdent()
+
+	// Go spec: The scope of a type identifier declared inside a function begins
+	// at the identifier in the TypeSpec and ends at the end of the innermost
+	// containing block.
+	// (Global identifiers are resolved in a separate phase after parsing.)
+	spec := &ast.TypeSpec{Doc: doc, Name: ident}
+	p.declare(spec, nil, p.topScope, ast.Typ, ident)
+
+	spec.Type = p.parseType()
+	p.expectSemi() // call before accessing p.linecomment
+	spec.Comment = p.lineComment
+
+	return spec
+}
+
+func (p *parser) parseGenDecl(keyword token.Token, f parseSpecFunction) *ast.GenDecl {
+	if p.trace {
+		defer un(trace(p, "GenDecl("+keyword.String()+")"))
+	}
+
+	doc := p.leadComment
+	pos := p.expect(keyword)
+	var lparen, rparen token.Pos
+	var list []ast.Spec
+	if p.tok == token.LPAREN {
+		lparen = p.pos
+		p.next()
+		for iota := 0; p.tok != token.RPAREN && p.tok != token.EOF; iota++ {
+			list = append(list, f(p.leadComment, keyword, iota))
+		}
+		rparen = p.expect(token.RPAREN)
+		p.expectSemi()
+	} else {
+		list = append(list, f(nil, keyword, 0))
+	}
+
+	return &ast.GenDecl{
+		Doc:    doc,
+		TokPos: pos,
+		Tok:    keyword,
+		Lparen: lparen,
+		Specs:  list,
+		Rparen: rparen,
+	}
+}
+
+func (p *parser) parseFuncDecl() *ast.FuncDecl {
+	if p.trace {
+		defer un(trace(p, "FunctionDecl"))
+	}
+
+	doc := p.leadComment
+	pos := p.expect(token.FUNC)
+	scope := ast.NewScope(p.topScope) // function scope
+
+	var recv *ast.FieldList
+	if p.tok == token.LPAREN {
+		recv = p.parseParameters(scope, false)
+	}
+
+	ident := p.parseIdent()
+
+	params, results := p.parseSignature(scope)
+
+	var body *ast.BlockStmt
+	if p.tok == token.LBRACE {
+		body = p.parseBody(scope)
+	}
+	p.expectSemi()
+
+	decl := &ast.FuncDecl{
+		Doc:  doc,
+		Recv: recv,
+		Name: ident,
+		Type: &ast.FuncType{
+			Func:    pos,
+			Params:  params,
+			Results: results,
+		},
+		Body: body,
+	}
+	if recv == nil {
+		// Go spec: The scope of an identifier denoting a constant, type,
+		// variable, or function (but not method) declared at top level
+		// (outside any function) is the package block.
+		//
+		// init() functions cannot be referred to and there may
+		// be more than one - don't put them in the pkgScope
+		if ident.Name != "init" {
+			p.declare(decl, nil, p.pkgScope, ast.Fun, ident)
+		}
+	}
+
+	return decl
+}
+
+func (p *parser) parseDecl(sync func(*parser)) ast.Decl {
+	if p.trace {
+		defer un(trace(p, "Declaration"))
+	}
+
+	var f parseSpecFunction
+	switch p.tok {
+	case token.CONST, token.VAR:
+		f = p.parseValueSpec
+
+	case token.TYPE:
+		f = p.parseTypeSpec
+
+	case token.FUNC:
+		return p.parseFuncDecl()
+
+	default:
+		pos := p.pos
+		p.errorExpected(pos, "declaration")
+		sync(p)
+		return &ast.BadDecl{From: pos, To: p.pos}
+	}
+
+	return p.parseGenDecl(p.tok, f)
+}
+
+// ----------------------------------------------------------------------------
+// Source files
+
+func (p *parser) parseFile() *ast.File {
+	if p.trace {
+		defer un(trace(p, "File"))
+	}
+
+	// Don't bother parsing the rest if we had errors scanning the first token.
+	// Likely not a Go source file at all.
+	if p.errors.Len() != 0 {
+		return nil
+	}
+
+	// package clause
+	doc := p.leadComment
+	pos := p.expect(token.PACKAGE)
+	// Go spec: The package clause is not a declaration;
+	// the package name does not appear in any scope.
+	ident := p.parseIdent()
+	if ident.Name == "_" && p.mode&DeclarationErrors != 0 {
+		p.error(p.pos, "invalid package name _")
+	}
+	p.expectSemi()
+
+	// Don't bother parsing the rest if we had errors parsing the package clause.
+	// Likely not a Go source file at all.
+	if p.errors.Len() != 0 {
+		return nil
+	}
+
+	p.openScope()
+	p.pkgScope = p.topScope
+	var decls []ast.Decl
+	if p.mode&PackageClauseOnly == 0 {
+		// import decls
+		for p.tok == token.IMPORT {
+			decls = append(decls, p.parseGenDecl(token.IMPORT, p.parseImportSpec))
+		}
+
+		if p.mode&ImportsOnly == 0 {
+			// rest of package body
+			for p.tok != token.EOF {
+				decls = append(decls, p.parseDecl(syncDecl))
+			}
+		}
+	}
+	p.closeScope()
+	assert(p.topScope == nil, "unbalanced scopes")
+	assert(p.labelScope == nil, "unbalanced label scopes")
+
+	// resolve global identifiers within the same file
+	i := 0
+	for _, ident := range p.unresolved {
+		// i <= index for current ident
+		assert(ident.Obj == unresolved, "object already resolved")
+		ident.Obj = p.pkgScope.Lookup(ident.Name) // also removes unresolved sentinel
+		if ident.Obj == nil {
+			p.unresolved[i] = ident
+			i++
+		}
+	}
+
+	return &ast.File{
+		Doc:        doc,
+		Package:    pos,
+		Name:       ident,
+		Decls:      decls,
+		Scope:      p.pkgScope,
+		Imports:    p.imports,
+		Unresolved: p.unresolved[0:i],
+		Comments:   p.comments,
+	}
+}
diff --git a/src/go/parser/parser_test.go b/src/go/parser/parser_test.go
new file mode 100644
index 000000000..85065fd18
--- /dev/null
+++ b/src/go/parser/parser_test.go
@@ -0,0 +1,449 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package parser
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/token"
+	"os"
+	"strings"
+	"testing"
+)
+
+var fset = token.NewFileSet()
+
+var validFiles = []string{
+	"parser.go",
+	"parser_test.go",
+	"error_test.go",
+	"short_test.go",
+}
+
+func TestParse(t *testing.T) {
+	for _, filename := range validFiles {
+		_, err := ParseFile(fset, filename, nil, DeclarationErrors)
+		if err != nil {
+			t.Fatalf("ParseFile(%s): %v", filename, err)
+		}
+	}
+}
+
+func nameFilter(filename string) bool {
+	switch filename {
+	case "parser.go", "interface.go", "parser_test.go":
+		return true
+	case "parser.go.orig":
+		return true // permit but should be ignored by ParseDir
+	}
+	return false
+}
+
+func dirFilter(f os.FileInfo) bool { return nameFilter(f.Name()) }
+
+func TestParseDir(t *testing.T) {
+	path := "."
+	pkgs, err := ParseDir(fset, path, dirFilter, 0)
+	if err != nil {
+		t.Fatalf("ParseDir(%s): %v", path, err)
+	}
+	if n := len(pkgs); n != 1 {
+		t.Errorf("got %d packages; want 1", n)
+	}
+	pkg := pkgs["parser"]
+	if pkg == nil {
+		t.Errorf(`package "parser" not found`)
+		return
+	}
+	if n := len(pkg.Files); n != 3 {
+		t.Errorf("got %d package files; want 3", n)
+	}
+	for filename := range pkg.Files {
+		if !nameFilter(filename) {
+			t.Errorf("unexpected package file: %s", filename)
+		}
+	}
+}
+
+func TestParseExpr(t *testing.T) {
+	// just kicking the tires:
+	// a valid arithmetic expression
+	src := "a + b"
+	x, err := ParseExpr(src)
+	if err != nil {
+		t.Errorf("ParseExpr(%q): %v", src, err)
+	}
+	// sanity check
+	if _, ok := x.(*ast.BinaryExpr); !ok {
+		t.Errorf("ParseExpr(%q): got %T, want *ast.BinaryExpr", src, x)
+	}
+
+	// a valid type expression
+	src = "struct{x *int}"
+	x, err = ParseExpr(src)
+	if err != nil {
+		t.Errorf("ParseExpr(%q): %v", src, err)
+	}
+	// sanity check
+	if _, ok := x.(*ast.StructType); !ok {
+		t.Errorf("ParseExpr(%q): got %T, want *ast.StructType", src, x)
+	}
+
+	// an invalid expression
+	src = "a + *"
+	if _, err := ParseExpr(src); err == nil {
+		t.Errorf("ParseExpr(%q): got no error", src)
+	}
+
+	// a valid expression followed by extra tokens is invalid
+	src = "a[i] := x"
+	if _, err := ParseExpr(src); err == nil {
+		t.Errorf("ParseExpr(%q): got no error", src)
+	}
+
+	// a semicolon is not permitted unless automatically inserted
+	src = "a + b\n"
+	if _, err := ParseExpr(src); err != nil {
+		t.Errorf("ParseExpr(%q): got error %s", src, err)
+	}
+	src = "a + b;"
+	if _, err := ParseExpr(src); err == nil {
+		t.Errorf("ParseExpr(%q): got no error", src)
+	}
+
+	// various other stuff following a valid expression
+	const validExpr = "a + b"
+	const anything = "dh3*#D)#_"
+	for _, c := range "!)]};," {
+		src := validExpr + string(c) + anything
+		if _, err := ParseExpr(src); err == nil {
+			t.Errorf("ParseExpr(%q): got no error", src)
+		}
+	}
+
+	// ParseExpr must not crash
+	for _, src := range valids {
+		ParseExpr(src)
+	}
+}
+
+func TestColonEqualsScope(t *testing.T) {
+	f, err := ParseFile(fset, "", `package p; func f() { x, y, z := x, y, z }`, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// RHS refers to undefined globals; LHS does not.
+	as := f.Decls[0].(*ast.FuncDecl).Body.List[0].(*ast.AssignStmt)
+	for _, v := range as.Rhs {
+		id := v.(*ast.Ident)
+		if id.Obj != nil {
+			t.Errorf("rhs %s has Obj, should not", id.Name)
+		}
+	}
+	for _, v := range as.Lhs {
+		id := v.(*ast.Ident)
+		if id.Obj == nil {
+			t.Errorf("lhs %s does not have Obj, should", id.Name)
+		}
+	}
+}
+
+func TestVarScope(t *testing.T) {
+	f, err := ParseFile(fset, "", `package p; func f() { var x, y, z = x, y, z }`, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// RHS refers to undefined globals; LHS does not.
+	as := f.Decls[0].(*ast.FuncDecl).Body.List[0].(*ast.DeclStmt).Decl.(*ast.GenDecl).Specs[0].(*ast.ValueSpec)
+	for _, v := range as.Values {
+		id := v.(*ast.Ident)
+		if id.Obj != nil {
+			t.Errorf("rhs %s has Obj, should not", id.Name)
+		}
+	}
+	for _, id := range as.Names {
+		if id.Obj == nil {
+			t.Errorf("lhs %s does not have Obj, should", id.Name)
+		}
+	}
+}
+
+func TestObjects(t *testing.T) {
+	const src = `
+package p
+import fmt "fmt"
+const pi = 3.14
+type T struct{}
+var x int
+func f() { L: }
+`
+
+	f, err := ParseFile(fset, "", src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	objects := map[string]ast.ObjKind{
+		"p":   ast.Bad, // not in a scope
+		"fmt": ast.Bad, // not resolved yet
+		"pi":  ast.Con,
+		"T":   ast.Typ,
+		"x":   ast.Var,
+		"int": ast.Bad, // not resolved yet
+		"f":   ast.Fun,
+		"L":   ast.Lbl,
+	}
+
+	ast.Inspect(f, func(n ast.Node) bool {
+		if ident, ok := n.(*ast.Ident); ok {
+			obj := ident.Obj
+			if obj == nil {
+				if objects[ident.Name] != ast.Bad {
+					t.Errorf("no object for %s", ident.Name)
+				}
+				return true
+			}
+			if obj.Name != ident.Name {
+				t.Errorf("names don't match: obj.Name = %s, ident.Name = %s", obj.Name, ident.Name)
+			}
+			kind := objects[ident.Name]
+			if obj.Kind != kind {
+				t.Errorf("%s: obj.Kind = %s; want %s", ident.Name, obj.Kind, kind)
+			}
+		}
+		return true
+	})
+}
+
+func TestUnresolved(t *testing.T) {
+	f, err := ParseFile(fset, "", `
+package p
+//
+func f1a(int)
+func f2a(byte, int, float)
+func f3a(a, b int, c float)
+func f4a(...complex)
+func f5a(a s1a, b ...complex)
+//
+func f1b(*int)
+func f2b([]byte, (int), *float)
+func f3b(a, b *int, c []float)
+func f4b(...*complex)
+func f5b(a s1a, b ...[]complex)
+//
+type s1a struct { int }
+type s2a struct { byte; int; s1a }
+type s3a struct { a, b int; c float }
+//
+type s1b struct { *int }
+type s2b struct { byte; int; *float }
+type s3b struct { a, b *s3b; c []float }
+`, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	want := "int " + // f1a
+		"byte int float " + // f2a
+		"int float " + // f3a
+		"complex " + // f4a
+		"complex " + // f5a
+		//
+		"int " + // f1b
+		"byte int float " + // f2b
+		"int float " + // f3b
+		"complex " + // f4b
+		"complex " + // f5b
+		//
+		"int " + // s1a
+		"byte int " + // s2a
+		"int float " + // s3a
+		//
+		"int " + // s1a
+		"byte int float " + // s2a
+		"float " // s3a
+
+	// collect unresolved identifiers
+	var buf bytes.Buffer
+	for _, u := range f.Unresolved {
+		buf.WriteString(u.Name)
+		buf.WriteByte(' ')
+	}
+	got := buf.String()
+
+	if got != want {
+		t.Errorf("\ngot:  %s\nwant: %s", got, want)
+	}
+}
+
+var imports = map[string]bool{
+	`"a"`:        true,
+	"`a`":        true,
+	`"a/b"`:      true,
+	`"a.b"`:      true,
+	`"m\x61th"`:  true,
+	`"greek/αβ"`: true,
+	`""`:         false,
+
+	// Each of these pairs tests both `` vs "" strings
+	// and also use of invalid characters spelled out as
+	// escape sequences and written directly.
+	// For example `"\x00"` tests import "\x00"
+	// while "`\x00`" tests import `<actual-NUL-byte>`.
+	`"\x00"`:     false,
+	"`\x00`":     false,
+	`"\x7f"`:     false,
+	"`\x7f`":     false,
+	`"a!"`:       false,
+	"`a!`":       false,
+	`"a b"`:      false,
+	"`a b`":      false,
+	`"a\\b"`:     false,
+	"`a\\b`":     false,
+	"\"`a`\"":    false,
+	"`\"a\"`":    false,
+	`"\x80\x80"`: false,
+	"`\x80\x80`": false,
+	`"\xFFFD"`:   false,
+	"`\xFFFD`":   false,
+}
+
+func TestImports(t *testing.T) {
+	for path, isValid := range imports {
+		src := fmt.Sprintf("package p; import %s", path)
+		_, err := ParseFile(fset, "", src, 0)
+		switch {
+		case err != nil && isValid:
+			t.Errorf("ParseFile(%s): got %v; expected no error", src, err)
+		case err == nil && !isValid:
+			t.Errorf("ParseFile(%s): got no error; expected one", src)
+		}
+	}
+}
+
+func TestCommentGroups(t *testing.T) {
+	f, err := ParseFile(fset, "", `
+package p /* 1a */ /* 1b */      /* 1c */ // 1d
+/* 2a
+*/
+// 2b
+const pi = 3.1415
+/* 3a */ // 3b
+/* 3c */ const e = 2.7182
+
+// Example from issue 3139
+func ExampleCount() {
+	fmt.Println(strings.Count("cheese", "e"))
+	fmt.Println(strings.Count("five", "")) // before & after each rune
+	// Output:
+	// 3
+	// 5
+}
+`, ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expected := [][]string{
+		{"/* 1a */", "/* 1b */", "/* 1c */", "// 1d"},
+		{"/* 2a\n*/", "// 2b"},
+		{"/* 3a */", "// 3b", "/* 3c */"},
+		{"// Example from issue 3139"},
+		{"// before & after each rune"},
+		{"// Output:", "// 3", "// 5"},
+	}
+	if len(f.Comments) != len(expected) {
+		t.Fatalf("got %d comment groups; expected %d", len(f.Comments), len(expected))
+	}
+	for i, exp := range expected {
+		got := f.Comments[i].List
+		if len(got) != len(exp) {
+			t.Errorf("got %d comments in group %d; expected %d", len(got), i, len(exp))
+			continue
+		}
+		for j, exp := range exp {
+			got := got[j].Text
+			if got != exp {
+				t.Errorf("got %q in group %d; expected %q", got, i, exp)
+			}
+		}
+	}
+}
+
+func getField(file *ast.File, fieldname string) *ast.Field {
+	parts := strings.Split(fieldname, ".")
+	for _, d := range file.Decls {
+		if d, ok := d.(*ast.GenDecl); ok && d.Tok == token.TYPE {
+			for _, s := range d.Specs {
+				if s, ok := s.(*ast.TypeSpec); ok && s.Name.Name == parts[0] {
+					if s, ok := s.Type.(*ast.StructType); ok {
+						for _, f := range s.Fields.List {
+							for _, name := range f.Names {
+								if name.Name == parts[1] {
+									return f
+								}
+							}
+						}
+					}
+				}
+			}
+		}
+	}
+	return nil
+}
+
+// Don't use ast.CommentGroup.Text() - we want to see exact comment text.
+func commentText(c *ast.CommentGroup) string {
+	var buf bytes.Buffer
+	if c != nil {
+		for _, c := range c.List {
+			buf.WriteString(c.Text)
+		}
+	}
+	return buf.String()
+}
+
+func checkFieldComments(t *testing.T, file *ast.File, fieldname, lead, line string) {
+	f := getField(file, fieldname)
+	if f == nil {
+		t.Fatalf("field not found: %s", fieldname)
+	}
+	if got := commentText(f.Doc); got != lead {
+		t.Errorf("got lead comment %q; expected %q", got, lead)
+	}
+	if got := commentText(f.Comment); got != line {
+		t.Errorf("got line comment %q; expected %q", got, line)
+	}
+}
+
+func TestLeadAndLineComments(t *testing.T) {
+	f, err := ParseFile(fset, "", `
+package p
+type T struct {
+	/* F1 lead comment */
+	//
+	F1 int  /* F1 */ // line comment
+	// F2 lead
+	// comment
+	F2 int  // F2 line comment
+	// f3 lead comment
+	f3 int  // f3 line comment
+}
+`, ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	checkFieldComments(t, f, "T.F1", "/* F1 lead comment *///", "/* F1 */// line comment")
+	checkFieldComments(t, f, "T.F2", "// F2 lead// comment", "// F2 line comment")
+	checkFieldComments(t, f, "T.f3", "// f3 lead comment", "// f3 line comment")
+	ast.FileExports(f)
+	checkFieldComments(t, f, "T.F1", "/* F1 lead comment *///", "/* F1 */// line comment")
+	checkFieldComments(t, f, "T.F2", "// F2 lead// comment", "// F2 line comment")
+	if getField(f, "T.f3") != nil {
+		t.Error("not expected to find T.f3")
+	}
+}
diff --git a/src/go/parser/performance_test.go b/src/go/parser/performance_test.go
new file mode 100644
index 000000000..f2732c0e2
--- /dev/null
+++ b/src/go/parser/performance_test.go
@@ -0,0 +1,30 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package parser
+
+import (
+	"go/token"
+	"io/ioutil"
+	"testing"
+)
+
+var src = readFile("parser.go")
+
+func readFile(filename string) []byte {
+	data, err := ioutil.ReadFile(filename)
+	if err != nil {
+		panic(err)
+	}
+	return data
+}
+
+func BenchmarkParse(b *testing.B) {
+	b.SetBytes(int64(len(src)))
+	for i := 0; i < b.N; i++ {
+		if _, err := ParseFile(token.NewFileSet(), "", src, ParseComments); err != nil {
+			b.Fatalf("benchmark failed due to parse error: %s", err)
+		}
+	}
+}
diff --git a/src/go/parser/short_test.go b/src/go/parser/short_test.go
new file mode 100644
index 000000000..f861086dd
--- /dev/null
+++ b/src/go/parser/short_test.go
@@ -0,0 +1,103 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains test cases for short valid and invalid programs.
+
+package parser
+
+import "testing"
+
+var valids = []string{
+	"package p\n",
+	`package p;`,
+	`package p; import "fmt"; func f() { fmt.Println("Hello, World!") };`,
+	`package p; func f() { if f(T{}) {} };`,
+	`package p; func f() { _ = <-chan int(nil) };`,
+	`package p; func f() { _ = (<-chan int)(nil) };`,
+	`package p; func f() { _ = (<-chan <-chan int)(nil) };`,
+	`package p; func f() { _ = <-chan <-chan <-chan <-chan <-int(nil) };`,
+	`package p; func f(func() func() func());`,
+	`package p; func f(...T);`,
+	`package p; func f(float, ...int);`,
+	`package p; func f(x int, a ...int) { f(0, a...); f(1, a...,) };`,
+	`package p; func f(int,) {};`,
+	`package p; func f(...int,) {};`,
+	`package p; func f(x ...int,) {};`,
+	`package p; type T []int; var a []bool; func f() { if a[T{42}[0]] {} };`,
+	`package p; type T []int; func g(int) bool { return true }; func f() { if g(T{42}[0]) {} };`,
+	`package p; type T []int; func f() { for _ = range []int{T{42}[0]} {} };`,
+	`package p; var a = T{{1, 2}, {3, 4}}`,
+	`package p; func f() { select { case <- c: case c <- d: case c <- <- d: case <-c <- d: } };`,
+	`package p; func f() { select { case x := (<-c): } };`,
+	`package p; func f() { if ; true {} };`,
+	`package p; func f() { switch ; {} };`,
+	`package p; func f() { for _ = range "foo" + "bar" {} };`,
+	`package p; func f() { var s []int; g(s[:], s[i:], s[:j], s[i:j], s[i:j:k], s[:j:k]) };`,
+	`package p; var ( _ = (struct {*T}).m; _ = (interface {T}).m )`,
+	`package p; func ((T),) m() {}`,
+	`package p; func ((*T),) m() {}`,
+	`package p; func (*(T),) m() {}`,
+	`package p; func _(x []int) { for range x {} }`,
+}
+
+func TestValid(t *testing.T) {
+	for _, src := range valids {
+		checkErrors(t, src, src)
+	}
+}
+
+var invalids = []string{
+	`foo /* ERROR "expected 'package'" */ !`,
+	`package p; func f() { if { /* ERROR "expected operand" */ } };`,
+	`package p; func f() { if ; { /* ERROR "expected operand" */ } };`,
+	`package p; func f() { if f(); { /* ERROR "expected operand" */ } };`,
+	`package p; func f() { if _ /* ERROR "expected boolean expression" */ = range x; true {} };`,
+	`package p; func f() { switch _ /* ERROR "expected switch expression" */ = range x; true {} };`,
+	`package p; func f() { for _ = range x ; /* ERROR "expected '{'" */ ; {} };`,
+	`package p; func f() { for ; ; _ = range /* ERROR "expected operand" */ x {} };`,
+	`package p; func f() { for ; _ /* ERROR "expected boolean or range expression" */ = range x ; {} };`,
+	`package p; func f() { switch t /* ERROR "expected switch expression" */ = t.(type) {} };`,
+	`package p; func f() { switch t /* ERROR "expected switch expression" */ , t = t.(type) {} };`,
+	`package p; func f() { switch t /* ERROR "expected switch expression" */ = t.(type), t {} };`,
+	`package p; var a = [ /* ERROR "expected expression" */ 1]int;`,
+	`package p; var a = [ /* ERROR "expected expression" */ ...]int;`,
+	`package p; var a = struct /* ERROR "expected expression" */ {}`,
+	`package p; var a = func /* ERROR "expected expression" */ ();`,
+	`package p; var a = interface /* ERROR "expected expression" */ {}`,
+	`package p; var a = [ /* ERROR "expected expression" */ ]int`,
+	`package p; var a = map /* ERROR "expected expression" */ [int]int`,
+	`package p; var a = chan /* ERROR "expected expression" */ int;`,
+	`package p; var a = []int{[ /* ERROR "expected expression" */ ]int};`,
+	`package p; var a = ( /* ERROR "expected expression" */ []int);`,
+	`package p; var a = a[[ /* ERROR "expected expression" */ ]int:[]int];`,
+	`package p; var a = <- /* ERROR "expected expression" */ chan int;`,
+	`package p; func f() { select { case _ <- chan /* ERROR "expected expression" */ int: } };`,
+	`package p; func f() { _ = (<-<- /* ERROR "expected 'chan'" */ chan int)(nil) };`,
+	`package p; func f() { _ = (<-chan<-chan<-chan<-chan<-chan<- /* ERROR "expected channel type" */ int)(nil) };`,
+	`package p; func f() { var t []int; t /* ERROR "expected identifier on left side of :=" */ [0] := 0 };`,
+	`package p; func f() { if x := g(); x = /* ERROR "expected '=='" */ 0 {}};`,
+	`package p; func f() { _ = x = /* ERROR "expected '=='" */ 0 {}};`,
+	`package p; func f() { _ = 1 == func()int { var x bool; x = x = /* ERROR "expected '=='" */ true; return x }() };`,
+	`package p; func f() { var s []int; _ = s[] /* ERROR "expected operand" */ };`,
+	`package p; func f() { var s []int; _ = s[i:j: /* ERROR "3rd index required" */ ] };`,
+	`package p; func f() { var s []int; _ = s[i: /* ERROR "2nd index required" */ :k] };`,
+	`package p; func f() { var s []int; _ = s[i: /* ERROR "2nd index required" */ :] };`,
+	`package p; func f() { var s []int; _ = s[: /* ERROR "2nd index required" */ :] };`,
+	`package p; func f() { var s []int; _ = s[: /* ERROR "2nd index required" */ ::] };`,
+	`package p; func f() { var s []int; _ = s[i:j:k: /* ERROR "expected ']'" */ l] };`,
+	`package p; func f() { for x /* ERROR "boolean or range expression" */ = []string {} }`,
+	`package p; func f() { for x /* ERROR "boolean or range expression" */ := []string {} }`,
+	`package p; func f() { for i /* ERROR "boolean or range expression" */ , x = []string {} }`,
+	`package p; func f() { for i /* ERROR "boolean or range expression" */ , x := []string {} }`,
+	`package p; func f() { go f /* ERROR HERE "function must be invoked" */ }`,
+	`package p; func f() { defer func() {} /* ERROR HERE "function must be invoked" */ }`,
+	`package p; func f() { go func() { func() { f(x func /* ERROR "expected '\)'" */ (){}) } } }`,
+	`package p; func f() (a b string /* ERROR "expected '\)'" */ , ok bool) // issue 8656`,
+}
+
+func TestInvalid(t *testing.T) {
+	for _, src := range invalids {
+		checkErrors(t, src, src)
+	}
+}
diff --git a/src/go/parser/testdata/commas.src b/src/go/parser/testdata/commas.src
new file mode 100644
index 000000000..af6e70645
--- /dev/null
+++ b/src/go/parser/testdata/commas.src
@@ -0,0 +1,19 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test case for error messages/parser synchronization
+// after missing commas.
+
+package p
+
+var _ = []int{
+	0 /* ERROR "missing ','" */
+}
+
+var _ = []int{
+	0,
+	1,
+	2,
+	3 /* ERROR "missing ','" */
+}
diff --git a/src/go/parser/testdata/issue3106.src b/src/go/parser/testdata/issue3106.src
new file mode 100644
index 000000000..82796c8ce
--- /dev/null
+++ b/src/go/parser/testdata/issue3106.src
@@ -0,0 +1,46 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test case for issue 3106: Better synchronization of
+// parser after certain syntax errors.
+
+package main
+
+func f() {
+	var m Mutex
+	c := MakeCond(&m)
+	percent := 0
+	const step = 10
+	for i := 0; i < 5; i++ {
+		go func() {
+			for {
+				// Emulates some useful work.
+				time.Sleep(1e8)
+				m.Lock()
+				defer
+				if /* ERROR "expected operand, found 'if'" */ percent == 100 {
+					m.Unlock()
+					break
+				}
+				percent++
+				if percent % step == 0 {
+					//c.Signal()
+				}
+				m.Unlock()
+			}
+		}()
+	}
+	for {
+		m.Lock()
+		if percent == 0 || percent % step != 0 {
+			c.Wait()
+		}
+		fmt.Print(",")
+		if percent == 100 {
+			m.Unlock()
+			break
+		}
+		m.Unlock()
+	}
+}
diff --git a/src/go/printer/example_test.go b/src/go/printer/example_test.go
new file mode 100644
index 000000000..e570040ba
--- /dev/null
+++ b/src/go/printer/example_test.go
@@ -0,0 +1,67 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package printer_test
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/printer"
+	"go/token"
+	"strings"
+	"testing"
+)
+
+// Dummy test function so that godoc does not use the entire file as example.
+func Test(*testing.T) {}
+
+func parseFunc(filename, functionname string) (fun *ast.FuncDecl, fset *token.FileSet) {
+	fset = token.NewFileSet()
+	if file, err := parser.ParseFile(fset, filename, nil, 0); err == nil {
+		for _, d := range file.Decls {
+			if f, ok := d.(*ast.FuncDecl); ok && f.Name.Name == functionname {
+				fun = f
+				return
+			}
+		}
+	}
+	panic("function not found")
+}
+
+func ExampleFprint() {
+	// Parse source file and extract the AST without comments for
+	// this function, with position information referring to the
+	// file set fset.
+	funcAST, fset := parseFunc("example_test.go", "ExampleFprint")
+
+	// Print the function body into buffer buf.
+	// The file set is provided to the printer so that it knows
+	// about the original source formatting and can add additional
+	// line breaks where they were present in the source.
+	var buf bytes.Buffer
+	printer.Fprint(&buf, fset, funcAST.Body)
+
+	// Remove braces {} enclosing the function body, unindent,
+	// and trim leading and trailing white space.
+	s := buf.String()
+	s = s[1 : len(s)-1]
+	s = strings.TrimSpace(strings.Replace(s, "\n\t", "\n", -1))
+
+	// Print the cleaned-up body text to stdout.
+	fmt.Println(s)
+
+	// output:
+	// funcAST, fset := parseFunc("example_test.go", "ExampleFprint")
+	//
+	// var buf bytes.Buffer
+	// printer.Fprint(&buf, fset, funcAST.Body)
+	//
+	// s := buf.String()
+	// s = s[1 : len(s)-1]
+	// s = strings.TrimSpace(strings.Replace(s, "\n\t", "\n", -1))
+	//
+	// fmt.Println(s)
+}
diff --git a/src/go/printer/nodes.go b/src/go/printer/nodes.go
new file mode 100644
index 000000000..6e26f9a63
--- /dev/null
+++ b/src/go/printer/nodes.go
@@ -0,0 +1,1602 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements printing of AST nodes; specifically
+// expressions, statements, declarations, and files. It uses
+// the print functionality implemented in printer.go.
+
+package printer
+
+import (
+	"bytes"
+	"go/ast"
+	"go/token"
+	"unicode/utf8"
+)
+
+// Formatting issues:
+// - better comment formatting for /*-style comments at the end of a line (e.g. a declaration)
+//   when the comment spans multiple lines; if such a comment is just two lines, formatting is
+//   not idempotent
+// - formatting of expression lists
+// - should use blank instead of tab to separate one-line function bodies from
+//   the function header unless there is a group of consecutive one-liners
+
+// ----------------------------------------------------------------------------
+// Common AST nodes.
+
+// Print as many newlines as necessary (but at least min newlines) to get to
+// the current line. ws is printed before the first line break. If newSection
+// is set, the first line break is printed as formfeed. Returns true if any
+// line break was printed; returns false otherwise.
+//
+// TODO(gri): linebreak may add too many lines if the next statement at "line"
+//            is preceded by comments because the computation of n assumes
+//            the current position before the comment and the target position
+//            after the comment. Thus, after interspersing such comments, the
+//            space taken up by them is not considered to reduce the number of
+//            linebreaks. At the moment there is no easy way to know about
+//            future (not yet interspersed) comments in this function.
+//
+func (p *printer) linebreak(line, min int, ws whiteSpace, newSection bool) (printedBreak bool) {
+	n := nlimit(line - p.pos.Line)
+	if n < min {
+		n = min
+	}
+	if n > 0 {
+		p.print(ws)
+		if newSection {
+			p.print(formfeed)
+			n--
+		}
+		for ; n > 0; n-- {
+			p.print(newline)
+		}
+		printedBreak = true
+	}
+	return
+}
+
+// setComment sets g as the next comment if g != nil and if node comments
+// are enabled - this mode is used when printing source code fragments such
+// as exports only. It assumes that there is no pending comment in p.comments
+// and at most one pending comment in the p.comment cache.
+func (p *printer) setComment(g *ast.CommentGroup) {
+	if g == nil || !p.useNodeComments {
+		return
+	}
+	if p.comments == nil {
+		// initialize p.comments lazily
+		p.comments = make([]*ast.CommentGroup, 1)
+	} else if p.cindex < len(p.comments) {
+		// for some reason there are pending comments; this
+		// should never happen - handle gracefully and flush
+		// all comments up to g, ignore anything after that
+		p.flush(p.posFor(g.List[0].Pos()), token.ILLEGAL)
+		p.comments = p.comments[0:1]
+		// in debug mode, report error
+		p.internalError("setComment found pending comments")
+	}
+	p.comments[0] = g
+	p.cindex = 0
+	// don't overwrite any pending comment in the p.comment cache
+	// (there may be a pending comment when a line comment is
+	// immediately followed by a lead comment with no other
+	// tokens between)
+	if p.commentOffset == infinity {
+		p.nextComment() // get comment ready for use
+	}
+}
+
+type exprListMode uint
+
+const (
+	commaTerm exprListMode = 1 << iota // list is optionally terminated by a comma
+	noIndent                           // no extra indentation in multi-line lists
+)
+
+// If indent is set, a multi-line identifier list is indented after the
+// first linebreak encountered.
+func (p *printer) identList(list []*ast.Ident, indent bool) {
+	// convert into an expression list so we can re-use exprList formatting
+	xlist := make([]ast.Expr, len(list))
+	for i, x := range list {
+		xlist[i] = x
+	}
+	var mode exprListMode
+	if !indent {
+		mode = noIndent
+	}
+	p.exprList(token.NoPos, xlist, 1, mode, token.NoPos)
+}
+
+// Print a list of expressions. If the list spans multiple
+// source lines, the original line breaks are respected between
+// expressions.
+//
+// TODO(gri) Consider rewriting this to be independent of []ast.Expr
+//           so that we can use the algorithm for any kind of list
+//           (e.g., pass list via a channel over which to range).
+func (p *printer) exprList(prev0 token.Pos, list []ast.Expr, depth int, mode exprListMode, next0 token.Pos) {
+	if len(list) == 0 {
+		return
+	}
+
+	prev := p.posFor(prev0)
+	next := p.posFor(next0)
+	line := p.lineFor(list[0].Pos())
+	endLine := p.lineFor(list[len(list)-1].End())
+
+	if prev.IsValid() && prev.Line == line && line == endLine {
+		// all list entries on a single line
+		for i, x := range list {
+			if i > 0 {
+				// use position of expression following the comma as
+				// comma position for correct comment placement
+				p.print(x.Pos(), token.COMMA, blank)
+			}
+			p.expr0(x, depth)
+		}
+		return
+	}
+
+	// list entries span multiple lines;
+	// use source code positions to guide line breaks
+
+	// don't add extra indentation if noIndent is set;
+	// i.e., pretend that the first line is already indented
+	ws := ignore
+	if mode&noIndent == 0 {
+		ws = indent
+	}
+
+	// the first linebreak is always a formfeed since this section must not
+	// depend on any previous formatting
+	prevBreak := -1 // index of last expression that was followed by a linebreak
+	if prev.IsValid() && prev.Line < line && p.linebreak(line, 0, ws, true) {
+		ws = ignore
+		prevBreak = 0
+	}
+
+	// initialize expression/key size: a zero value indicates expr/key doesn't fit on a single line
+	size := 0
+
+	// print all list elements
+	for i, x := range list {
+		prevLine := line
+		line = p.lineFor(x.Pos())
+
+		// determine if the next linebreak, if any, needs to use formfeed:
+		// in general, use the entire node size to make the decision; for
+		// key:value expressions, use the key size
+		// TODO(gri) for a better result, should probably incorporate both
+		//           the key and the node size into the decision process
+		useFF := true
+
+		// determine element size: all bets are off if we don't have
+		// position information for the previous and next token (likely
+		// generated code - simply ignore the size in this case by setting
+		// it to 0)
+		prevSize := size
+		const infinity = 1e6 // larger than any source line
+		size = p.nodeSize(x, infinity)
+		pair, isPair := x.(*ast.KeyValueExpr)
+		if size <= infinity && prev.IsValid() && next.IsValid() {
+			// x fits on a single line
+			if isPair {
+				size = p.nodeSize(pair.Key, infinity) // size <= infinity
+			}
+		} else {
+			// size too large or we don't have good layout information
+			size = 0
+		}
+
+		// if the previous line and the current line had single-
+		// line-expressions and the key sizes are small or the
+		// the ratio between the key sizes does not exceed a
+		// threshold, align columns and do not use formfeed
+		if prevSize > 0 && size > 0 {
+			const smallSize = 20
+			if prevSize <= smallSize && size <= smallSize {
+				useFF = false
+			} else {
+				const r = 4 // threshold
+				ratio := float64(size) / float64(prevSize)
+				useFF = ratio <= 1.0/r || r <= ratio
+			}
+		}
+
+		if i > 0 {
+			needsLinebreak := prevLine < line && prevLine > 0 && line > 0
+			// use position of expression following the comma as
+			// comma position for correct comment placement, but
+			// only if the expression is on the same line
+			if !needsLinebreak {
+				p.print(x.Pos())
+			}
+			p.print(token.COMMA)
+			needsBlank := true
+			if needsLinebreak {
+				// lines are broken using newlines so comments remain aligned
+				// unless forceFF is set or there are multiple expressions on
+				// the same line in which case formfeed is used
+				if p.linebreak(line, 0, ws, useFF || prevBreak+1 < i) {
+					ws = ignore
+					prevBreak = i
+					needsBlank = false // we got a line break instead
+				}
+			}
+			if needsBlank {
+				p.print(blank)
+			}
+		}
+
+		if isPair && size > 0 && len(list) > 1 {
+			// we have a key:value expression that fits onto one line and
+			// is in a list with more then one entry: use a column for the
+			// key such that consecutive entries can align if possible
+			p.expr(pair.Key)
+			p.print(pair.Colon, token.COLON, vtab)
+			p.expr(pair.Value)
+		} else {
+			p.expr0(x, depth)
+		}
+	}
+
+	if mode&commaTerm != 0 && next.IsValid() && p.pos.Line < next.Line {
+		// print a terminating comma if the next token is on a new line
+		p.print(token.COMMA)
+		if ws == ignore && mode&noIndent == 0 {
+			// unindent if we indented
+			p.print(unindent)
+		}
+		p.print(formfeed) // terminating comma needs a line break to look good
+		return
+	}
+
+	if ws == ignore && mode&noIndent == 0 {
+		// unindent if we indented
+		p.print(unindent)
+	}
+}
+
+func (p *printer) parameters(fields *ast.FieldList) {
+	p.print(fields.Opening, token.LPAREN)
+	if len(fields.List) > 0 {
+		prevLine := p.lineFor(fields.Opening)
+		ws := indent
+		for i, par := range fields.List {
+			// determine par begin and end line (may be different
+			// if there are multiple parameter names for this par
+			// or the type is on a separate line)
+			var parLineBeg int
+			if len(par.Names) > 0 {
+				parLineBeg = p.lineFor(par.Names[0].Pos())
+			} else {
+				parLineBeg = p.lineFor(par.Type.Pos())
+			}
+			var parLineEnd = p.lineFor(par.Type.End())
+			// separating "," if needed
+			needsLinebreak := 0 < prevLine && prevLine < parLineBeg
+			if i > 0 {
+				// use position of parameter following the comma as
+				// comma position for correct comma placement, but
+				// only if the next parameter is on the same line
+				if !needsLinebreak {
+					p.print(par.Pos())
+				}
+				p.print(token.COMMA)
+			}
+			// separator if needed (linebreak or blank)
+			if needsLinebreak && p.linebreak(parLineBeg, 0, ws, true) {
+				// break line if the opening "(" or previous parameter ended on a different line
+				ws = ignore
+			} else if i > 0 {
+				p.print(blank)
+			}
+			// parameter names
+			if len(par.Names) > 0 {
+				// Very subtle: If we indented before (ws == ignore), identList
+				// won't indent again. If we didn't (ws == indent), identList will
+				// indent if the identList spans multiple lines, and it will outdent
+				// again at the end (and still ws == indent). Thus, a subsequent indent
+				// by a linebreak call after a type, or in the next multi-line identList
+				// will do the right thing.
+				p.identList(par.Names, ws == indent)
+				p.print(blank)
+			}
+			// parameter type
+			p.expr(stripParensAlways(par.Type))
+			prevLine = parLineEnd
+		}
+		// if the closing ")" is on a separate line from the last parameter,
+		// print an additional "," and line break
+		if closing := p.lineFor(fields.Closing); 0 < prevLine && prevLine < closing {
+			p.print(token.COMMA)
+			p.linebreak(closing, 0, ignore, true)
+		}
+		// unindent if we indented
+		if ws == ignore {
+			p.print(unindent)
+		}
+	}
+	p.print(fields.Closing, token.RPAREN)
+}
+
+func (p *printer) signature(params, result *ast.FieldList) {
+	if params != nil {
+		p.parameters(params)
+	} else {
+		p.print(token.LPAREN, token.RPAREN)
+	}
+	n := result.NumFields()
+	if n > 0 {
+		// result != nil
+		p.print(blank)
+		if n == 1 && result.List[0].Names == nil {
+			// single anonymous result; no ()'s
+			p.expr(stripParensAlways(result.List[0].Type))
+			return
+		}
+		p.parameters(result)
+	}
+}
+
+func identListSize(list []*ast.Ident, maxSize int) (size int) {
+	for i, x := range list {
+		if i > 0 {
+			size += len(", ")
+		}
+		size += utf8.RuneCountInString(x.Name)
+		if size >= maxSize {
+			break
+		}
+	}
+	return
+}
+
+func (p *printer) isOneLineFieldList(list []*ast.Field) bool {
+	if len(list) != 1 {
+		return false // allow only one field
+	}
+	f := list[0]
+	if f.Tag != nil || f.Comment != nil {
+		return false // don't allow tags or comments
+	}
+	// only name(s) and type
+	const maxSize = 30 // adjust as appropriate, this is an approximate value
+	namesSize := identListSize(f.Names, maxSize)
+	if namesSize > 0 {
+		namesSize = 1 // blank between names and types
+	}
+	typeSize := p.nodeSize(f.Type, maxSize)
+	return namesSize+typeSize <= maxSize
+}
+
+func (p *printer) setLineComment(text string) {
+	p.setComment(&ast.CommentGroup{List: []*ast.Comment{{Slash: token.NoPos, Text: text}}})
+}
+
+func (p *printer) fieldList(fields *ast.FieldList, isStruct, isIncomplete bool) {
+	lbrace := fields.Opening
+	list := fields.List
+	rbrace := fields.Closing
+	hasComments := isIncomplete || p.commentBefore(p.posFor(rbrace))
+	srcIsOneLine := lbrace.IsValid() && rbrace.IsValid() && p.lineFor(lbrace) == p.lineFor(rbrace)
+
+	if !hasComments && srcIsOneLine {
+		// possibly a one-line struct/interface
+		if len(list) == 0 {
+			// no blank between keyword and {} in this case
+			p.print(lbrace, token.LBRACE, rbrace, token.RBRACE)
+			return
+		} else if isStruct && p.isOneLineFieldList(list) { // for now ignore interfaces
+			// small enough - print on one line
+			// (don't use identList and ignore source line breaks)
+			p.print(lbrace, token.LBRACE, blank)
+			f := list[0]
+			for i, x := range f.Names {
+				if i > 0 {
+					// no comments so no need for comma position
+					p.print(token.COMMA, blank)
+				}
+				p.expr(x)
+			}
+			if len(f.Names) > 0 {
+				p.print(blank)
+			}
+			p.expr(f.Type)
+			p.print(blank, rbrace, token.RBRACE)
+			return
+		}
+	}
+	// hasComments || !srcIsOneLine
+
+	p.print(blank, lbrace, token.LBRACE, indent)
+	if hasComments || len(list) > 0 {
+		p.print(formfeed)
+	}
+
+	if isStruct {
+
+		sep := vtab
+		if len(list) == 1 {
+			sep = blank
+		}
+		var line int
+		for i, f := range list {
+			if i > 0 {
+				p.linebreak(p.lineFor(f.Pos()), 1, ignore, p.linesFrom(line) > 0)
+			}
+			extraTabs := 0
+			p.setComment(f.Doc)
+			p.recordLine(&line)
+			if len(f.Names) > 0 {
+				// named fields
+				p.identList(f.Names, false)
+				p.print(sep)
+				p.expr(f.Type)
+				extraTabs = 1
+			} else {
+				// anonymous field
+				p.expr(f.Type)
+				extraTabs = 2
+			}
+			if f.Tag != nil {
+				if len(f.Names) > 0 && sep == vtab {
+					p.print(sep)
+				}
+				p.print(sep)
+				p.expr(f.Tag)
+				extraTabs = 0
+			}
+			if f.Comment != nil {
+				for ; extraTabs > 0; extraTabs-- {
+					p.print(sep)
+				}
+				p.setComment(f.Comment)
+			}
+		}
+		if isIncomplete {
+			if len(list) > 0 {
+				p.print(formfeed)
+			}
+			p.flush(p.posFor(rbrace), token.RBRACE) // make sure we don't lose the last line comment
+			p.setLineComment("// contains filtered or unexported fields")
+		}
+
+	} else { // interface
+
+		var line int
+		for i, f := range list {
+			if i > 0 {
+				p.linebreak(p.lineFor(f.Pos()), 1, ignore, p.linesFrom(line) > 0)
+			}
+			p.setComment(f.Doc)
+			p.recordLine(&line)
+			if ftyp, isFtyp := f.Type.(*ast.FuncType); isFtyp {
+				// method
+				p.expr(f.Names[0])
+				p.signature(ftyp.Params, ftyp.Results)
+			} else {
+				// embedded interface
+				p.expr(f.Type)
+			}
+			p.setComment(f.Comment)
+		}
+		if isIncomplete {
+			if len(list) > 0 {
+				p.print(formfeed)
+			}
+			p.flush(p.posFor(rbrace), token.RBRACE) // make sure we don't lose the last line comment
+			p.setLineComment("// contains filtered or unexported methods")
+		}
+
+	}
+	p.print(unindent, formfeed, rbrace, token.RBRACE)
+}
+
+// ----------------------------------------------------------------------------
+// Expressions
+
+func walkBinary(e *ast.BinaryExpr) (has4, has5 bool, maxProblem int) {
+	switch e.Op.Precedence() {
+	case 4:
+		has4 = true
+	case 5:
+		has5 = true
+	}
+
+	switch l := e.X.(type) {
+	case *ast.BinaryExpr:
+		if l.Op.Precedence() < e.Op.Precedence() {
+			// parens will be inserted.
+			// pretend this is an *ast.ParenExpr and do nothing.
+			break
+		}
+		h4, h5, mp := walkBinary(l)
+		has4 = has4 || h4
+		has5 = has5 || h5
+		if maxProblem < mp {
+			maxProblem = mp
+		}
+	}
+
+	switch r := e.Y.(type) {
+	case *ast.BinaryExpr:
+		if r.Op.Precedence() <= e.Op.Precedence() {
+			// parens will be inserted.
+			// pretend this is an *ast.ParenExpr and do nothing.
+			break
+		}
+		h4, h5, mp := walkBinary(r)
+		has4 = has4 || h4
+		has5 = has5 || h5
+		if maxProblem < mp {
+			maxProblem = mp
+		}
+
+	case *ast.StarExpr:
+		if e.Op == token.QUO { // `*/`
+			maxProblem = 5
+		}
+
+	case *ast.UnaryExpr:
+		switch e.Op.String() + r.Op.String() {
+		case "/*", "&&", "&^":
+			maxProblem = 5
+		case "++", "--":
+			if maxProblem < 4 {
+				maxProblem = 4
+			}
+		}
+	}
+	return
+}
+
+func cutoff(e *ast.BinaryExpr, depth int) int {
+	has4, has5, maxProblem := walkBinary(e)
+	if maxProblem > 0 {
+		return maxProblem + 1
+	}
+	if has4 && has5 {
+		if depth == 1 {
+			return 5
+		}
+		return 4
+	}
+	if depth == 1 {
+		return 6
+	}
+	return 4
+}
+
+func diffPrec(expr ast.Expr, prec int) int {
+	x, ok := expr.(*ast.BinaryExpr)
+	if !ok || prec != x.Op.Precedence() {
+		return 1
+	}
+	return 0
+}
+
+func reduceDepth(depth int) int {
+	depth--
+	if depth < 1 {
+		depth = 1
+	}
+	return depth
+}
+
+// Format the binary expression: decide the cutoff and then format.
+// Let's call depth == 1 Normal mode, and depth > 1 Compact mode.
+// (Algorithm suggestion by Russ Cox.)
+//
+// The precedences are:
+//	5             *  /  %  <<  >>  &  &^
+//	4             +  -  |  ^
+//	3             ==  !=  <  <=  >  >=
+//	2             &&
+//	1             ||
+//
+// The only decision is whether there will be spaces around levels 4 and 5.
+// There are never spaces at level 6 (unary), and always spaces at levels 3 and below.
+//
+// To choose the cutoff, look at the whole expression but excluding primary
+// expressions (function calls, parenthesized exprs), and apply these rules:
+//
+//	1) If there is a binary operator with a right side unary operand
+//	   that would clash without a space, the cutoff must be (in order):
+//
+//		/*	6
+//		&&	6
+//		&^	6
+//		++	5
+//		--	5
+//
+//         (Comparison operators always have spaces around them.)
+//
+//	2) If there is a mix of level 5 and level 4 operators, then the cutoff
+//	   is 5 (use spaces to distinguish precedence) in Normal mode
+//	   and 4 (never use spaces) in Compact mode.
+//
+//	3) If there are no level 4 operators or no level 5 operators, then the
+//	   cutoff is 6 (always use spaces) in Normal mode
+//	   and 4 (never use spaces) in Compact mode.
+//
+func (p *printer) binaryExpr(x *ast.BinaryExpr, prec1, cutoff, depth int) {
+	prec := x.Op.Precedence()
+	if prec < prec1 {
+		// parenthesis needed
+		// Note: The parser inserts an ast.ParenExpr node; thus this case
+		//       can only occur if the AST is created in a different way.
+		p.print(token.LPAREN)
+		p.expr0(x, reduceDepth(depth)) // parentheses undo one level of depth
+		p.print(token.RPAREN)
+		return
+	}
+
+	printBlank := prec < cutoff
+
+	ws := indent
+	p.expr1(x.X, prec, depth+diffPrec(x.X, prec))
+	if printBlank {
+		p.print(blank)
+	}
+	xline := p.pos.Line // before the operator (it may be on the next line!)
+	yline := p.lineFor(x.Y.Pos())
+	p.print(x.OpPos, x.Op)
+	if xline != yline && xline > 0 && yline > 0 {
+		// at least one line break, but respect an extra empty line
+		// in the source
+		if p.linebreak(yline, 1, ws, true) {
+			ws = ignore
+			printBlank = false // no blank after line break
+		}
+	}
+	if printBlank {
+		p.print(blank)
+	}
+	p.expr1(x.Y, prec+1, depth+1)
+	if ws == ignore {
+		p.print(unindent)
+	}
+}
+
+func isBinary(expr ast.Expr) bool {
+	_, ok := expr.(*ast.BinaryExpr)
+	return ok
+}
+
+func (p *printer) expr1(expr ast.Expr, prec1, depth int) {
+	p.print(expr.Pos())
+
+	switch x := expr.(type) {
+	case *ast.BadExpr:
+		p.print("BadExpr")
+
+	case *ast.Ident:
+		p.print(x)
+
+	case *ast.BinaryExpr:
+		if depth < 1 {
+			p.internalError("depth < 1:", depth)
+			depth = 1
+		}
+		p.binaryExpr(x, prec1, cutoff(x, depth), depth)
+
+	case *ast.KeyValueExpr:
+		p.expr(x.Key)
+		p.print(x.Colon, token.COLON, blank)
+		p.expr(x.Value)
+
+	case *ast.StarExpr:
+		const prec = token.UnaryPrec
+		if prec < prec1 {
+			// parenthesis needed
+			p.print(token.LPAREN)
+			p.print(token.MUL)
+			p.expr(x.X)
+			p.print(token.RPAREN)
+		} else {
+			// no parenthesis needed
+			p.print(token.MUL)
+			p.expr(x.X)
+		}
+
+	case *ast.UnaryExpr:
+		const prec = token.UnaryPrec
+		if prec < prec1 {
+			// parenthesis needed
+			p.print(token.LPAREN)
+			p.expr(x)
+			p.print(token.RPAREN)
+		} else {
+			// no parenthesis needed
+			p.print(x.Op)
+			if x.Op == token.RANGE {
+				// TODO(gri) Remove this code if it cannot be reached.
+				p.print(blank)
+			}
+			p.expr1(x.X, prec, depth)
+		}
+
+	case *ast.BasicLit:
+		p.print(x)
+
+	case *ast.FuncLit:
+		p.expr(x.Type)
+		p.adjBlock(p.distanceFrom(x.Type.Pos()), blank, x.Body)
+
+	case *ast.ParenExpr:
+		if _, hasParens := x.X.(*ast.ParenExpr); hasParens {
+			// don't print parentheses around an already parenthesized expression
+			// TODO(gri) consider making this more general and incorporate precedence levels
+			p.expr0(x.X, reduceDepth(depth)) // parentheses undo one level of depth
+		} else {
+			p.print(token.LPAREN)
+			p.expr0(x.X, reduceDepth(depth)) // parentheses undo one level of depth
+			p.print(x.Rparen, token.RPAREN)
+		}
+
+	case *ast.SelectorExpr:
+		p.expr1(x.X, token.HighestPrec, depth)
+		p.print(token.PERIOD)
+		if line := p.lineFor(x.Sel.Pos()); p.pos.IsValid() && p.pos.Line < line {
+			p.print(indent, newline, x.Sel.Pos(), x.Sel, unindent)
+		} else {
+			p.print(x.Sel.Pos(), x.Sel)
+		}
+
+	case *ast.TypeAssertExpr:
+		p.expr1(x.X, token.HighestPrec, depth)
+		p.print(token.PERIOD, x.Lparen, token.LPAREN)
+		if x.Type != nil {
+			p.expr(x.Type)
+		} else {
+			p.print(token.TYPE)
+		}
+		p.print(x.Rparen, token.RPAREN)
+
+	case *ast.IndexExpr:
+		// TODO(gri): should treat[] like parentheses and undo one level of depth
+		p.expr1(x.X, token.HighestPrec, 1)
+		p.print(x.Lbrack, token.LBRACK)
+		p.expr0(x.Index, depth+1)
+		p.print(x.Rbrack, token.RBRACK)
+
+	case *ast.SliceExpr:
+		// TODO(gri): should treat[] like parentheses and undo one level of depth
+		p.expr1(x.X, token.HighestPrec, 1)
+		p.print(x.Lbrack, token.LBRACK)
+		indices := []ast.Expr{x.Low, x.High}
+		if x.Max != nil {
+			indices = append(indices, x.Max)
+		}
+		for i, y := range indices {
+			if i > 0 {
+				// blanks around ":" if both sides exist and either side is a binary expression
+				// TODO(gri) once we have committed a variant of a[i:j:k] we may want to fine-
+				//           tune the formatting here
+				x := indices[i-1]
+				if depth <= 1 && x != nil && y != nil && (isBinary(x) || isBinary(y)) {
+					p.print(blank, token.COLON, blank)
+				} else {
+					p.print(token.COLON)
+				}
+			}
+			if y != nil {
+				p.expr0(y, depth+1)
+			}
+		}
+		p.print(x.Rbrack, token.RBRACK)
+
+	case *ast.CallExpr:
+		if len(x.Args) > 1 {
+			depth++
+		}
+		if _, ok := x.Fun.(*ast.FuncType); ok {
+			// conversions to literal function types require parentheses around the type
+			p.print(token.LPAREN)
+			p.expr1(x.Fun, token.HighestPrec, depth)
+			p.print(token.RPAREN)
+		} else {
+			p.expr1(x.Fun, token.HighestPrec, depth)
+		}
+		p.print(x.Lparen, token.LPAREN)
+		if x.Ellipsis.IsValid() {
+			p.exprList(x.Lparen, x.Args, depth, 0, x.Ellipsis)
+			p.print(x.Ellipsis, token.ELLIPSIS)
+			if x.Rparen.IsValid() && p.lineFor(x.Ellipsis) < p.lineFor(x.Rparen) {
+				p.print(token.COMMA, formfeed)
+			}
+		} else {
+			p.exprList(x.Lparen, x.Args, depth, commaTerm, x.Rparen)
+		}
+		p.print(x.Rparen, token.RPAREN)
+
+	case *ast.CompositeLit:
+		// composite literal elements that are composite literals themselves may have the type omitted
+		if x.Type != nil {
+			p.expr1(x.Type, token.HighestPrec, depth)
+		}
+		p.print(x.Lbrace, token.LBRACE)
+		p.exprList(x.Lbrace, x.Elts, 1, commaTerm, x.Rbrace)
+		// do not insert extra line break following a /*-style comment
+		// before the closing '}' as it might break the code if there
+		// is no trailing ','
+		mode := noExtraLinebreak
+		// do not insert extra blank following a /*-style comment
+		// before the closing '}' unless the literal is empty
+		if len(x.Elts) > 0 {
+			mode |= noExtraBlank
+		}
+		p.print(mode, x.Rbrace, token.RBRACE, mode)
+
+	case *ast.Ellipsis:
+		p.print(token.ELLIPSIS)
+		if x.Elt != nil {
+			p.expr(x.Elt)
+		}
+
+	case *ast.ArrayType:
+		p.print(token.LBRACK)
+		if x.Len != nil {
+			p.expr(x.Len)
+		}
+		p.print(token.RBRACK)
+		p.expr(x.Elt)
+
+	case *ast.StructType:
+		p.print(token.STRUCT)
+		p.fieldList(x.Fields, true, x.Incomplete)
+
+	case *ast.FuncType:
+		p.print(token.FUNC)
+		p.signature(x.Params, x.Results)
+
+	case *ast.InterfaceType:
+		p.print(token.INTERFACE)
+		p.fieldList(x.Methods, false, x.Incomplete)
+
+	case *ast.MapType:
+		p.print(token.MAP, token.LBRACK)
+		p.expr(x.Key)
+		p.print(token.RBRACK)
+		p.expr(x.Value)
+
+	case *ast.ChanType:
+		switch x.Dir {
+		case ast.SEND | ast.RECV:
+			p.print(token.CHAN)
+		case ast.RECV:
+			p.print(token.ARROW, token.CHAN) // x.Arrow and x.Pos() are the same
+		case ast.SEND:
+			p.print(token.CHAN, x.Arrow, token.ARROW)
+		}
+		p.print(blank)
+		p.expr(x.Value)
+
+	default:
+		panic("unreachable")
+	}
+
+	return
+}
+
+func (p *printer) expr0(x ast.Expr, depth int) {
+	p.expr1(x, token.LowestPrec, depth)
+}
+
+func (p *printer) expr(x ast.Expr) {
+	const depth = 1
+	p.expr1(x, token.LowestPrec, depth)
+}
+
+// ----------------------------------------------------------------------------
+// Statements
+
+// Print the statement list indented, but without a newline after the last statement.
+// Extra line breaks between statements in the source are respected but at most one
+// empty line is printed between statements.
+func (p *printer) stmtList(list []ast.Stmt, nindent int, nextIsRBrace bool) {
+	if nindent > 0 {
+		p.print(indent)
+	}
+	var line int
+	i := 0
+	for _, s := range list {
+		// ignore empty statements (was issue 3466)
+		if _, isEmpty := s.(*ast.EmptyStmt); !isEmpty {
+			// nindent == 0 only for lists of switch/select case clauses;
+			// in those cases each clause is a new section
+			if len(p.output) > 0 {
+				// only print line break if we are not at the beginning of the output
+				// (i.e., we are not printing only a partial program)
+				p.linebreak(p.lineFor(s.Pos()), 1, ignore, i == 0 || nindent == 0 || p.linesFrom(line) > 0)
+			}
+			p.recordLine(&line)
+			p.stmt(s, nextIsRBrace && i == len(list)-1)
+			// labeled statements put labels on a separate line, but here
+			// we only care about the start line of the actual statement
+			// without label - correct line for each label
+			for t := s; ; {
+				lt, _ := t.(*ast.LabeledStmt)
+				if lt == nil {
+					break
+				}
+				line++
+				t = lt.Stmt
+			}
+			i++
+		}
+	}
+	if nindent > 0 {
+		p.print(unindent)
+	}
+}
+
+// block prints an *ast.BlockStmt; it always spans at least two lines.
+func (p *printer) block(b *ast.BlockStmt, nindent int) {
+	p.print(b.Lbrace, token.LBRACE)
+	p.stmtList(b.List, nindent, true)
+	p.linebreak(p.lineFor(b.Rbrace), 1, ignore, true)
+	p.print(b.Rbrace, token.RBRACE)
+}
+
+func isTypeName(x ast.Expr) bool {
+	switch t := x.(type) {
+	case *ast.Ident:
+		return true
+	case *ast.SelectorExpr:
+		return isTypeName(t.X)
+	}
+	return false
+}
+
+func stripParens(x ast.Expr) ast.Expr {
+	if px, strip := x.(*ast.ParenExpr); strip {
+		// parentheses must not be stripped if there are any
+		// unparenthesized composite literals starting with
+		// a type name
+		ast.Inspect(px.X, func(node ast.Node) bool {
+			switch x := node.(type) {
+			case *ast.ParenExpr:
+				// parentheses protect enclosed composite literals
+				return false
+			case *ast.CompositeLit:
+				if isTypeName(x.Type) {
+					strip = false // do not strip parentheses
+				}
+				return false
+			}
+			// in all other cases, keep inspecting
+			return true
+		})
+		if strip {
+			return stripParens(px.X)
+		}
+	}
+	return x
+}
+
+func stripParensAlways(x ast.Expr) ast.Expr {
+	if x, ok := x.(*ast.ParenExpr); ok {
+		return stripParensAlways(x.X)
+	}
+	return x
+}
+
+func (p *printer) controlClause(isForStmt bool, init ast.Stmt, expr ast.Expr, post ast.Stmt) {
+	p.print(blank)
+	needsBlank := false
+	if init == nil && post == nil {
+		// no semicolons required
+		if expr != nil {
+			p.expr(stripParens(expr))
+			needsBlank = true
+		}
+	} else {
+		// all semicolons required
+		// (they are not separators, print them explicitly)
+		if init != nil {
+			p.stmt(init, false)
+		}
+		p.print(token.SEMICOLON, blank)
+		if expr != nil {
+			p.expr(stripParens(expr))
+			needsBlank = true
+		}
+		if isForStmt {
+			p.print(token.SEMICOLON, blank)
+			needsBlank = false
+			if post != nil {
+				p.stmt(post, false)
+				needsBlank = true
+			}
+		}
+	}
+	if needsBlank {
+		p.print(blank)
+	}
+}
+
+// indentList reports whether an expression list would look better if it
+// were indented wholesale (starting with the very first element, rather
+// than starting at the first line break).
+//
+func (p *printer) indentList(list []ast.Expr) bool {
+	// Heuristic: indentList returns true if there are more than one multi-
+	// line element in the list, or if there is any element that is not
+	// starting on the same line as the previous one ends.
+	if len(list) >= 2 {
+		var b = p.lineFor(list[0].Pos())
+		var e = p.lineFor(list[len(list)-1].End())
+		if 0 < b && b < e {
+			// list spans multiple lines
+			n := 0 // multi-line element count
+			line := b
+			for _, x := range list {
+				xb := p.lineFor(x.Pos())
+				xe := p.lineFor(x.End())
+				if line < xb {
+					// x is not starting on the same
+					// line as the previous one ended
+					return true
+				}
+				if xb < xe {
+					// x is a multi-line element
+					n++
+				}
+				line = xe
+			}
+			return n > 1
+		}
+	}
+	return false
+}
+
+func (p *printer) stmt(stmt ast.Stmt, nextIsRBrace bool) {
+	p.print(stmt.Pos())
+
+	switch s := stmt.(type) {
+	case *ast.BadStmt:
+		p.print("BadStmt")
+
+	case *ast.DeclStmt:
+		p.decl(s.Decl)
+
+	case *ast.EmptyStmt:
+		// nothing to do
+
+	case *ast.LabeledStmt:
+		// a "correcting" unindent immediately following a line break
+		// is applied before the line break if there is no comment
+		// between (see writeWhitespace)
+		p.print(unindent)
+		p.expr(s.Label)
+		p.print(s.Colon, token.COLON, indent)
+		if e, isEmpty := s.Stmt.(*ast.EmptyStmt); isEmpty {
+			if !nextIsRBrace {
+				p.print(newline, e.Pos(), token.SEMICOLON)
+				break
+			}
+		} else {
+			p.linebreak(p.lineFor(s.Stmt.Pos()), 1, ignore, true)
+		}
+		p.stmt(s.Stmt, nextIsRBrace)
+
+	case *ast.ExprStmt:
+		const depth = 1
+		p.expr0(s.X, depth)
+
+	case *ast.SendStmt:
+		const depth = 1
+		p.expr0(s.Chan, depth)
+		p.print(blank, s.Arrow, token.ARROW, blank)
+		p.expr0(s.Value, depth)
+
+	case *ast.IncDecStmt:
+		const depth = 1
+		p.expr0(s.X, depth+1)
+		p.print(s.TokPos, s.Tok)
+
+	case *ast.AssignStmt:
+		var depth = 1
+		if len(s.Lhs) > 1 && len(s.Rhs) > 1 {
+			depth++
+		}
+		p.exprList(s.Pos(), s.Lhs, depth, 0, s.TokPos)
+		p.print(blank, s.TokPos, s.Tok, blank)
+		p.exprList(s.TokPos, s.Rhs, depth, 0, token.NoPos)
+
+	case *ast.GoStmt:
+		p.print(token.GO, blank)
+		p.expr(s.Call)
+
+	case *ast.DeferStmt:
+		p.print(token.DEFER, blank)
+		p.expr(s.Call)
+
+	case *ast.ReturnStmt:
+		p.print(token.RETURN)
+		if s.Results != nil {
+			p.print(blank)
+			// Use indentList heuristic to make corner cases look
+			// better (issue 1207). A more systematic approach would
+			// always indent, but this would cause significant
+			// reformatting of the code base and not necessarily
+			// lead to more nicely formatted code in general.
+			if p.indentList(s.Results) {
+				p.print(indent)
+				p.exprList(s.Pos(), s.Results, 1, noIndent, token.NoPos)
+				p.print(unindent)
+			} else {
+				p.exprList(s.Pos(), s.Results, 1, 0, token.NoPos)
+			}
+		}
+
+	case *ast.BranchStmt:
+		p.print(s.Tok)
+		if s.Label != nil {
+			p.print(blank)
+			p.expr(s.Label)
+		}
+
+	case *ast.BlockStmt:
+		p.block(s, 1)
+
+	case *ast.IfStmt:
+		p.print(token.IF)
+		p.controlClause(false, s.Init, s.Cond, nil)
+		p.block(s.Body, 1)
+		if s.Else != nil {
+			p.print(blank, token.ELSE, blank)
+			switch s.Else.(type) {
+			case *ast.BlockStmt, *ast.IfStmt:
+				p.stmt(s.Else, nextIsRBrace)
+			default:
+				p.print(token.LBRACE, indent, formfeed)
+				p.stmt(s.Else, true)
+				p.print(unindent, formfeed, token.RBRACE)
+			}
+		}
+
+	case *ast.CaseClause:
+		if s.List != nil {
+			p.print(token.CASE, blank)
+			p.exprList(s.Pos(), s.List, 1, 0, s.Colon)
+		} else {
+			p.print(token.DEFAULT)
+		}
+		p.print(s.Colon, token.COLON)
+		p.stmtList(s.Body, 1, nextIsRBrace)
+
+	case *ast.SwitchStmt:
+		p.print(token.SWITCH)
+		p.controlClause(false, s.Init, s.Tag, nil)
+		p.block(s.Body, 0)
+
+	case *ast.TypeSwitchStmt:
+		p.print(token.SWITCH)
+		if s.Init != nil {
+			p.print(blank)
+			p.stmt(s.Init, false)
+			p.print(token.SEMICOLON)
+		}
+		p.print(blank)
+		p.stmt(s.Assign, false)
+		p.print(blank)
+		p.block(s.Body, 0)
+
+	case *ast.CommClause:
+		if s.Comm != nil {
+			p.print(token.CASE, blank)
+			p.stmt(s.Comm, false)
+		} else {
+			p.print(token.DEFAULT)
+		}
+		p.print(s.Colon, token.COLON)
+		p.stmtList(s.Body, 1, nextIsRBrace)
+
+	case *ast.SelectStmt:
+		p.print(token.SELECT, blank)
+		body := s.Body
+		if len(body.List) == 0 && !p.commentBefore(p.posFor(body.Rbrace)) {
+			// print empty select statement w/o comments on one line
+			p.print(body.Lbrace, token.LBRACE, body.Rbrace, token.RBRACE)
+		} else {
+			p.block(body, 0)
+		}
+
+	case *ast.ForStmt:
+		p.print(token.FOR)
+		p.controlClause(true, s.Init, s.Cond, s.Post)
+		p.block(s.Body, 1)
+
+	case *ast.RangeStmt:
+		p.print(token.FOR, blank)
+		if s.Key != nil {
+			p.expr(s.Key)
+			if s.Value != nil {
+				// use position of value following the comma as
+				// comma position for correct comment placement
+				p.print(s.Value.Pos(), token.COMMA, blank)
+				p.expr(s.Value)
+			}
+			p.print(blank, s.TokPos, s.Tok, blank)
+		}
+		p.print(token.RANGE, blank)
+		p.expr(stripParens(s.X))
+		p.print(blank)
+		p.block(s.Body, 1)
+
+	default:
+		panic("unreachable")
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Declarations
+
+// The keepTypeColumn function determines if the type column of a series of
+// consecutive const or var declarations must be kept, or if initialization
+// values (V) can be placed in the type column (T) instead. The i'th entry
+// in the result slice is true if the type column in spec[i] must be kept.
+//
+// For example, the declaration:
+//
+//	const (
+//		foobar int = 42 // comment
+//		x          = 7  // comment
+//		foo
+//              bar = 991
+//	)
+//
+// leads to the type/values matrix below. A run of value columns (V) can
+// be moved into the type column if there is no type for any of the values
+// in that column (we only move entire columns so that they align properly).
+//
+//	matrix        formatted     result
+//                    matrix
+//	T  V    ->    T  V     ->   true      there is a T and so the type
+//	-  V          -  V          true      column must be kept
+//	-  -          -  -          false
+//	-  V          V  -          false     V is moved into T column
+//
+func keepTypeColumn(specs []ast.Spec) []bool {
+	m := make([]bool, len(specs))
+
+	populate := func(i, j int, keepType bool) {
+		if keepType {
+			for ; i < j; i++ {
+				m[i] = true
+			}
+		}
+	}
+
+	i0 := -1 // if i0 >= 0 we are in a run and i0 is the start of the run
+	var keepType bool
+	for i, s := range specs {
+		t := s.(*ast.ValueSpec)
+		if t.Values != nil {
+			if i0 < 0 {
+				// start of a run of ValueSpecs with non-nil Values
+				i0 = i
+				keepType = false
+			}
+		} else {
+			if i0 >= 0 {
+				// end of a run
+				populate(i0, i, keepType)
+				i0 = -1
+			}
+		}
+		if t.Type != nil {
+			keepType = true
+		}
+	}
+	if i0 >= 0 {
+		// end of a run
+		populate(i0, len(specs), keepType)
+	}
+
+	return m
+}
+
+func (p *printer) valueSpec(s *ast.ValueSpec, keepType bool) {
+	p.setComment(s.Doc)
+	p.identList(s.Names, false) // always present
+	extraTabs := 3
+	if s.Type != nil || keepType {
+		p.print(vtab)
+		extraTabs--
+	}
+	if s.Type != nil {
+		p.expr(s.Type)
+	}
+	if s.Values != nil {
+		p.print(vtab, token.ASSIGN, blank)
+		p.exprList(token.NoPos, s.Values, 1, 0, token.NoPos)
+		extraTabs--
+	}
+	if s.Comment != nil {
+		for ; extraTabs > 0; extraTabs-- {
+			p.print(vtab)
+		}
+		p.setComment(s.Comment)
+	}
+}
+
+// The parameter n is the number of specs in the group. If doIndent is set,
+// multi-line identifier lists in the spec are indented when the first
+// linebreak is encountered.
+//
+func (p *printer) spec(spec ast.Spec, n int, doIndent bool) {
+	switch s := spec.(type) {
+	case *ast.ImportSpec:
+		p.setComment(s.Doc)
+		if s.Name != nil {
+			p.expr(s.Name)
+			p.print(blank)
+		}
+		p.expr(s.Path)
+		p.setComment(s.Comment)
+		p.print(s.EndPos)
+
+	case *ast.ValueSpec:
+		if n != 1 {
+			p.internalError("expected n = 1; got", n)
+		}
+		p.setComment(s.Doc)
+		p.identList(s.Names, doIndent) // always present
+		if s.Type != nil {
+			p.print(blank)
+			p.expr(s.Type)
+		}
+		if s.Values != nil {
+			p.print(blank, token.ASSIGN, blank)
+			p.exprList(token.NoPos, s.Values, 1, 0, token.NoPos)
+		}
+		p.setComment(s.Comment)
+
+	case *ast.TypeSpec:
+		p.setComment(s.Doc)
+		p.expr(s.Name)
+		if n == 1 {
+			p.print(blank)
+		} else {
+			p.print(vtab)
+		}
+		p.expr(s.Type)
+		p.setComment(s.Comment)
+
+	default:
+		panic("unreachable")
+	}
+}
+
+func (p *printer) genDecl(d *ast.GenDecl) {
+	p.setComment(d.Doc)
+	p.print(d.Pos(), d.Tok, blank)
+
+	if d.Lparen.IsValid() {
+		// group of parenthesized declarations
+		p.print(d.Lparen, token.LPAREN)
+		if n := len(d.Specs); n > 0 {
+			p.print(indent, formfeed)
+			if n > 1 && (d.Tok == token.CONST || d.Tok == token.VAR) {
+				// two or more grouped const/var declarations:
+				// determine if the type column must be kept
+				keepType := keepTypeColumn(d.Specs)
+				var line int
+				for i, s := range d.Specs {
+					if i > 0 {
+						p.linebreak(p.lineFor(s.Pos()), 1, ignore, p.linesFrom(line) > 0)
+					}
+					p.recordLine(&line)
+					p.valueSpec(s.(*ast.ValueSpec), keepType[i])
+				}
+			} else {
+				var line int
+				for i, s := range d.Specs {
+					if i > 0 {
+						p.linebreak(p.lineFor(s.Pos()), 1, ignore, p.linesFrom(line) > 0)
+					}
+					p.recordLine(&line)
+					p.spec(s, n, false)
+				}
+			}
+			p.print(unindent, formfeed)
+		}
+		p.print(d.Rparen, token.RPAREN)
+
+	} else {
+		// single declaration
+		p.spec(d.Specs[0], 1, true)
+	}
+}
+
+// nodeSize determines the size of n in chars after formatting.
+// The result is <= maxSize if the node fits on one line with at
+// most maxSize chars and the formatted output doesn't contain
+// any control chars. Otherwise, the result is > maxSize.
+//
+func (p *printer) nodeSize(n ast.Node, maxSize int) (size int) {
+	// nodeSize invokes the printer, which may invoke nodeSize
+	// recursively. For deep composite literal nests, this can
+	// lead to an exponential algorithm. Remember previous
+	// results to prune the recursion (was issue 1628).
+	if size, found := p.nodeSizes[n]; found {
+		return size
+	}
+
+	size = maxSize + 1 // assume n doesn't fit
+	p.nodeSizes[n] = size
+
+	// nodeSize computation must be independent of particular
+	// style so that we always get the same decision; print
+	// in RawFormat
+	cfg := Config{Mode: RawFormat}
+	var buf bytes.Buffer
+	if err := cfg.fprint(&buf, p.fset, n, p.nodeSizes); err != nil {
+		return
+	}
+	if buf.Len() <= maxSize {
+		for _, ch := range buf.Bytes() {
+			if ch < ' ' {
+				return
+			}
+		}
+		size = buf.Len() // n fits
+		p.nodeSizes[n] = size
+	}
+	return
+}
+
+// bodySize is like nodeSize but it is specialized for *ast.BlockStmt's.
+func (p *printer) bodySize(b *ast.BlockStmt, maxSize int) int {
+	pos1 := b.Pos()
+	pos2 := b.Rbrace
+	if pos1.IsValid() && pos2.IsValid() && p.lineFor(pos1) != p.lineFor(pos2) {
+		// opening and closing brace are on different lines - don't make it a one-liner
+		return maxSize + 1
+	}
+	if len(b.List) > 5 {
+		// too many statements - don't make it a one-liner
+		return maxSize + 1
+	}
+	// otherwise, estimate body size
+	bodySize := p.commentSizeBefore(p.posFor(pos2))
+	for i, s := range b.List {
+		if bodySize > maxSize {
+			break // no need to continue
+		}
+		if i > 0 {
+			bodySize += 2 // space for a semicolon and blank
+		}
+		bodySize += p.nodeSize(s, maxSize)
+	}
+	return bodySize
+}
+
+// adjBlock prints an "adjacent" block (e.g., a for-loop or function body) following
+// a header (e.g., a for-loop control clause or function signature) of given headerSize.
+// If the header's and block's size are "small enough" and the block is "simple enough",
+// the block is printed on the current line, without line breaks, spaced from the header
+// by sep. Otherwise the block's opening "{" is printed on the current line, followed by
+// lines for the block's statements and its closing "}".
+//
+func (p *printer) adjBlock(headerSize int, sep whiteSpace, b *ast.BlockStmt) {
+	if b == nil {
+		return
+	}
+
+	const maxSize = 100
+	if headerSize+p.bodySize(b, maxSize) <= maxSize {
+		p.print(sep, b.Lbrace, token.LBRACE)
+		if len(b.List) > 0 {
+			p.print(blank)
+			for i, s := range b.List {
+				if i > 0 {
+					p.print(token.SEMICOLON, blank)
+				}
+				p.stmt(s, i == len(b.List)-1)
+			}
+			p.print(blank)
+		}
+		p.print(noExtraLinebreak, b.Rbrace, token.RBRACE, noExtraLinebreak)
+		return
+	}
+
+	if sep != ignore {
+		p.print(blank) // always use blank
+	}
+	p.block(b, 1)
+}
+
+// distanceFrom returns the column difference between from and p.pos (the current
+// estimated position) if both are on the same line; if they are on different lines
+// (or unknown) the result is infinity.
+func (p *printer) distanceFrom(from token.Pos) int {
+	if from.IsValid() && p.pos.IsValid() {
+		if f := p.posFor(from); f.Line == p.pos.Line {
+			return p.pos.Column - f.Column
+		}
+	}
+	return infinity
+}
+
+func (p *printer) funcDecl(d *ast.FuncDecl) {
+	p.setComment(d.Doc)
+	p.print(d.Pos(), token.FUNC, blank)
+	if d.Recv != nil {
+		p.parameters(d.Recv) // method: print receiver
+		p.print(blank)
+	}
+	p.expr(d.Name)
+	p.signature(d.Type.Params, d.Type.Results)
+	p.adjBlock(p.distanceFrom(d.Pos()), vtab, d.Body)
+}
+
+func (p *printer) decl(decl ast.Decl) {
+	switch d := decl.(type) {
+	case *ast.BadDecl:
+		p.print(d.Pos(), "BadDecl")
+	case *ast.GenDecl:
+		p.genDecl(d)
+	case *ast.FuncDecl:
+		p.funcDecl(d)
+	default:
+		panic("unreachable")
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Files
+
+func declToken(decl ast.Decl) (tok token.Token) {
+	tok = token.ILLEGAL
+	switch d := decl.(type) {
+	case *ast.GenDecl:
+		tok = d.Tok
+	case *ast.FuncDecl:
+		tok = token.FUNC
+	}
+	return
+}
+
+func (p *printer) declList(list []ast.Decl) {
+	tok := token.ILLEGAL
+	for _, d := range list {
+		prev := tok
+		tok = declToken(d)
+		// If the declaration token changed (e.g., from CONST to TYPE)
+		// or the next declaration has documentation associated with it,
+		// print an empty line between top-level declarations.
+		// (because p.linebreak is called with the position of d, which
+		// is past any documentation, the minimum requirement is satisfied
+		// even w/o the extra getDoc(d) nil-check - leave it in case the
+		// linebreak logic improves - there's already a TODO).
+		if len(p.output) > 0 {
+			// only print line break if we are not at the beginning of the output
+			// (i.e., we are not printing only a partial program)
+			min := 1
+			if prev != tok || getDoc(d) != nil {
+				min = 2
+			}
+			p.linebreak(p.lineFor(d.Pos()), min, ignore, false)
+		}
+		p.decl(d)
+	}
+}
+
+func (p *printer) file(src *ast.File) {
+	p.setComment(src.Doc)
+	p.print(src.Pos(), token.PACKAGE, blank)
+	p.expr(src.Name)
+	p.declList(src.Decls)
+	p.print(newline)
+}
diff --git a/src/go/printer/performance_test.go b/src/go/printer/performance_test.go
new file mode 100644
index 000000000..5b29affcb
--- /dev/null
+++ b/src/go/printer/performance_test.go
@@ -0,0 +1,58 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements a simple printer performance benchmark:
+// go test -bench=BenchmarkPrint
+
+package printer
+
+import (
+	"bytes"
+	"go/ast"
+	"go/parser"
+	"io"
+	"io/ioutil"
+	"log"
+	"testing"
+)
+
+var testfile *ast.File
+
+func testprint(out io.Writer, file *ast.File) {
+	if err := (&Config{TabIndent | UseSpaces, 8, 0}).Fprint(out, fset, file); err != nil {
+		log.Fatalf("print error: %s", err)
+	}
+}
+
+// cannot initialize in init because (printer) Fprint launches goroutines.
+func initialize() {
+	const filename = "testdata/parser.go"
+
+	src, err := ioutil.ReadFile(filename)
+	if err != nil {
+		log.Fatalf("%s", err)
+	}
+
+	file, err := parser.ParseFile(fset, filename, src, parser.ParseComments)
+	if err != nil {
+		log.Fatalf("%s", err)
+	}
+
+	var buf bytes.Buffer
+	testprint(&buf, file)
+	if !bytes.Equal(buf.Bytes(), src) {
+		log.Fatalf("print error: %s not idempotent", filename)
+	}
+
+	testfile = file
+}
+
+func BenchmarkPrint(b *testing.B) {
+	if testfile == nil {
+		initialize()
+	}
+	for i := 0; i < b.N; i++ {
+		testprint(ioutil.Discard, testfile)
+	}
+}
diff --git a/src/go/printer/printer.go b/src/go/printer/printer.go
new file mode 100644
index 000000000..280c697a0
--- /dev/null
+++ b/src/go/printer/printer.go
@@ -0,0 +1,1292 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package printer implements printing of AST nodes.
+package printer
+
+import (
+	"fmt"
+	"go/ast"
+	"go/token"
+	"io"
+	"os"
+	"strconv"
+	"strings"
+	"text/tabwriter"
+	"unicode"
+)
+
+const (
+	maxNewlines = 2     // max. number of newlines between source text
+	debug       = false // enable for debugging
+	infinity    = 1 << 30
+)
+
+type whiteSpace byte
+
+const (
+	ignore   = whiteSpace(0)
+	blank    = whiteSpace(' ')
+	vtab     = whiteSpace('\v')
+	newline  = whiteSpace('\n')
+	formfeed = whiteSpace('\f')
+	indent   = whiteSpace('>')
+	unindent = whiteSpace('<')
+)
+
+// A pmode value represents the current printer mode.
+type pmode int
+
+const (
+	noExtraBlank     pmode = 1 << iota // disables extra blank after /*-style comment
+	noExtraLinebreak                   // disables extra line break after /*-style comment
+)
+
+type commentInfo struct {
+	cindex         int               // current comment index
+	comment        *ast.CommentGroup // = printer.comments[cindex]; or nil
+	commentOffset  int               // = printer.posFor(printer.comments[cindex].List[0].Pos()).Offset; or infinity
+	commentNewline bool              // true if the comment group contains newlines
+}
+
+type printer struct {
+	// Configuration (does not change after initialization)
+	Config
+	fset *token.FileSet
+
+	// Current state
+	output      []byte       // raw printer result
+	indent      int          // current indentation
+	mode        pmode        // current printer mode
+	impliedSemi bool         // if set, a linebreak implies a semicolon
+	lastTok     token.Token  // last token printed (token.ILLEGAL if it's whitespace)
+	prevOpen    token.Token  // previous non-brace "open" token (, [, or token.ILLEGAL
+	wsbuf       []whiteSpace // delayed white space
+
+	// Positions
+	// The out position differs from the pos position when the result
+	// formatting differs from the source formatting (in the amount of
+	// white space). If there's a difference and SourcePos is set in
+	// ConfigMode, //line comments are used in the output to restore
+	// original source positions for a reader.
+	pos     token.Position // current position in AST (source) space
+	out     token.Position // current position in output space
+	last    token.Position // value of pos after calling writeString
+	linePtr *int           // if set, record out.Line for the next token in *linePtr
+
+	// The list of all source comments, in order of appearance.
+	comments        []*ast.CommentGroup // may be nil
+	useNodeComments bool                // if not set, ignore lead and line comments of nodes
+
+	// Information about p.comments[p.cindex]; set up by nextComment.
+	commentInfo
+
+	// Cache of already computed node sizes.
+	nodeSizes map[ast.Node]int
+
+	// Cache of most recently computed line position.
+	cachedPos  token.Pos
+	cachedLine int // line corresponding to cachedPos
+}
+
+func (p *printer) init(cfg *Config, fset *token.FileSet, nodeSizes map[ast.Node]int) {
+	p.Config = *cfg
+	p.fset = fset
+	p.pos = token.Position{Line: 1, Column: 1}
+	p.out = token.Position{Line: 1, Column: 1}
+	p.wsbuf = make([]whiteSpace, 0, 16) // whitespace sequences are short
+	p.nodeSizes = nodeSizes
+	p.cachedPos = -1
+}
+
+func (p *printer) internalError(msg ...interface{}) {
+	if debug {
+		fmt.Print(p.pos.String() + ": ")
+		fmt.Println(msg...)
+		panic("go/printer")
+	}
+}
+
+// commentsHaveNewline reports whether a list of comments belonging to
+// an *ast.CommentGroup contains newlines. Because the position information
+// may only be partially correct, we also have to read the comment text.
+func (p *printer) commentsHaveNewline(list []*ast.Comment) bool {
+	// len(list) > 0
+	line := p.lineFor(list[0].Pos())
+	for i, c := range list {
+		if i > 0 && p.lineFor(list[i].Pos()) != line {
+			// not all comments on the same line
+			return true
+		}
+		if t := c.Text; len(t) >= 2 && (t[1] == '/' || strings.Contains(t, "\n")) {
+			return true
+		}
+	}
+	_ = line
+	return false
+}
+
+func (p *printer) nextComment() {
+	for p.cindex < len(p.comments) {
+		c := p.comments[p.cindex]
+		p.cindex++
+		if list := c.List; len(list) > 0 {
+			p.comment = c
+			p.commentOffset = p.posFor(list[0].Pos()).Offset
+			p.commentNewline = p.commentsHaveNewline(list)
+			return
+		}
+		// we should not reach here (correct ASTs don't have empty
+		// ast.CommentGroup nodes), but be conservative and try again
+	}
+	// no more comments
+	p.commentOffset = infinity
+}
+
+// commentBefore returns true iff the current comment group occurs
+// before the next position in the source code and printing it does
+// not introduce implicit semicolons.
+//
+func (p *printer) commentBefore(next token.Position) bool {
+	return p.commentOffset < next.Offset && (!p.impliedSemi || !p.commentNewline)
+}
+
+// commentSizeBefore returns the estimated size of the
+// comments on the same line before the next position.
+//
+func (p *printer) commentSizeBefore(next token.Position) int {
+	// save/restore current p.commentInfo (p.nextComment() modifies it)
+	defer func(info commentInfo) {
+		p.commentInfo = info
+	}(p.commentInfo)
+
+	size := 0
+	for p.commentBefore(next) {
+		for _, c := range p.comment.List {
+			size += len(c.Text)
+		}
+		p.nextComment()
+	}
+	return size
+}
+
+// recordLine records the output line number for the next non-whitespace
+// token in *linePtr. It is used to compute an accurate line number for a
+// formatted construct, independent of pending (not yet emitted) whitespace
+// or comments.
+//
+func (p *printer) recordLine(linePtr *int) {
+	p.linePtr = linePtr
+}
+
+// linesFrom returns the number of output lines between the current
+// output line and the line argument, ignoring any pending (not yet
+// emitted) whitespace or comments. It is used to compute an accurate
+// size (in number of lines) for a formatted construct.
+//
+func (p *printer) linesFrom(line int) int {
+	return p.out.Line - line
+}
+
+func (p *printer) posFor(pos token.Pos) token.Position {
+	// not used frequently enough to cache entire token.Position
+	return p.fset.Position(pos)
+}
+
+func (p *printer) lineFor(pos token.Pos) int {
+	if pos != p.cachedPos {
+		p.cachedPos = pos
+		p.cachedLine = p.fset.Position(pos).Line
+	}
+	return p.cachedLine
+}
+
+// atLineBegin emits a //line comment if necessary and prints indentation.
+func (p *printer) atLineBegin(pos token.Position) {
+	// write a //line comment if necessary
+	if p.Config.Mode&SourcePos != 0 && pos.IsValid() && (p.out.Line != pos.Line || p.out.Filename != pos.Filename) {
+		p.output = append(p.output, tabwriter.Escape) // protect '\n' in //line from tabwriter interpretation
+		p.output = append(p.output, fmt.Sprintf("//line %s:%d\n", pos.Filename, pos.Line)...)
+		p.output = append(p.output, tabwriter.Escape)
+		// p.out must match the //line comment
+		p.out.Filename = pos.Filename
+		p.out.Line = pos.Line
+	}
+
+	// write indentation
+	// use "hard" htabs - indentation columns
+	// must not be discarded by the tabwriter
+	n := p.Config.Indent + p.indent // include base indentation
+	for i := 0; i < n; i++ {
+		p.output = append(p.output, '\t')
+	}
+
+	// update positions
+	p.pos.Offset += n
+	p.pos.Column += n
+	p.out.Column += n
+}
+
+// writeByte writes ch n times to p.output and updates p.pos.
+func (p *printer) writeByte(ch byte, n int) {
+	if p.out.Column == 1 {
+		p.atLineBegin(p.pos)
+	}
+
+	for i := 0; i < n; i++ {
+		p.output = append(p.output, ch)
+	}
+
+	// update positions
+	p.pos.Offset += n
+	if ch == '\n' || ch == '\f' {
+		p.pos.Line += n
+		p.out.Line += n
+		p.pos.Column = 1
+		p.out.Column = 1
+		return
+	}
+	p.pos.Column += n
+	p.out.Column += n
+}
+
+// writeString writes the string s to p.output and updates p.pos, p.out,
+// and p.last. If isLit is set, s is escaped w/ tabwriter.Escape characters
+// to protect s from being interpreted by the tabwriter.
+//
+// Note: writeString is only used to write Go tokens, literals, and
+// comments, all of which must be written literally. Thus, it is correct
+// to always set isLit = true. However, setting it explicitly only when
+// needed (i.e., when we don't know that s contains no tabs or line breaks)
+// avoids processing extra escape characters and reduces run time of the
+// printer benchmark by up to 10%.
+//
+func (p *printer) writeString(pos token.Position, s string, isLit bool) {
+	if p.out.Column == 1 {
+		p.atLineBegin(pos)
+	}
+
+	if pos.IsValid() {
+		// update p.pos (if pos is invalid, continue with existing p.pos)
+		// Note: Must do this after handling line beginnings because
+		// atLineBegin updates p.pos if there's indentation, but p.pos
+		// is the position of s.
+		p.pos = pos
+	}
+
+	if isLit {
+		// Protect s such that is passes through the tabwriter
+		// unchanged. Note that valid Go programs cannot contain
+		// tabwriter.Escape bytes since they do not appear in legal
+		// UTF-8 sequences.
+		p.output = append(p.output, tabwriter.Escape)
+	}
+
+	if debug {
+		p.output = append(p.output, fmt.Sprintf("/*%s*/", pos)...) // do not update p.pos!
+	}
+	p.output = append(p.output, s...)
+
+	// update positions
+	nlines := 0
+	var li int // index of last newline; valid if nlines > 0
+	for i := 0; i < len(s); i++ {
+		// Go tokens cannot contain '\f' - no need to look for it
+		if s[i] == '\n' {
+			nlines++
+			li = i
+		}
+	}
+	p.pos.Offset += len(s)
+	if nlines > 0 {
+		p.pos.Line += nlines
+		p.out.Line += nlines
+		c := len(s) - li
+		p.pos.Column = c
+		p.out.Column = c
+	} else {
+		p.pos.Column += len(s)
+		p.out.Column += len(s)
+	}
+
+	if isLit {
+		p.output = append(p.output, tabwriter.Escape)
+	}
+
+	p.last = p.pos
+}
+
+// writeCommentPrefix writes the whitespace before a comment.
+// If there is any pending whitespace, it consumes as much of
+// it as is likely to help position the comment nicely.
+// pos is the comment position, next the position of the item
+// after all pending comments, prev is the previous comment in
+// a group of comments (or nil), and tok is the next token.
+//
+func (p *printer) writeCommentPrefix(pos, next token.Position, prev, comment *ast.Comment, tok token.Token) {
+	if len(p.output) == 0 {
+		// the comment is the first item to be printed - don't write any whitespace
+		return
+	}
+
+	if pos.IsValid() && pos.Filename != p.last.Filename {
+		// comment in a different file - separate with newlines
+		p.writeByte('\f', maxNewlines)
+		return
+	}
+
+	if pos.Line == p.last.Line && (prev == nil || prev.Text[1] != '/') {
+		// comment on the same line as last item:
+		// separate with at least one separator
+		hasSep := false
+		if prev == nil {
+			// first comment of a comment group
+			j := 0
+			for i, ch := range p.wsbuf {
+				switch ch {
+				case blank:
+					// ignore any blanks before a comment
+					p.wsbuf[i] = ignore
+					continue
+				case vtab:
+					// respect existing tabs - important
+					// for proper formatting of commented structs
+					hasSep = true
+					continue
+				case indent:
+					// apply pending indentation
+					continue
+				}
+				j = i
+				break
+			}
+			p.writeWhitespace(j)
+		}
+		// make sure there is at least one separator
+		if !hasSep {
+			sep := byte('\t')
+			if pos.Line == next.Line {
+				// next item is on the same line as the comment
+				// (which must be a /*-style comment): separate
+				// with a blank instead of a tab
+				sep = ' '
+			}
+			p.writeByte(sep, 1)
+		}
+
+	} else {
+		// comment on a different line:
+		// separate with at least one line break
+		droppedLinebreak := false
+		j := 0
+		for i, ch := range p.wsbuf {
+			switch ch {
+			case blank, vtab:
+				// ignore any horizontal whitespace before line breaks
+				p.wsbuf[i] = ignore
+				continue
+			case indent:
+				// apply pending indentation
+				continue
+			case unindent:
+				// if this is not the last unindent, apply it
+				// as it is (likely) belonging to the last
+				// construct (e.g., a multi-line expression list)
+				// and is not part of closing a block
+				if i+1 < len(p.wsbuf) && p.wsbuf[i+1] == unindent {
+					continue
+				}
+				// if the next token is not a closing }, apply the unindent
+				// if it appears that the comment is aligned with the
+				// token; otherwise assume the unindent is part of a
+				// closing block and stop (this scenario appears with
+				// comments before a case label where the comments
+				// apply to the next case instead of the current one)
+				if tok != token.RBRACE && pos.Column == next.Column {
+					continue
+				}
+			case newline, formfeed:
+				p.wsbuf[i] = ignore
+				droppedLinebreak = prev == nil // record only if first comment of a group
+			}
+			j = i
+			break
+		}
+		p.writeWhitespace(j)
+
+		// determine number of linebreaks before the comment
+		n := 0
+		if pos.IsValid() && p.last.IsValid() {
+			n = pos.Line - p.last.Line
+			if n < 0 { // should never happen
+				n = 0
+			}
+		}
+
+		// at the package scope level only (p.indent == 0),
+		// add an extra newline if we dropped one before:
+		// this preserves a blank line before documentation
+		// comments at the package scope level (issue 2570)
+		if p.indent == 0 && droppedLinebreak {
+			n++
+		}
+
+		// make sure there is at least one line break
+		// if the previous comment was a line comment
+		if n == 0 && prev != nil && prev.Text[1] == '/' {
+			n = 1
+		}
+
+		if n > 0 {
+			// use formfeeds to break columns before a comment;
+			// this is analogous to using formfeeds to separate
+			// individual lines of /*-style comments
+			p.writeByte('\f', nlimit(n))
+		}
+	}
+}
+
+// Returns true if s contains only white space
+// (only tabs and blanks can appear in the printer's context).
+//
+func isBlank(s string) bool {
+	for i := 0; i < len(s); i++ {
+		if s[i] > ' ' {
+			return false
+		}
+	}
+	return true
+}
+
+// commonPrefix returns the common prefix of a and b.
+func commonPrefix(a, b string) string {
+	i := 0
+	for i < len(a) && i < len(b) && a[i] == b[i] && (a[i] <= ' ' || a[i] == '*') {
+		i++
+	}
+	return a[0:i]
+}
+
+// trimRight returns s with trailing whitespace removed.
+func trimRight(s string) string {
+	return strings.TrimRightFunc(s, unicode.IsSpace)
+}
+
+// stripCommonPrefix removes a common prefix from /*-style comment lines (unless no
+// comment line is indented, all but the first line have some form of space prefix).
+// The prefix is computed using heuristics such that is likely that the comment
+// contents are nicely laid out after re-printing each line using the printer's
+// current indentation.
+//
+func stripCommonPrefix(lines []string) {
+	if len(lines) <= 1 {
+		return // at most one line - nothing to do
+	}
+	// len(lines) > 1
+
+	// The heuristic in this function tries to handle a few
+	// common patterns of /*-style comments: Comments where
+	// the opening /* and closing */ are aligned and the
+	// rest of the comment text is aligned and indented with
+	// blanks or tabs, cases with a vertical "line of stars"
+	// on the left, and cases where the closing */ is on the
+	// same line as the last comment text.
+
+	// Compute maximum common white prefix of all but the first,
+	// last, and blank lines, and replace blank lines with empty
+	// lines (the first line starts with /* and has no prefix).
+	// In case of two-line comments, consider the last line for
+	// the prefix computation since otherwise the prefix would
+	// be empty.
+	//
+	// Note that the first and last line are never empty (they
+	// contain the opening /* and closing */ respectively) and
+	// thus they can be ignored by the blank line check.
+	var prefix string
+	if len(lines) > 2 {
+		first := true
+		for i, line := range lines[1 : len(lines)-1] {
+			switch {
+			case isBlank(line):
+				lines[1+i] = "" // range starts with lines[1]
+			case first:
+				prefix = commonPrefix(line, line)
+				first = false
+			default:
+				prefix = commonPrefix(prefix, line)
+			}
+		}
+	} else { // len(lines) == 2, lines cannot be blank (contain /* and */)
+		line := lines[1]
+		prefix = commonPrefix(line, line)
+	}
+
+	/*
+	 * Check for vertical "line of stars" and correct prefix accordingly.
+	 */
+	lineOfStars := false
+	if i := strings.Index(prefix, "*"); i >= 0 {
+		// Line of stars present.
+		if i > 0 && prefix[i-1] == ' ' {
+			i-- // remove trailing blank from prefix so stars remain aligned
+		}
+		prefix = prefix[0:i]
+		lineOfStars = true
+	} else {
+		// No line of stars present.
+		// Determine the white space on the first line after the /*
+		// and before the beginning of the comment text, assume two
+		// blanks instead of the /* unless the first character after
+		// the /* is a tab. If the first comment line is empty but
+		// for the opening /*, assume up to 3 blanks or a tab. This
+		// whitespace may be found as suffix in the common prefix.
+		first := lines[0]
+		if isBlank(first[2:]) {
+			// no comment text on the first line:
+			// reduce prefix by up to 3 blanks or a tab
+			// if present - this keeps comment text indented
+			// relative to the /* and */'s if it was indented
+			// in the first place
+			i := len(prefix)
+			for n := 0; n < 3 && i > 0 && prefix[i-1] == ' '; n++ {
+				i--
+			}
+			if i == len(prefix) && i > 0 && prefix[i-1] == '\t' {
+				i--
+			}
+			prefix = prefix[0:i]
+		} else {
+			// comment text on the first line
+			suffix := make([]byte, len(first))
+			n := 2 // start after opening /*
+			for n < len(first) && first[n] <= ' ' {
+				suffix[n] = first[n]
+				n++
+			}
+			if n > 2 && suffix[2] == '\t' {
+				// assume the '\t' compensates for the /*
+				suffix = suffix[2:n]
+			} else {
+				// otherwise assume two blanks
+				suffix[0], suffix[1] = ' ', ' '
+				suffix = suffix[0:n]
+			}
+			// Shorten the computed common prefix by the length of
+			// suffix, if it is found as suffix of the prefix.
+			prefix = strings.TrimSuffix(prefix, string(suffix))
+		}
+	}
+
+	// Handle last line: If it only contains a closing */, align it
+	// with the opening /*, otherwise align the text with the other
+	// lines.
+	last := lines[len(lines)-1]
+	closing := "*/"
+	i := strings.Index(last, closing) // i >= 0 (closing is always present)
+	if isBlank(last[0:i]) {
+		// last line only contains closing */
+		if lineOfStars {
+			closing = " */" // add blank to align final star
+		}
+		lines[len(lines)-1] = prefix + closing
+	} else {
+		// last line contains more comment text - assume
+		// it is aligned like the other lines and include
+		// in prefix computation
+		prefix = commonPrefix(prefix, last)
+	}
+
+	// Remove the common prefix from all but the first and empty lines.
+	for i, line := range lines {
+		if i > 0 && line != "" {
+			lines[i] = line[len(prefix):]
+		}
+	}
+}
+
+func (p *printer) writeComment(comment *ast.Comment) {
+	text := comment.Text
+	pos := p.posFor(comment.Pos())
+
+	const linePrefix = "//line "
+	if strings.HasPrefix(text, linePrefix) && (!pos.IsValid() || pos.Column == 1) {
+		// possibly a line directive
+		ldir := strings.TrimSpace(text[len(linePrefix):])
+		if i := strings.LastIndex(ldir, ":"); i >= 0 {
+			if line, err := strconv.Atoi(ldir[i+1:]); err == nil && line > 0 {
+				// The line directive we are about to print changed
+				// the Filename and Line number used for subsequent
+				// tokens. We have to update our AST-space position
+				// accordingly and suspend indentation temporarily.
+				indent := p.indent
+				p.indent = 0
+				defer func() {
+					p.pos.Filename = ldir[:i]
+					p.pos.Line = line
+					p.pos.Column = 1
+					p.indent = indent
+				}()
+			}
+		}
+	}
+
+	// shortcut common case of //-style comments
+	if text[1] == '/' {
+		p.writeString(pos, trimRight(text), true)
+		return
+	}
+
+	// for /*-style comments, print line by line and let the
+	// write function take care of the proper indentation
+	lines := strings.Split(text, "\n")
+
+	// The comment started in the first column but is going
+	// to be indented. For an idempotent result, add indentation
+	// to all lines such that they look like they were indented
+	// before - this will make sure the common prefix computation
+	// is the same independent of how many times formatting is
+	// applied (was issue 1835).
+	if pos.IsValid() && pos.Column == 1 && p.indent > 0 {
+		for i, line := range lines[1:] {
+			lines[1+i] = "   " + line
+		}
+	}
+
+	stripCommonPrefix(lines)
+
+	// write comment lines, separated by formfeed,
+	// without a line break after the last line
+	for i, line := range lines {
+		if i > 0 {
+			p.writeByte('\f', 1)
+			pos = p.pos
+		}
+		if len(line) > 0 {
+			p.writeString(pos, trimRight(line), true)
+		}
+	}
+}
+
+// writeCommentSuffix writes a line break after a comment if indicated
+// and processes any leftover indentation information. If a line break
+// is needed, the kind of break (newline vs formfeed) depends on the
+// pending whitespace. The writeCommentSuffix result indicates if a
+// newline was written or if a formfeed was dropped from the whitespace
+// buffer.
+//
+func (p *printer) writeCommentSuffix(needsLinebreak bool) (wroteNewline, droppedFF bool) {
+	for i, ch := range p.wsbuf {
+		switch ch {
+		case blank, vtab:
+			// ignore trailing whitespace
+			p.wsbuf[i] = ignore
+		case indent, unindent:
+			// don't lose indentation information
+		case newline, formfeed:
+			// if we need a line break, keep exactly one
+			// but remember if we dropped any formfeeds
+			if needsLinebreak {
+				needsLinebreak = false
+				wroteNewline = true
+			} else {
+				if ch == formfeed {
+					droppedFF = true
+				}
+				p.wsbuf[i] = ignore
+			}
+		}
+	}
+	p.writeWhitespace(len(p.wsbuf))
+
+	// make sure we have a line break
+	if needsLinebreak {
+		p.writeByte('\n', 1)
+		wroteNewline = true
+	}
+
+	return
+}
+
+// intersperseComments consumes all comments that appear before the next token
+// tok and prints it together with the buffered whitespace (i.e., the whitespace
+// that needs to be written before the next token). A heuristic is used to mix
+// the comments and whitespace. The intersperseComments result indicates if a
+// newline was written or if a formfeed was dropped from the whitespace buffer.
+//
+func (p *printer) intersperseComments(next token.Position, tok token.Token) (wroteNewline, droppedFF bool) {
+	var last *ast.Comment
+	for p.commentBefore(next) {
+		for _, c := range p.comment.List {
+			p.writeCommentPrefix(p.posFor(c.Pos()), next, last, c, tok)
+			p.writeComment(c)
+			last = c
+		}
+		p.nextComment()
+	}
+
+	if last != nil {
+		// if the last comment is a /*-style comment and the next item
+		// follows on the same line but is not a comma, and not a "closing"
+		// token immediately following its corresponding "opening" token,
+		// add an extra blank for separation unless explicitly disabled
+		if p.mode&noExtraBlank == 0 &&
+			last.Text[1] == '*' && p.lineFor(last.Pos()) == next.Line &&
+			tok != token.COMMA &&
+			(tok != token.RPAREN || p.prevOpen == token.LPAREN) &&
+			(tok != token.RBRACK || p.prevOpen == token.LBRACK) {
+			p.writeByte(' ', 1)
+		}
+		// ensure that there is a line break after a //-style comment,
+		// before a closing '}' unless explicitly disabled, or at eof
+		needsLinebreak :=
+			last.Text[1] == '/' ||
+				tok == token.RBRACE && p.mode&noExtraLinebreak == 0 ||
+				tok == token.EOF
+		return p.writeCommentSuffix(needsLinebreak)
+	}
+
+	// no comment was written - we should never reach here since
+	// intersperseComments should not be called in that case
+	p.internalError("intersperseComments called without pending comments")
+	return
+}
+
+// whiteWhitespace writes the first n whitespace entries.
+func (p *printer) writeWhitespace(n int) {
+	// write entries
+	for i := 0; i < n; i++ {
+		switch ch := p.wsbuf[i]; ch {
+		case ignore:
+			// ignore!
+		case indent:
+			p.indent++
+		case unindent:
+			p.indent--
+			if p.indent < 0 {
+				p.internalError("negative indentation:", p.indent)
+				p.indent = 0
+			}
+		case newline, formfeed:
+			// A line break immediately followed by a "correcting"
+			// unindent is swapped with the unindent - this permits
+			// proper label positioning. If a comment is between
+			// the line break and the label, the unindent is not
+			// part of the comment whitespace prefix and the comment
+			// will be positioned correctly indented.
+			if i+1 < n && p.wsbuf[i+1] == unindent {
+				// Use a formfeed to terminate the current section.
+				// Otherwise, a long label name on the next line leading
+				// to a wide column may increase the indentation column
+				// of lines before the label; effectively leading to wrong
+				// indentation.
+				p.wsbuf[i], p.wsbuf[i+1] = unindent, formfeed
+				i-- // do it again
+				continue
+			}
+			fallthrough
+		default:
+			p.writeByte(byte(ch), 1)
+		}
+	}
+
+	// shift remaining entries down
+	l := copy(p.wsbuf, p.wsbuf[n:])
+	p.wsbuf = p.wsbuf[:l]
+}
+
+// ----------------------------------------------------------------------------
+// Printing interface
+
+// nlines limits n to maxNewlines.
+func nlimit(n int) int {
+	if n > maxNewlines {
+		n = maxNewlines
+	}
+	return n
+}
+
+func mayCombine(prev token.Token, next byte) (b bool) {
+	switch prev {
+	case token.INT:
+		b = next == '.' // 1.
+	case token.ADD:
+		b = next == '+' // ++
+	case token.SUB:
+		b = next == '-' // --
+	case token.QUO:
+		b = next == '*' // /*
+	case token.LSS:
+		b = next == '-' || next == '<' // <- or <<
+	case token.AND:
+		b = next == '&' || next == '^' // && or &^
+	}
+	return
+}
+
+// print prints a list of "items" (roughly corresponding to syntactic
+// tokens, but also including whitespace and formatting information).
+// It is the only print function that should be called directly from
+// any of the AST printing functions in nodes.go.
+//
+// Whitespace is accumulated until a non-whitespace token appears. Any
+// comments that need to appear before that token are printed first,
+// taking into account the amount and structure of any pending white-
+// space for best comment placement. Then, any leftover whitespace is
+// printed, followed by the actual token.
+//
+func (p *printer) print(args ...interface{}) {
+	for _, arg := range args {
+		// information about the current arg
+		var data string
+		var isLit bool
+		var impliedSemi bool // value for p.impliedSemi after this arg
+
+		// record previous opening token, if any
+		switch p.lastTok {
+		case token.ILLEGAL:
+			// ignore (white space)
+		case token.LPAREN, token.LBRACK:
+			p.prevOpen = p.lastTok
+		default:
+			// other tokens followed any opening token
+			p.prevOpen = token.ILLEGAL
+		}
+
+		switch x := arg.(type) {
+		case pmode:
+			// toggle printer mode
+			p.mode ^= x
+			continue
+
+		case whiteSpace:
+			if x == ignore {
+				// don't add ignore's to the buffer; they
+				// may screw up "correcting" unindents (see
+				// LabeledStmt)
+				continue
+			}
+			i := len(p.wsbuf)
+			if i == cap(p.wsbuf) {
+				// Whitespace sequences are very short so this should
+				// never happen. Handle gracefully (but possibly with
+				// bad comment placement) if it does happen.
+				p.writeWhitespace(i)
+				i = 0
+			}
+			p.wsbuf = p.wsbuf[0 : i+1]
+			p.wsbuf[i] = x
+			if x == newline || x == formfeed {
+				// newlines affect the current state (p.impliedSemi)
+				// and not the state after printing arg (impliedSemi)
+				// because comments can be interspersed before the arg
+				// in this case
+				p.impliedSemi = false
+			}
+			p.lastTok = token.ILLEGAL
+			continue
+
+		case *ast.Ident:
+			data = x.Name
+			impliedSemi = true
+			p.lastTok = token.IDENT
+
+		case *ast.BasicLit:
+			data = x.Value
+			isLit = true
+			impliedSemi = true
+			p.lastTok = x.Kind
+
+		case token.Token:
+			s := x.String()
+			if mayCombine(p.lastTok, s[0]) {
+				// the previous and the current token must be
+				// separated by a blank otherwise they combine
+				// into a different incorrect token sequence
+				// (except for token.INT followed by a '.' this
+				// should never happen because it is taken care
+				// of via binary expression formatting)
+				if len(p.wsbuf) != 0 {
+					p.internalError("whitespace buffer not empty")
+				}
+				p.wsbuf = p.wsbuf[0:1]
+				p.wsbuf[0] = ' '
+			}
+			data = s
+			// some keywords followed by a newline imply a semicolon
+			switch x {
+			case token.BREAK, token.CONTINUE, token.FALLTHROUGH, token.RETURN,
+				token.INC, token.DEC, token.RPAREN, token.RBRACK, token.RBRACE:
+				impliedSemi = true
+			}
+			p.lastTok = x
+
+		case token.Pos:
+			if x.IsValid() {
+				p.pos = p.posFor(x) // accurate position of next item
+			}
+			continue
+
+		case string:
+			// incorrect AST - print error message
+			data = x
+			isLit = true
+			impliedSemi = true
+			p.lastTok = token.STRING
+
+		default:
+			fmt.Fprintf(os.Stderr, "print: unsupported argument %v (%T)\n", arg, arg)
+			panic("go/printer type")
+		}
+		// data != ""
+
+		next := p.pos // estimated/accurate position of next item
+		wroteNewline, droppedFF := p.flush(next, p.lastTok)
+
+		// intersperse extra newlines if present in the source and
+		// if they don't cause extra semicolons (don't do this in
+		// flush as it will cause extra newlines at the end of a file)
+		if !p.impliedSemi {
+			n := nlimit(next.Line - p.pos.Line)
+			// don't exceed maxNewlines if we already wrote one
+			if wroteNewline && n == maxNewlines {
+				n = maxNewlines - 1
+			}
+			if n > 0 {
+				ch := byte('\n')
+				if droppedFF {
+					ch = '\f' // use formfeed since we dropped one before
+				}
+				p.writeByte(ch, n)
+				impliedSemi = false
+			}
+		}
+
+		// the next token starts now - record its line number if requested
+		if p.linePtr != nil {
+			*p.linePtr = p.out.Line
+			p.linePtr = nil
+		}
+
+		p.writeString(next, data, isLit)
+		p.impliedSemi = impliedSemi
+	}
+}
+
+// flush prints any pending comments and whitespace occurring textually
+// before the position of the next token tok. The flush result indicates
+// if a newline was written or if a formfeed was dropped from the whitespace
+// buffer.
+//
+func (p *printer) flush(next token.Position, tok token.Token) (wroteNewline, droppedFF bool) {
+	if p.commentBefore(next) {
+		// if there are comments before the next item, intersperse them
+		wroteNewline, droppedFF = p.intersperseComments(next, tok)
+	} else {
+		// otherwise, write any leftover whitespace
+		p.writeWhitespace(len(p.wsbuf))
+	}
+	return
+}
+
+// getNode returns the ast.CommentGroup associated with n, if any.
+func getDoc(n ast.Node) *ast.CommentGroup {
+	switch n := n.(type) {
+	case *ast.Field:
+		return n.Doc
+	case *ast.ImportSpec:
+		return n.Doc
+	case *ast.ValueSpec:
+		return n.Doc
+	case *ast.TypeSpec:
+		return n.Doc
+	case *ast.GenDecl:
+		return n.Doc
+	case *ast.FuncDecl:
+		return n.Doc
+	case *ast.File:
+		return n.Doc
+	}
+	return nil
+}
+
+func (p *printer) printNode(node interface{}) error {
+	// unpack *CommentedNode, if any
+	var comments []*ast.CommentGroup
+	if cnode, ok := node.(*CommentedNode); ok {
+		node = cnode.Node
+		comments = cnode.Comments
+	}
+
+	if comments != nil {
+		// commented node - restrict comment list to relevant range
+		n, ok := node.(ast.Node)
+		if !ok {
+			goto unsupported
+		}
+		beg := n.Pos()
+		end := n.End()
+		// if the node has associated documentation,
+		// include that commentgroup in the range
+		// (the comment list is sorted in the order
+		// of the comment appearance in the source code)
+		if doc := getDoc(n); doc != nil {
+			beg = doc.Pos()
+		}
+		// token.Pos values are global offsets, we can
+		// compare them directly
+		i := 0
+		for i < len(comments) && comments[i].End() < beg {
+			i++
+		}
+		j := i
+		for j < len(comments) && comments[j].Pos() < end {
+			j++
+		}
+		if i < j {
+			p.comments = comments[i:j]
+		}
+	} else if n, ok := node.(*ast.File); ok {
+		// use ast.File comments, if any
+		p.comments = n.Comments
+	}
+
+	// if there are no comments, use node comments
+	p.useNodeComments = p.comments == nil
+
+	// get comments ready for use
+	p.nextComment()
+
+	// format node
+	switch n := node.(type) {
+	case ast.Expr:
+		p.expr(n)
+	case ast.Stmt:
+		// A labeled statement will un-indent to position the label.
+		// Set p.indent to 1 so we don't get indent "underflow".
+		if _, ok := n.(*ast.LabeledStmt); ok {
+			p.indent = 1
+		}
+		p.stmt(n, false)
+	case ast.Decl:
+		p.decl(n)
+	case ast.Spec:
+		p.spec(n, 1, false)
+	case []ast.Stmt:
+		// A labeled statement will un-indent to position the label.
+		// Set p.indent to 1 so we don't get indent "underflow".
+		for _, s := range n {
+			if _, ok := s.(*ast.LabeledStmt); ok {
+				p.indent = 1
+			}
+		}
+		p.stmtList(n, 0, false)
+	case []ast.Decl:
+		p.declList(n)
+	case *ast.File:
+		p.file(n)
+	default:
+		goto unsupported
+	}
+
+	return nil
+
+unsupported:
+	return fmt.Errorf("go/printer: unsupported node type %T", node)
+}
+
+// ----------------------------------------------------------------------------
+// Trimmer
+
+// A trimmer is an io.Writer filter for stripping tabwriter.Escape
+// characters, trailing blanks and tabs, and for converting formfeed
+// and vtab characters into newlines and htabs (in case no tabwriter
+// is used). Text bracketed by tabwriter.Escape characters is passed
+// through unchanged.
+//
+type trimmer struct {
+	output io.Writer
+	state  int
+	space  []byte
+}
+
+// trimmer is implemented as a state machine.
+// It can be in one of the following states:
+const (
+	inSpace  = iota // inside space
+	inEscape        // inside text bracketed by tabwriter.Escapes
+	inText          // inside text
+)
+
+func (p *trimmer) resetSpace() {
+	p.state = inSpace
+	p.space = p.space[0:0]
+}
+
+// Design note: It is tempting to eliminate extra blanks occurring in
+//              whitespace in this function as it could simplify some
+//              of the blanks logic in the node printing functions.
+//              However, this would mess up any formatting done by
+//              the tabwriter.
+
+var aNewline = []byte("\n")
+
+func (p *trimmer) Write(data []byte) (n int, err error) {
+	// invariants:
+	// p.state == inSpace:
+	//	p.space is unwritten
+	// p.state == inEscape, inText:
+	//	data[m:n] is unwritten
+	m := 0
+	var b byte
+	for n, b = range data {
+		if b == '\v' {
+			b = '\t' // convert to htab
+		}
+		switch p.state {
+		case inSpace:
+			switch b {
+			case '\t', ' ':
+				p.space = append(p.space, b)
+			case '\n', '\f':
+				p.resetSpace() // discard trailing space
+				_, err = p.output.Write(aNewline)
+			case tabwriter.Escape:
+				_, err = p.output.Write(p.space)
+				p.state = inEscape
+				m = n + 1 // +1: skip tabwriter.Escape
+			default:
+				_, err = p.output.Write(p.space)
+				p.state = inText
+				m = n
+			}
+		case inEscape:
+			if b == tabwriter.Escape {
+				_, err = p.output.Write(data[m:n])
+				p.resetSpace()
+			}
+		case inText:
+			switch b {
+			case '\t', ' ':
+				_, err = p.output.Write(data[m:n])
+				p.resetSpace()
+				p.space = append(p.space, b)
+			case '\n', '\f':
+				_, err = p.output.Write(data[m:n])
+				p.resetSpace()
+				_, err = p.output.Write(aNewline)
+			case tabwriter.Escape:
+				_, err = p.output.Write(data[m:n])
+				p.state = inEscape
+				m = n + 1 // +1: skip tabwriter.Escape
+			}
+		default:
+			panic("unreachable")
+		}
+		if err != nil {
+			return
+		}
+	}
+	n = len(data)
+
+	switch p.state {
+	case inEscape, inText:
+		_, err = p.output.Write(data[m:n])
+		p.resetSpace()
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Public interface
+
+// A Mode value is a set of flags (or 0). They control printing.
+type Mode uint
+
+const (
+	RawFormat Mode = 1 << iota // do not use a tabwriter; if set, UseSpaces is ignored
+	TabIndent                  // use tabs for indentation independent of UseSpaces
+	UseSpaces                  // use spaces instead of tabs for alignment
+	SourcePos                  // emit //line comments to preserve original source positions
+)
+
+// A Config node controls the output of Fprint.
+type Config struct {
+	Mode     Mode // default: 0
+	Tabwidth int  // default: 8
+	Indent   int  // default: 0 (all code is indented at least by this much)
+}
+
+// fprint implements Fprint and takes a nodesSizes map for setting up the printer state.
+func (cfg *Config) fprint(output io.Writer, fset *token.FileSet, node interface{}, nodeSizes map[ast.Node]int) (err error) {
+	// print node
+	var p printer
+	p.init(cfg, fset, nodeSizes)
+	if err = p.printNode(node); err != nil {
+		return
+	}
+	// print outstanding comments
+	p.impliedSemi = false // EOF acts like a newline
+	p.flush(token.Position{Offset: infinity, Line: infinity}, token.EOF)
+
+	// redirect output through a trimmer to eliminate trailing whitespace
+	// (Input to a tabwriter must be untrimmed since trailing tabs provide
+	// formatting information. The tabwriter could provide trimming
+	// functionality but no tabwriter is used when RawFormat is set.)
+	output = &trimmer{output: output}
+
+	// redirect output through a tabwriter if necessary
+	if cfg.Mode&RawFormat == 0 {
+		minwidth := cfg.Tabwidth
+
+		padchar := byte('\t')
+		if cfg.Mode&UseSpaces != 0 {
+			padchar = ' '
+		}
+
+		twmode := tabwriter.DiscardEmptyColumns
+		if cfg.Mode&TabIndent != 0 {
+			minwidth = 0
+			twmode |= tabwriter.TabIndent
+		}
+
+		output = tabwriter.NewWriter(output, minwidth, cfg.Tabwidth, 1, padchar, twmode)
+	}
+
+	// write printer result via tabwriter/trimmer to output
+	if _, err = output.Write(p.output); err != nil {
+		return
+	}
+
+	// flush tabwriter, if any
+	if tw, _ := output.(*tabwriter.Writer); tw != nil {
+		err = tw.Flush()
+	}
+
+	return
+}
+
+// A CommentedNode bundles an AST node and corresponding comments.
+// It may be provided as argument to any of the Fprint functions.
+//
+type CommentedNode struct {
+	Node     interface{} // *ast.File, or ast.Expr, ast.Decl, ast.Spec, or ast.Stmt
+	Comments []*ast.CommentGroup
+}
+
+// Fprint "pretty-prints" an AST node to output for a given configuration cfg.
+// Position information is interpreted relative to the file set fset.
+// The node type must be *ast.File, *CommentedNode, []ast.Decl, []ast.Stmt,
+// or assignment-compatible to ast.Expr, ast.Decl, ast.Spec, or ast.Stmt.
+//
+func (cfg *Config) Fprint(output io.Writer, fset *token.FileSet, node interface{}) error {
+	return cfg.fprint(output, fset, node, make(map[ast.Node]int))
+}
+
+// Fprint "pretty-prints" an AST node to output.
+// It calls Config.Fprint with default settings.
+//
+func Fprint(output io.Writer, fset *token.FileSet, node interface{}) error {
+	return (&Config{Tabwidth: 8}).Fprint(output, fset, node)
+}
diff --git a/src/go/printer/printer_test.go b/src/go/printer/printer_test.go
new file mode 100644
index 000000000..3b0570e5b
--- /dev/null
+++ b/src/go/printer/printer_test.go
@@ -0,0 +1,562 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package printer
+
+import (
+	"bytes"
+	"errors"
+	"flag"
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"io/ioutil"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+const (
+	dataDir  = "testdata"
+	tabwidth = 8
+)
+
+var update = flag.Bool("update", false, "update golden files")
+
+var fset = token.NewFileSet()
+
+type checkMode uint
+
+const (
+	export checkMode = 1 << iota
+	rawFormat
+	idempotent
+)
+
+// format parses src, prints the corresponding AST, verifies the resulting
+// src is syntactically correct, and returns the resulting src or an error
+// if any.
+func format(src []byte, mode checkMode) ([]byte, error) {
+	// parse src
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		return nil, fmt.Errorf("parse: %s\n%s", err, src)
+	}
+
+	// filter exports if necessary
+	if mode&export != 0 {
+		ast.FileExports(f) // ignore result
+		f.Comments = nil   // don't print comments that are not in AST
+	}
+
+	// determine printer configuration
+	cfg := Config{Tabwidth: tabwidth}
+	if mode&rawFormat != 0 {
+		cfg.Mode |= RawFormat
+	}
+
+	// print AST
+	var buf bytes.Buffer
+	if err := cfg.Fprint(&buf, fset, f); err != nil {
+		return nil, fmt.Errorf("print: %s", err)
+	}
+
+	// make sure formatted output is syntactically correct
+	res := buf.Bytes()
+	if _, err := parser.ParseFile(fset, "", res, 0); err != nil {
+		return nil, fmt.Errorf("re-parse: %s\n%s", err, buf.Bytes())
+	}
+
+	return res, nil
+}
+
+// lineAt returns the line in text starting at offset offs.
+func lineAt(text []byte, offs int) []byte {
+	i := offs
+	for i < len(text) && text[i] != '\n' {
+		i++
+	}
+	return text[offs:i]
+}
+
+// diff compares a and b.
+func diff(aname, bname string, a, b []byte) error {
+	var buf bytes.Buffer // holding long error message
+
+	// compare lengths
+	if len(a) != len(b) {
+		fmt.Fprintf(&buf, "\nlength changed: len(%s) = %d, len(%s) = %d", aname, len(a), bname, len(b))
+	}
+
+	// compare contents
+	line := 1
+	offs := 1
+	for i := 0; i < len(a) && i < len(b); i++ {
+		ch := a[i]
+		if ch != b[i] {
+			fmt.Fprintf(&buf, "\n%s:%d:%d: %s", aname, line, i-offs+1, lineAt(a, offs))
+			fmt.Fprintf(&buf, "\n%s:%d:%d: %s", bname, line, i-offs+1, lineAt(b, offs))
+			fmt.Fprintf(&buf, "\n\n")
+			break
+		}
+		if ch == '\n' {
+			line++
+			offs = i + 1
+		}
+	}
+
+	if buf.Len() > 0 {
+		return errors.New(buf.String())
+	}
+	return nil
+}
+
+func runcheck(t *testing.T, source, golden string, mode checkMode) {
+	src, err := ioutil.ReadFile(source)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	res, err := format(src, mode)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	// update golden files if necessary
+	if *update {
+		if err := ioutil.WriteFile(golden, res, 0644); err != nil {
+			t.Error(err)
+		}
+		return
+	}
+
+	// get golden
+	gld, err := ioutil.ReadFile(golden)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	// formatted source and golden must be the same
+	if err := diff(source, golden, res, gld); err != nil {
+		t.Error(err)
+		return
+	}
+
+	if mode&idempotent != 0 {
+		// formatting golden must be idempotent
+		// (This is very difficult to achieve in general and for now
+		// it is only checked for files explicitly marked as such.)
+		res, err = format(gld, mode)
+		if err := diff(golden, fmt.Sprintf("format(%s)", golden), gld, res); err != nil {
+			t.Errorf("golden is not idempotent: %s", err)
+		}
+	}
+}
+
+func check(t *testing.T, source, golden string, mode checkMode) {
+	// run the test
+	cc := make(chan int)
+	go func() {
+		runcheck(t, source, golden, mode)
+		cc <- 0
+	}()
+
+	// wait with timeout
+	select {
+	case <-time.After(10 * time.Second): // plenty of a safety margin, even for very slow machines
+		// test running past time out
+		t.Errorf("%s: running too slowly", source)
+	case <-cc:
+		// test finished within allotted time margin
+	}
+}
+
+type entry struct {
+	source, golden string
+	mode           checkMode
+}
+
+// Use go test -update to create/update the respective golden files.
+var data = []entry{
+	{"empty.input", "empty.golden", idempotent},
+	{"comments.input", "comments.golden", 0},
+	{"comments.input", "comments.x", export},
+	{"comments2.input", "comments2.golden", idempotent},
+	{"linebreaks.input", "linebreaks.golden", idempotent},
+	{"expressions.input", "expressions.golden", idempotent},
+	{"expressions.input", "expressions.raw", rawFormat | idempotent},
+	{"declarations.input", "declarations.golden", 0},
+	{"statements.input", "statements.golden", 0},
+	{"slow.input", "slow.golden", idempotent},
+}
+
+func TestFiles(t *testing.T) {
+	for _, e := range data {
+		source := filepath.Join(dataDir, e.source)
+		golden := filepath.Join(dataDir, e.golden)
+		check(t, source, golden, e.mode)
+		// TODO(gri) check that golden is idempotent
+		//check(t, golden, golden, e.mode)
+	}
+}
+
+// TestLineComments, using a simple test case, checks that consecutive line
+// comments are properly terminated with a newline even if the AST position
+// information is incorrect.
+//
+func TestLineComments(t *testing.T) {
+	const src = `// comment 1
+	// comment 2
+	// comment 3
+	package main
+	`
+
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		panic(err) // error in test
+	}
+
+	var buf bytes.Buffer
+	fset = token.NewFileSet() // use the wrong file set
+	Fprint(&buf, fset, f)
+
+	nlines := 0
+	for _, ch := range buf.Bytes() {
+		if ch == '\n' {
+			nlines++
+		}
+	}
+
+	const expected = 3
+	if nlines < expected {
+		t.Errorf("got %d, expected %d\n", nlines, expected)
+		t.Errorf("result:\n%s", buf.Bytes())
+	}
+}
+
+// Verify that the printer can be invoked during initialization.
+func init() {
+	const name = "foobar"
+	var buf bytes.Buffer
+	if err := Fprint(&buf, fset, &ast.Ident{Name: name}); err != nil {
+		panic(err) // error in test
+	}
+	// in debug mode, the result contains additional information;
+	// ignore it
+	if s := buf.String(); !debug && s != name {
+		panic("got " + s + ", want " + name)
+	}
+}
+
+// Verify that the printer doesn't crash if the AST contains BadXXX nodes.
+func TestBadNodes(t *testing.T) {
+	const src = "package p\n("
+	const res = "package p\nBadDecl\n"
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err == nil {
+		t.Error("expected illegal program") // error in test
+	}
+	var buf bytes.Buffer
+	Fprint(&buf, fset, f)
+	if buf.String() != res {
+		t.Errorf("got %q, expected %q", buf.String(), res)
+	}
+}
+
+// testComment verifies that f can be parsed again after printing it
+// with its first comment set to comment at any possible source offset.
+func testComment(t *testing.T, f *ast.File, srclen int, comment *ast.Comment) {
+	f.Comments[0].List[0] = comment
+	var buf bytes.Buffer
+	for offs := 0; offs <= srclen; offs++ {
+		buf.Reset()
+		// Printing f should result in a correct program no
+		// matter what the (incorrect) comment position is.
+		if err := Fprint(&buf, fset, f); err != nil {
+			t.Error(err)
+		}
+		if _, err := parser.ParseFile(fset, "", buf.Bytes(), 0); err != nil {
+			t.Fatalf("incorrect program for pos = %d:\n%s", comment.Slash, buf.String())
+		}
+		// Position information is just an offset.
+		// Move comment one byte down in the source.
+		comment.Slash++
+	}
+}
+
+// Verify that the printer produces a correct program
+// even if the position information of comments introducing newlines
+// is incorrect.
+func TestBadComments(t *testing.T) {
+	const src = `
+// first comment - text and position changed by test
+package p
+import "fmt"
+const pi = 3.14 // rough circle
+var (
+	x, y, z int = 1, 2, 3
+	u, v float64
+)
+func fibo(n int) {
+	if n < 2 {
+		return n /* seed values */
+	}
+	return fibo(n-1) + fibo(n-2)
+}
+`
+
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		t.Error(err) // error in test
+	}
+
+	comment := f.Comments[0].List[0]
+	pos := comment.Pos()
+	if fset.Position(pos).Offset != 1 {
+		t.Error("expected offset 1") // error in test
+	}
+
+	testComment(t, f, len(src), &ast.Comment{Slash: pos, Text: "//-style comment"})
+	testComment(t, f, len(src), &ast.Comment{Slash: pos, Text: "/*-style comment */"})
+	testComment(t, f, len(src), &ast.Comment{Slash: pos, Text: "/*-style \n comment */"})
+	testComment(t, f, len(src), &ast.Comment{Slash: pos, Text: "/*-style comment \n\n\n */"})
+}
+
+type visitor chan *ast.Ident
+
+func (v visitor) Visit(n ast.Node) (w ast.Visitor) {
+	if ident, ok := n.(*ast.Ident); ok {
+		v <- ident
+	}
+	return v
+}
+
+// idents is an iterator that returns all idents in f via the result channel.
+func idents(f *ast.File) <-chan *ast.Ident {
+	v := make(visitor)
+	go func() {
+		ast.Walk(v, f)
+		close(v)
+	}()
+	return v
+}
+
+// identCount returns the number of identifiers found in f.
+func identCount(f *ast.File) int {
+	n := 0
+	for range idents(f) {
+		n++
+	}
+	return n
+}
+
+// Verify that the SourcePos mode emits correct //line comments
+// by testing that position information for matching identifiers
+// is maintained.
+func TestSourcePos(t *testing.T) {
+	const src = `
+package p
+import ( "go/printer"; "math" )
+const pi = 3.14; var x = 0
+type t struct{ x, y, z int; u, v, w float32 }
+func (t *t) foo(a, b, c int) int {
+	return a*t.x + b*t.y +
+		// two extra lines here
+		// ...
+		c*t.z
+}
+`
+
+	// parse original
+	f1, err := parser.ParseFile(fset, "src", src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// pretty-print original
+	var buf bytes.Buffer
+	err = (&Config{Mode: UseSpaces | SourcePos, Tabwidth: 8}).Fprint(&buf, fset, f1)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// parse pretty printed original
+	// (//line comments must be interpreted even w/o parser.ParseComments set)
+	f2, err := parser.ParseFile(fset, "", buf.Bytes(), 0)
+	if err != nil {
+		t.Fatalf("%s\n%s", err, buf.Bytes())
+	}
+
+	// At this point the position information of identifiers in f2 should
+	// match the position information of corresponding identifiers in f1.
+
+	// number of identifiers must be > 0 (test should run) and must match
+	n1 := identCount(f1)
+	n2 := identCount(f2)
+	if n1 == 0 {
+		t.Fatal("got no idents")
+	}
+	if n2 != n1 {
+		t.Errorf("got %d idents; want %d", n2, n1)
+	}
+
+	// verify that all identifiers have correct line information
+	i2range := idents(f2)
+	for i1 := range idents(f1) {
+		i2 := <-i2range
+
+		if i2.Name != i1.Name {
+			t.Errorf("got ident %s; want %s", i2.Name, i1.Name)
+		}
+
+		l1 := fset.Position(i1.Pos()).Line
+		l2 := fset.Position(i2.Pos()).Line
+		if l2 != l1 {
+			t.Errorf("got line %d; want %d for %s", l2, l1, i1.Name)
+		}
+	}
+
+	if t.Failed() {
+		t.Logf("\n%s", buf.Bytes())
+	}
+}
+
+var decls = []string{
+	`import "fmt"`,
+	"const pi = 3.1415\nconst e = 2.71828\n\nvar x = pi",
+	"func sum(x, y int) int\t{ return x + y }",
+}
+
+func TestDeclLists(t *testing.T) {
+	for _, src := range decls {
+		file, err := parser.ParseFile(fset, "", "package p;"+src, parser.ParseComments)
+		if err != nil {
+			panic(err) // error in test
+		}
+
+		var buf bytes.Buffer
+		err = Fprint(&buf, fset, file.Decls) // only print declarations
+		if err != nil {
+			panic(err) // error in test
+		}
+
+		out := buf.String()
+		if out != src {
+			t.Errorf("\ngot : %q\nwant: %q\n", out, src)
+		}
+	}
+}
+
+var stmts = []string{
+	"i := 0",
+	"select {}\nvar a, b = 1, 2\nreturn a + b",
+	"go f()\ndefer func() {}()",
+}
+
+func TestStmtLists(t *testing.T) {
+	for _, src := range stmts {
+		file, err := parser.ParseFile(fset, "", "package p; func _() {"+src+"}", parser.ParseComments)
+		if err != nil {
+			panic(err) // error in test
+		}
+
+		var buf bytes.Buffer
+		err = Fprint(&buf, fset, file.Decls[0].(*ast.FuncDecl).Body.List) // only print statements
+		if err != nil {
+			panic(err) // error in test
+		}
+
+		out := buf.String()
+		if out != src {
+			t.Errorf("\ngot : %q\nwant: %q\n", out, src)
+		}
+	}
+}
+
+func TestBaseIndent(t *testing.T) {
+	// The testfile must not contain multi-line raw strings since those
+	// are not indented (because their values must not change) and make
+	// this test fail.
+	const filename = "printer.go"
+	src, err := ioutil.ReadFile(filename)
+	if err != nil {
+		panic(err) // error in test
+	}
+
+	file, err := parser.ParseFile(fset, filename, src, 0)
+	if err != nil {
+		panic(err) // error in test
+	}
+
+	var buf bytes.Buffer
+	for indent := 0; indent < 4; indent++ {
+		buf.Reset()
+		(&Config{Tabwidth: tabwidth, Indent: indent}).Fprint(&buf, fset, file)
+		// all code must be indented by at least 'indent' tabs
+		lines := bytes.Split(buf.Bytes(), []byte{'\n'})
+		for i, line := range lines {
+			if len(line) == 0 {
+				continue // empty lines don't have indentation
+			}
+			n := 0
+			for j, b := range line {
+				if b != '\t' {
+					// end of indentation
+					n = j
+					break
+				}
+			}
+			if n < indent {
+				t.Errorf("line %d: got only %d tabs; want at least %d: %q", i, n, indent, line)
+			}
+		}
+	}
+}
+
+// TestFuncType tests that an ast.FuncType with a nil Params field
+// can be printed (per go/ast specification). Test case for issue 3870.
+func TestFuncType(t *testing.T) {
+	src := &ast.File{
+		Name: &ast.Ident{Name: "p"},
+		Decls: []ast.Decl{
+			&ast.FuncDecl{
+				Name: &ast.Ident{Name: "f"},
+				Type: &ast.FuncType{},
+			},
+		},
+	}
+
+	var buf bytes.Buffer
+	if err := Fprint(&buf, fset, src); err != nil {
+		t.Fatal(err)
+	}
+	got := buf.String()
+
+	const want = `package p
+
+func f()
+`
+
+	if got != want {
+		t.Fatalf("got:\n%s\nwant:\n%s\n", got, want)
+	}
+}
+
+// TextX is a skeleton test that can be filled in for debugging one-off cases.
+// Do not remove.
+func TestX(t *testing.T) {
+	const src = `
+package p
+func _() {}
+`
+	_, err := format([]byte(src), 0)
+	if err != nil {
+		t.Error(err)
+	}
+}
diff --git a/src/go/printer/testdata/comments.golden b/src/go/printer/testdata/comments.golden
new file mode 100644
index 000000000..b1af7958a
--- /dev/null
+++ b/src/go/printer/testdata/comments.golden
@@ -0,0 +1,643 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This is a package for testing comment placement by go/printer.
+//
+package main
+
+import "fmt"	// fmt
+
+const c0 = 0	// zero
+const (
+	c1	= iota	// c1
+	c2		// c2
+)
+
+// Alignment of comments in declarations>
+const (
+	_	T	= iota	// comment
+	_			// comment
+	_			// comment
+	_	= iota + 10
+	_	// comments
+
+	_		= 10	// comment
+	_	T	= 20	// comment
+)
+
+const (
+	_____	= iota	// foo
+	_		// bar
+	_	= 0	// bal
+	_		// bat
+)
+
+const (
+	_	T	= iota	// comment
+	_			// comment
+	_			// comment
+	_	= iota + 10
+	_		// comment
+	_		= 10
+	_		= 20	// comment
+	_	T	= 0	// comment
+)
+
+// The SZ struct; it is empty.
+type SZ struct{}
+
+// The S0 struct; no field is exported.
+type S0 struct {
+	int
+	x, y, z	int	// 3 unexported fields
+}
+
+// The S1 struct; some fields are not exported.
+type S1 struct {
+	S0
+	A, B, C	float	// 3 exported fields
+	D, b, c	int	// 2 unexported fields
+}
+
+// The S2 struct; all fields are exported.
+type S2 struct {
+	S1
+	A, B, C	float	// 3 exported fields
+}
+
+// The IZ interface; it is empty.
+type SZ interface{}
+
+// The I0 interface; no method is exported.
+type I0 interface {
+	f(x int) int	// unexported method
+}
+
+// The I1 interface; some methods are not exported.
+type I1 interface {
+	I0
+	F(x float) float	// exported methods
+	g(x int) int		// unexported method
+}
+
+// The I2 interface; all methods are exported.
+type I2 interface {
+	I0
+	F(x float) float	// exported method
+	G(x float) float	// exported method
+}
+
+// The S3 struct; all comments except for the last one must appear in the export.
+type S3 struct {
+	// lead comment for F1
+	F1	int	// line comment for F1
+	// lead comment for F2
+	F2	int	// line comment for F2
+	f3	int	// f3 is not exported
+}
+
+// This comment group should be separated
+// with a newline from the next comment
+// group.
+
+// This comment should NOT be associated with the next declaration.
+
+var x int	// x
+var ()
+
+// This comment SHOULD be associated with f0.
+func f0() {
+	const pi = 3.14	// pi
+	var s1 struct{}	/* an empty struct */	/* foo */
+	// a struct constructor
+	// --------------------
+	var s2 struct{} = struct{}{}
+	x := pi
+}
+
+//
+// This comment should be associated with f1, with one blank line before the comment.
+//
+func f1() {
+	f0()
+	/* 1 */
+	// 2
+	/* 3 */
+	/* 4 */
+	f0()
+}
+
+func _() {
+	// this comment should be properly indented
+}
+
+func _(x int) int {
+	if x < 0 {	// the tab printed before this comment's // must not affect the remaining lines
+		return -x	// this statement should be properly indented
+	}
+	if x < 0 {	/* the tab printed before this comment's /* must not affect the remaining lines */
+		return -x	// this statement should be properly indented
+	}
+	return x
+}
+
+func typeswitch(x interface{}) {
+	switch v := x.(type) {
+	case bool, int, float:
+	case string:
+	default:
+	}
+
+	switch x.(type) {
+	}
+
+	switch v0, ok := x.(int); v := x.(type) {
+	}
+
+	switch v0, ok := x.(int); x.(type) {
+	case byte:	// this comment should be on the same line as the keyword
+		// this comment should be normally indented
+		_ = 0
+	case bool, int, float:
+		// this comment should be indented
+	case string:
+	default:
+		// this comment should be indented
+	}
+	// this comment should not be indented
+}
+
+//
+// Indentation of comments after possibly indented multi-line constructs
+// (test cases for issue 3147).
+//
+
+func _() {
+	s := 1 +
+		2
+	// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2	// comment
+	// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2	// comment
+	// should be indented like s
+	_ = 0
+}
+
+func _() {
+	s := 1 +
+		2
+	// should be indented like s
+	_ = 0
+}
+
+func _() {
+	s := 1 +
+		2
+
+	// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2	// comment
+
+	// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2	// comment
+
+	// should be indented like s
+	_ = 0
+}
+
+func _() {
+	s := 1 +
+		2
+
+	// should be indented like s
+	_ = 0
+}
+
+// Test case from issue 3147.
+func f() {
+	templateText := "a" +	// A
+		"b" +	// B
+		"c"	// C
+
+	// should be aligned with f()
+	f()
+}
+
+// Modified test case from issue 3147.
+func f() {
+	templateText := "a" +	// A
+		"b" +	// B
+		"c"	// C
+
+		// may not be aligned with f() (source is not aligned)
+	f()
+}
+
+//
+// Test cases for alignment of lines in general comments.
+//
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line */
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line */
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line */
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line */
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line */
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line */
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line */
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line */
+}
+
+/*
+ * line
+ * of
+ * stars
+ */
+
+/* another line
+ * of
+ * stars */
+
+/*	and another line
+ *	of
+ *	stars */
+
+/* a line of
+ * stars */
+
+/*	and another line of
+ *	stars */
+
+/* a line of stars
+ */
+
+/*	and another line of
+ */
+
+/* a line of stars
+ */
+
+/*	and another line of
+ */
+
+/*
+aligned in middle
+here
+        not here
+*/
+
+/*
+blank line in middle:
+
+with no leading spaces on blank line.
+*/
+
+/*
+   aligned in middle
+   here
+           not here
+*/
+
+/*
+	blank line in middle:
+
+	with no leading spaces on blank line.
+*/
+
+func _() {
+	/*
+	 * line
+	 * of
+	 * stars
+	 */
+
+	/*
+		aligned in middle
+		here
+			not here
+	*/
+
+	/*
+		blank line in middle:
+
+		with no leading spaces on blank line.
+	*/
+}
+
+// Some interesting interspersed comments.
+// See below for more common cases.
+func _( /* this */ x /* is */ /* an */ int) {
+}
+
+func _( /* no params - extra blank before and after comment */ )	{}
+func _(a, b int /* params - no extra blank after comment */)		{}
+
+func _()	{ f( /* no args - extra blank before and after comment */ ) }
+func _()	{ f(a, b /* args - no extra blank after comment */) }
+
+func _() {
+	f( /* no args - extra blank before and after comment */ )
+	f(a, b /* args - no extra blank after comment */)
+}
+
+func ( /* comment1 */ T /* comment2 */) _()	{}
+
+func _()	{ /* "short-ish one-line functions with comments are formatted as multi-line functions */ }
+func _()	{ x := 0; /* comment */ y = x /* comment */ }
+
+func _() {
+	_ = 0
+	/* closing curly brace should be on new line */
+}
+
+func _() {
+	_ = []int{0, 1 /* don't introduce a newline after this comment - was issue 1365 */}
+}
+
+// Test cases from issue 1542:
+// Comments must not be placed before commas and cause invalid programs.
+func _() {
+	var a = []int{1, 2	/*jasldf*/}
+	_ = a
+}
+
+func _() {
+	var a = []int{1, 2}/*jasldf
+	 */
+
+	_ = a
+}
+
+func _() {
+	var a = []int{1, 2}// jasldf
+
+	_ = a
+}
+
+// Comments immediately adjacent to punctuation followed by a newline
+// remain after the punctuation (looks better and permits alignment of
+// comments).
+func _() {
+	_ = T{
+		1,	// comment after comma
+		2,	/* comment after comma */
+		3,	// comment after comma
+	}
+	_ = T{
+		1,	// comment after comma
+		2,	/* comment after comma */
+		3,	// comment after comma
+	}
+	_ = T{
+		/* comment before literal */ 1,
+		2,	/* comment before comma - ok to move after comma */
+		3,	/* comment before comma - ok to move after comma */
+	}
+
+	for i = 0;	// comment after semicolon
+	i < 9;		/* comment after semicolon */
+	i++ {		// comment after opening curly brace
+	}
+
+	// TODO(gri) the last comment in this example should be aligned */
+	for i = 0;	// comment after semicolon
+	i < 9;		/* comment before semicolon - ok to move after semicolon */
+	i++ /* comment before opening curly brace */ {
+	}
+}
+
+// If there is no newline following punctuation, commas move before the punctuation.
+// This way, commas interspersed in lists stay with the respective expression.
+func f(x /* comment */, y int, z int /* comment */, u, v, w int /* comment */) {
+	f(x /* comment */, y)
+	f(x,	/* comment */
+		y)
+	f(
+		x,	/* comment */
+	)
+}
+
+func g(
+	x int,	/* comment */
+) {
+}
+
+type _ struct {
+	a, b /* comment */, c int
+}
+
+type _ struct {
+	a, b /* comment */, c int
+}
+
+func _() {
+	for a /* comment */, b := range x {
+	}
+}
+
+// Print line directives correctly.
+
+// The following is a legal line directive.
+//line foo:1
+func _() {
+	_ = 0
+	// The following is a legal line directive. It must not be indented:
+//line foo:2
+	_ = 1
+
+	// The following is not a legal line directive (it doesn't start in column 1):
+	//line foo:2
+	_ = 2
+
+	// The following is not a legal line directive (negative line number):
+	//line foo:-3
+	_ = 3
+}
+
+// Line comments with tabs
+func _() {
+	var finput *bufio.Reader	// input file
+	var stderr *bufio.Writer
+	var ftable *bufio.Writer	// y.go file
+	var foutput *bufio.Writer	// y.output file
+
+	var oflag string	// -o [y.go]		- y.go file
+	var vflag string	// -v [y.output]	- y.output file
+	var lflag bool		// -l			- disable line directives
+}
+
+// Trailing white space in comments should be trimmed
+func _() {
+	// This comment has 4 blanks following that should be trimmed:
+	/* Each line of this comment has blanks or tabs following that should be trimmed:
+	   line 2:
+	   line 3:
+	*/
+}
+
+/* This comment is the last entry in this file. It must be printed and should be followed by a newline */
diff --git a/src/go/printer/testdata/comments.input b/src/go/printer/testdata/comments.input
new file mode 100644
index 000000000..983e2b2c9
--- /dev/null
+++ b/src/go/printer/testdata/comments.input
@@ -0,0 +1,648 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This is a package for testing comment placement by go/printer.
+//
+package main
+
+import "fmt"  // fmt
+
+const c0 = 0  // zero
+const (
+	c1 = iota  // c1
+	c2  // c2
+)
+
+// Alignment of comments in declarations>
+const (
+	_ T = iota  // comment
+	_  // comment
+	_  // comment
+	_ = iota+10
+	_  // comments
+
+	_ = 10  // comment
+	_ T = 20  // comment
+)
+
+const (
+	_____ = iota // foo
+	_ // bar
+	_  = 0    // bal
+	_ // bat
+)
+
+const (
+	_ T = iota // comment
+	_ // comment
+	_ // comment
+	_ = iota + 10
+	_ // comment
+	_ = 10
+	_ = 20 // comment
+	_ T = 0 // comment
+)
+
+// The SZ struct; it is empty.
+type SZ struct {}
+
+// The S0 struct; no field is exported.
+type S0 struct {
+	int
+	x, y, z int  // 3 unexported fields
+}
+
+// The S1 struct; some fields are not exported.
+type S1 struct {
+	S0
+	A, B, C float  // 3 exported fields
+	D, b, c int  // 2 unexported fields
+}
+
+// The S2 struct; all fields are exported.
+type S2 struct {
+	S1
+	A, B, C float  // 3 exported fields
+}
+
+// The IZ interface; it is empty.
+type SZ interface {}
+
+// The I0 interface; no method is exported.
+type I0 interface {
+	f(x int) int  // unexported method
+}
+
+// The I1 interface; some methods are not exported.
+type I1 interface {
+	I0
+	F(x float) float  // exported methods
+	g(x int) int  // unexported method
+}
+
+// The I2 interface; all methods are exported.
+type I2 interface {
+	I0
+	F(x float) float  // exported method
+	G(x float) float  // exported method
+}
+
+// The S3 struct; all comments except for the last one must appear in the export.
+type S3 struct {
+	// lead comment for F1
+	F1 int // line comment for F1
+	// lead comment for F2
+	F2 int // line comment for F2
+	f3 int // f3 is not exported
+}
+
+// This comment group should be separated
+// with a newline from the next comment
+// group.
+
+// This comment should NOT be associated with the next declaration.
+
+var x int  // x
+var ()
+
+
+// This comment SHOULD be associated with f0.
+func f0() {
+	const pi = 3.14  // pi
+	var s1 struct {}  /* an empty struct */ /* foo */
+	// a struct constructor
+	// --------------------
+	var s2 struct {} = struct {}{}
+	x := pi
+}
+//
+// This comment should be associated with f1, with one blank line before the comment.
+//
+func f1() {
+	f0()
+	/* 1 */
+	// 2
+	/* 3 */
+	/* 4 */
+	f0()
+}
+
+
+func _() {
+// this comment should be properly indented
+}
+
+
+func _(x int) int {
+	if x < 0 {  // the tab printed before this comment's // must not affect the remaining lines
+		return -x  // this statement should be properly indented
+	}
+	if x < 0 {  /* the tab printed before this comment's /* must not affect the remaining lines */
+		return -x  // this statement should be properly indented
+	}
+	return x
+}
+
+
+func typeswitch(x interface{}) {
+	switch v := x.(type) {
+	case bool, int, float:
+	case string:
+	default:
+	}
+
+	switch x.(type) {
+	}
+
+	switch v0, ok := x.(int); v := x.(type) {
+	}
+
+	switch v0, ok := x.(int); x.(type) {
+	case byte:  // this comment should be on the same line as the keyword
+		// this comment should be normally indented
+		_ = 0
+	case bool, int, float:
+		// this comment should be indented
+	case string:
+	default:
+		// this comment should be indented
+	}
+	// this comment should not be indented
+}
+
+//
+// Indentation of comments after possibly indented multi-line constructs
+// (test cases for issue 3147).
+//
+
+func _() {
+	s := 1 +
+		2
+// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2 // comment
+		// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2 // comment
+	// should be indented like s
+	_ = 0
+}
+
+func _() {
+	s := 1 +
+		2
+	// should be indented like s
+	_ = 0
+}
+
+func _() {
+	s := 1 +
+		2
+
+// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2 // comment
+
+		// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2 // comment
+
+	// should be indented like s
+	_ = 0
+}
+
+func _() {
+	s := 1 +
+		2
+
+	// should be indented like s
+	_ = 0
+}
+
+// Test case from issue 3147.
+func f() {
+	templateText := "a" + // A
+		"b" + // B
+		"c" // C
+
+	// should be aligned with f()
+	f()
+}
+
+// Modified test case from issue 3147.
+func f() {
+	templateText := "a" + // A
+		"b" + // B
+		"c" // C
+
+		// may not be aligned with f() (source is not aligned)
+	f()
+}
+
+//
+// Test cases for alignment of lines in general comments.
+//
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line
+	   */
+}
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line */
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line
+		*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line */
+}
+
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line
+	   */
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line */
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line
+		*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line */
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line
+	   */
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line */
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line
+		*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line */
+}
+
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line
+	   */
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line */
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line
+		*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line */
+}
+
+/*
+ * line
+ * of
+ * stars
+ */
+
+/* another line
+ * of
+ * stars */
+
+/*	and another line
+ *	of
+ *	stars */
+
+/* a line of
+ * stars */
+
+/*	and another line of
+ *	stars */
+
+/* a line of stars
+*/
+
+/*	and another line of
+*/
+
+/* a line of stars
+ */
+
+/*	and another line of
+ */
+
+/*
+aligned in middle
+here
+        not here
+*/
+
+/*
+blank line in middle:
+
+with no leading spaces on blank line.
+*/
+
+/*
+   aligned in middle
+   here
+           not here
+*/
+
+/*
+	blank line in middle:
+
+	with no leading spaces on blank line.
+*/
+
+func _() {
+	/*
+	 * line
+	 * of
+	 * stars
+	 */
+
+	/*
+	aligned in middle
+	here
+		not here
+	*/
+
+	/*
+	blank line in middle:
+
+	with no leading spaces on blank line.
+*/
+}
+
+
+// Some interesting interspersed comments.
+// See below for more common cases.
+func _(/* this */x/* is *//* an */ int) {
+}
+
+func _(/* no params - extra blank before and after comment */) {}
+func _(a, b int /* params - no extra blank after comment */) {}
+
+func _() { f(/* no args - extra blank before and after comment */) }
+func _() { f(a, b /* args - no extra blank after comment */) }
+
+func _() {
+	f(/* no args - extra blank before and after comment */)
+	f(a, b /* args - no extra blank after comment */)
+}
+
+func (/* comment1 */ T /* comment2 */) _() {}
+
+func _() { /* "short-ish one-line functions with comments are formatted as multi-line functions */ }
+func _() { x := 0; /* comment */ y = x /* comment */ }
+
+func _() {
+	_ = 0
+	/* closing curly brace should be on new line */ }
+
+func _() {
+	_ = []int{0, 1 /* don't introduce a newline after this comment - was issue 1365 */}
+}
+
+// Test cases from issue 1542:
+// Comments must not be placed before commas and cause invalid programs.
+func _() {
+	var a = []int{1, 2, /*jasldf*/
+	}
+	_ = a
+}
+
+func _() {
+	var a = []int{1, 2, /*jasldf
+						*/
+	}
+	_ = a
+}
+
+func _() {
+	var a = []int{1, 2, // jasldf
+	}
+	_ = a
+}
+
+// Comments immediately adjacent to punctuation followed by a newline
+// remain after the punctuation (looks better and permits alignment of
+// comments).
+func _() {
+	_ = T{
+		1,    // comment after comma
+		2,    /* comment after comma */
+		3  ,  // comment after comma
+	}
+	_ = T{
+		1  ,// comment after comma
+		2  ,/* comment after comma */
+		3,// comment after comma
+	}
+	_ = T{
+		/* comment before literal */1,
+		2/* comment before comma - ok to move after comma */,
+		3  /* comment before comma - ok to move after comma */  ,
+	}
+
+	for
+		i=0;// comment after semicolon
+		i<9;/* comment after semicolon */
+		i++{// comment after opening curly brace
+	}
+
+	// TODO(gri) the last comment in this example should be aligned */
+	for
+		i=0;// comment after semicolon
+		i<9/* comment before semicolon - ok to move after semicolon */;
+		i++ /* comment before opening curly brace */ {
+	}
+}
+
+// If there is no newline following punctuation, commas move before the punctuation.
+// This way, commas interspersed in lists stay with the respective expression.
+func f(x/* comment */, y int, z int /* comment */, u, v, w int /* comment */) {
+	f(x /* comment */, y)
+	f(x /* comment */, 
+	y)
+	f(
+		x /* comment */,
+	)
+}
+
+func g(
+	x int /* comment */,
+) {}
+
+type _ struct {
+	a, b /* comment */, c int
+}
+
+type _ struct { a, b /* comment */, c int }
+
+func _() {
+	for a /* comment */, b := range x {
+	}
+}
+
+// Print line directives correctly.
+
+// The following is a legal line directive.
+//line foo:1
+func _() {
+	_ = 0
+// The following is a legal line directive. It must not be indented:
+//line foo:2
+	_ = 1
+
+// The following is not a legal line directive (it doesn't start in column 1):
+	//line foo:2
+	_ = 2
+
+// The following is not a legal line directive (negative line number):
+//line foo:-3
+	_ = 3
+}
+
+// Line comments with tabs
+func _() {
+var	finput		*bufio.Reader			// input file
+var	stderr		*bufio.Writer
+var	ftable		*bufio.Writer			// y.go file
+var	foutput		*bufio.Writer			// y.output file
+
+var	oflag		string				// -o [y.go]		- y.go file
+var	vflag		string				// -v [y.output]	- y.output file
+var	lflag		bool				// -l			- disable line directives
+}
+
+// Trailing white space in comments should be trimmed
+func _() {
+// This comment has 4 blanks following that should be trimmed:    
+/* Each line of this comment has blanks or tabs following that should be trimmed:	
+   line 2:    
+   line 3:    			
+*/
+}
+
+/* This comment is the last entry in this file. It must be printed and should be followed by a newline */
diff --git a/src/go/printer/testdata/comments.x b/src/go/printer/testdata/comments.x
new file mode 100644
index 000000000..ae7729286
--- /dev/null
+++ b/src/go/printer/testdata/comments.x
@@ -0,0 +1,56 @@
+// This is a package for testing comment placement by go/printer.
+//
+package main
+
+// The SZ struct; it is empty.
+type SZ struct{}
+
+// The S0 struct; no field is exported.
+type S0 struct {
+	// contains filtered or unexported fields
+}
+
+// The S1 struct; some fields are not exported.
+type S1 struct {
+	S0
+	A, B, C	float	// 3 exported fields
+	D	int	// 2 unexported fields
+	// contains filtered or unexported fields
+}
+
+// The S2 struct; all fields are exported.
+type S2 struct {
+	S1
+	A, B, C	float	// 3 exported fields
+}
+
+// The IZ interface; it is empty.
+type SZ interface{}
+
+// The I0 interface; no method is exported.
+type I0 interface {
+	// contains filtered or unexported methods
+}
+
+// The I1 interface; some methods are not exported.
+type I1 interface {
+	I0
+	F(x float) float	// exported methods
+	// contains filtered or unexported methods
+}
+
+// The I2 interface; all methods are exported.
+type I2 interface {
+	I0
+	F(x float) float	// exported method
+	G(x float) float	// exported method
+}
+
+// The S3 struct; all comments except for the last one must appear in the export.
+type S3 struct {
+	// lead comment for F1
+	F1	int	// line comment for F1
+	// lead comment for F2
+	F2	int	// line comment for F2
+	// contains filtered or unexported fields
+}
diff --git a/src/go/printer/testdata/comments2.golden b/src/go/printer/testdata/comments2.golden
new file mode 100644
index 000000000..7676a26c1
--- /dev/null
+++ b/src/go/printer/testdata/comments2.golden
@@ -0,0 +1,105 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This is a package for testing comment placement by go/printer.
+//
+package main
+
+// Test cases for idempotent comment formatting (was issue 1835).
+/*
+c1a
+*/
+/*
+   c1b
+*/
+/* foo
+c1c
+*/
+/* foo
+   c1d
+*/
+/*
+c1e
+foo */
+/*
+   c1f
+   foo */
+
+func f() {
+	/*
+	   c2a
+	*/
+	/*
+	   c2b
+	*/
+	/* foo
+	   c2c
+	*/
+	/* foo
+	   c2d
+	*/
+	/*
+	   c2e
+	   foo */
+	/*
+	   c2f
+	   foo */
+}
+
+func g() {
+	/*
+	   c3a
+	*/
+	/*
+	   c3b
+	*/
+	/* foo
+	   c3c
+	*/
+	/* foo
+	   c3d
+	*/
+	/*
+	   c3e
+	   foo */
+	/*
+	   c3f
+	   foo */
+}
+
+// Test case taken literally from issue 1835.
+func main() {
+	/*
+	   prints test 5 times
+	*/
+	for i := 0; i < 5; i++ {
+		println("test")
+	}
+}
+
+func issue5623() {
+L:
+	_ = yyyyyyyyyyyyyyyy			// comment - should be aligned
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	/* comment */
+
+	_ = yyyyyyyyyyyyyyyy			/* comment - should be aligned */
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	// comment
+
+LLLLLLL:
+	_ = yyyyyyyyyyyyyyyy			// comment - should be aligned
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	// comment
+
+LL:
+LLLLL:
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	/* comment */
+	_ = yyyyyyyyyyyyyyyy			/* comment - should be aligned */
+
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	// comment
+	_ = yyyyyyyyyyyyyyyy			// comment - should be aligned
+
+	// test case from issue
+label:
+	mask := uint64(1)<<c - 1		// Allocation mask
+	used := atomic.LoadUint64(&h.used)	// Current allocations
+}
diff --git a/src/go/printer/testdata/comments2.input b/src/go/printer/testdata/comments2.input
new file mode 100644
index 000000000..4a055c827
--- /dev/null
+++ b/src/go/printer/testdata/comments2.input
@@ -0,0 +1,105 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This is a package for testing comment placement by go/printer.
+//
+package main
+
+// Test cases for idempotent comment formatting (was issue 1835).
+/*
+c1a
+*/
+/*
+   c1b
+*/
+/* foo
+c1c
+*/
+/* foo
+   c1d
+*/
+/*
+c1e
+foo */
+/*
+   c1f
+   foo */
+
+func f() {
+/*
+c2a
+*/
+/*
+   c2b
+*/
+/* foo
+c2c
+*/
+/* foo
+   c2d
+*/
+/*
+c2e
+foo */
+/*
+   c2f
+   foo */
+}
+
+func g() {
+/*
+c3a
+*/
+/*
+   c3b
+*/
+/* foo
+c3c
+*/
+/* foo
+   c3d
+*/
+/*
+c3e
+foo */
+/*
+   c3f
+   foo */
+}
+
+// Test case taken literally from issue 1835.
+func main() {
+/*
+prints test 5 times
+*/
+   for i := 0; i < 5; i++ {
+      println("test")
+   }
+}
+
+func issue5623() {
+L:
+   _ = yyyyyyyyyyyyyyyy // comment - should be aligned
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx /* comment */
+
+   _ = yyyyyyyyyyyyyyyy /* comment - should be aligned */
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx // comment
+
+LLLLLLL:
+   _ = yyyyyyyyyyyyyyyy // comment - should be aligned
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx // comment
+
+LL:
+LLLLL:
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx /* comment */
+   _ = yyyyyyyyyyyyyyyy /* comment - should be aligned */
+
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx // comment
+   _ = yyyyyyyyyyyyyyyy // comment - should be aligned
+
+// test case from issue
+label:
+   mask := uint64(1)<<c - 1 // Allocation mask
+   used := atomic.LoadUint64(&h.used) // Current allocations
+}
diff --git a/src/go/printer/testdata/declarations.golden b/src/go/printer/testdata/declarations.golden
new file mode 100644
index 000000000..a27f21fc8
--- /dev/null
+++ b/src/go/printer/testdata/declarations.golden
@@ -0,0 +1,955 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package imports
+
+import "io"
+
+import (
+	_ "io"
+)
+
+import _ "io"
+
+import (
+	"io"
+	"io"
+	"io"
+)
+
+import (
+	"io"
+	aLongRename "io"
+
+	b "io"
+)
+
+import (
+	"unrenamed"
+	renamed "renameMe"
+	. "io"
+	_ "io"
+	"io"
+	. "os"
+)
+
+// no newlines between consecutive single imports, but
+// respect extra line breaks in the source (at most one empty line)
+import _ "io"
+import _ "io"
+import _ "io"
+
+import _ "os"
+import _ "os"
+import _ "os"
+
+import _ "fmt"
+import _ "fmt"
+import _ "fmt"
+
+import "foo"	// a comment
+import "bar"	// a comment
+
+import (
+	_ "foo"
+	// a comment
+	"bar"
+	"foo"	// a comment
+	"bar"	// a comment
+)
+
+// comments + renames
+import (
+	"unrenamed"	// a comment
+	renamed "renameMe"
+	. "io"		/* a comment */
+	_ "io/ioutil"	// a comment
+	"io"		// testing alignment
+	. "os"
+	// a comment
+)
+
+// a case that caused problems in the past (comment placement)
+import (
+	. "fmt"
+	"io"
+	"malloc"	// for the malloc count test only
+	"math"
+	"strings"
+	"testing"
+)
+
+// more import examples
+import (
+	"xxx"
+	"much_longer_name"	// comment
+	"short_name"		// comment
+)
+
+import (
+	_ "xxx"
+	"much_longer_name"	// comment
+)
+
+import (
+	mymath "math"
+	"/foo/bar/long_package_path"	// a comment
+)
+
+import (
+	"package_a"	// comment
+	"package_b"
+	my_better_c "package_c"	// comment
+	"package_d"		// comment
+	my_e "package_e"	// comment
+
+	"package_a"	// comment
+	"package_bb"
+	"package_ccc"	// comment
+	"package_dddd"	// comment
+)
+
+// at least one empty line between declarations of different kind
+import _ "io"
+
+var _ int
+
+// at least one empty line between declarations of the same kind
+// if there is associated documentation (was issue 2570)
+type T1 struct{}
+
+// T2 comment
+type T2 struct {
+}	// should be a two-line struct
+
+// T3 comment
+type T2 struct {
+}	// should be a two-line struct
+
+// printing of constant literals
+const (
+	_	= "foobar"
+	_	= "a۰۱۸"
+	_	= "foo६४"
+	_	= "bar９８７６"
+	_	= 0
+	_	= 1
+	_	= 123456789012345678890
+	_	= 01234567
+	_	= 0xcafebabe
+	_	= 0.
+	_	= .0
+	_	= 3.14159265
+	_	= 1e0
+	_	= 1e+100
+	_	= 1e-100
+	_	= 2.71828e-1000
+	_	= 0i
+	_	= 1i
+	_	= 012345678901234567889i
+	_	= 123456789012345678890i
+	_	= 0.i
+	_	= .0i
+	_	= 3.14159265i
+	_	= 1e0i
+	_	= 1e+100i
+	_	= 1e-100i
+	_	= 2.71828e-1000i
+	_	= 'a'
+	_	= '\000'
+	_	= '\xFF'
+	_	= '\uff16'
+	_	= '\U0000ff16'
+	_	= `foobar`
+	_	= `foo
+---
+---
+bar`
+)
+
+func _() {
+	type _ int
+	type _ *int
+	type _ []int
+	type _ map[string]int
+	type _ chan int
+	type _ func() int
+
+	var _ int
+	var _ *int
+	var _ []int
+	var _ map[string]int
+	var _ chan int
+	var _ func() int
+
+	type _ struct{}
+	type _ *struct{}
+	type _ []struct{}
+	type _ map[string]struct{}
+	type _ chan struct{}
+	type _ func() struct{}
+
+	type _ interface{}
+	type _ *interface{}
+	type _ []interface{}
+	type _ map[string]interface{}
+	type _ chan interface{}
+	type _ func() interface{}
+
+	var _ struct{}
+	var _ *struct{}
+	var _ []struct{}
+	var _ map[string]struct{}
+	var _ chan struct{}
+	var _ func() struct{}
+
+	var _ interface{}
+	var _ *interface{}
+	var _ []interface{}
+	var _ map[string]interface{}
+	var _ chan interface{}
+	var _ func() interface{}
+}
+
+// don't lose blank lines in grouped declarations
+const (
+	_	int	= 0
+	_	float	= 1
+
+	_	string	= "foo"
+
+	_	= iota
+	_
+
+	// a comment
+	_
+
+	_
+)
+
+type (
+	_	int
+	_	struct{}
+
+	_	interface{}
+
+	// a comment
+	_	map[string]int
+)
+
+var (
+	_	int	= 0
+	_	float	= 1
+
+	_	string	= "foo"
+
+	_	bool
+
+	// a comment
+	_	bool
+)
+
+// don't lose blank lines in this struct
+type _ struct {
+	String	struct {
+		Str, Len int
+	}
+	Slice	struct {
+		Array, Len, Cap int
+	}
+	Eface	struct {
+		Typ, Ptr int
+	}
+
+	UncommonType	struct {
+		Name, PkgPath int
+	}
+	CommonType	struct {
+		Size, Hash, Alg, Align, FieldAlign, String, UncommonType int
+	}
+	Type	struct {
+		Typ, Ptr int
+	}
+	StructField	struct {
+		Name, PkgPath, Typ, Tag, Offset int
+	}
+	StructType	struct {
+		Fields int
+	}
+	PtrType	struct {
+		Elem int
+	}
+	SliceType	struct {
+		Elem int
+	}
+	ArrayType	struct {
+		Elem, Len int
+	}
+
+	Stktop	struct {
+		Stackguard, Stackbase, Gobuf int
+	}
+	Gobuf	struct {
+		Sp, Pc, G int
+	}
+	G	struct {
+		Stackbase, Sched, Status, Alllink int
+	}
+}
+
+// no blank lines in empty structs and interfaces, but leave 1- or 2-line layout alone
+type _ struct{}
+type _ struct {
+}
+
+type _ interface{}
+type _ interface {
+}
+
+// no tabs for single or ungrouped decls
+func _() {
+	const xxxxxx = 0
+	type x int
+	var xxx int
+	var yyyy float = 3.14
+	var zzzzz = "bar"
+
+	const (
+		xxxxxx = 0
+	)
+	type (
+		x int
+	)
+	var (
+		xxx int
+	)
+	var (
+		yyyy float = 3.14
+	)
+	var (
+		zzzzz = "bar"
+	)
+}
+
+// tabs for multiple or grouped decls
+func _() {
+	// no entry has a type
+	const (
+		zzzzzz	= 1
+		z	= 2
+		zzz	= 3
+	)
+	// some entries have a type
+	const (
+		xxxxxx			= 1
+		x			= 2
+		xxx			= 3
+		yyyyyyyy	float	= iota
+		yyyy			= "bar"
+		yyy
+		yy	= 2
+	)
+}
+
+func _() {
+	// no entry has a type
+	var (
+		zzzzzz	= 1
+		z	= 2
+		zzz	= 3
+	)
+	// no entry has a value
+	var (
+		_	int
+		_	float
+		_	string
+
+		_	int	// comment
+		_	float	// comment
+		_	string	// comment
+	)
+	// some entries have a type
+	var (
+		xxxxxx		int
+		x		float
+		xxx		string
+		yyyyyyyy	int	= 1234
+		y		float	= 3.14
+		yyyy			= "bar"
+		yyy		string	= "foo"
+	)
+	// mixed entries - all comments should be aligned
+	var (
+		a, b, c			int
+		x			= 10
+		d			int			// comment
+		y				= 20		// comment
+		f, ff, fff, ffff	int	= 0, 1, 2, 3	// comment
+	)
+	// respect original line breaks
+	var _ = []T{
+		T{0x20, "Telugu"},
+	}
+	var _ = []T{
+		// respect original line breaks
+		T{0x20, "Telugu"},
+	}
+}
+
+// use the formatted output rather than the input to decide when to align
+// (was issue 4505)
+const (
+	short		= 2 * (1 + 2)
+	aMuchLongerName	= 3
+)
+
+var (
+	short		= X{}
+	aMuchLongerName	= X{}
+
+	x1	= X{}	// foo
+	x2	= X{}	// foo
+)
+
+func _() {
+	type (
+		xxxxxx	int
+		x	float
+		xxx	string
+		xxxxx	[]x
+		xx	struct{}
+		xxxxxxx	struct {
+			_, _	int
+			_	float
+		}
+		xxxx	chan<- string
+	)
+}
+
+// alignment of "=" in consecutive lines (extended example from issue 1414)
+const (
+	umax	uint	= ^uint(0)		// maximum value for a uint
+	bpu		= 1 << (5 + umax>>63)	// bits per uint
+	foo
+	bar	= -1
+)
+
+// typical enum
+const (
+	a	MyType	= iota
+	abcd
+	b
+	c
+	def
+)
+
+// excerpt from godoc.go
+var (
+	goroot		= flag.String("goroot", runtime.GOROOT(), "Go root directory")
+	testDir		= flag.String("testdir", "", "Go root subdirectory - for testing only (faster startups)")
+	pkgPath		= flag.String("path", "", "additional package directories (colon-separated)")
+	filter		= flag.String("filter", "", "filter file containing permitted package directory paths")
+	filterMin	= flag.Int("filter_minutes", 0, "filter file update interval in minutes; disabled if <= 0")
+	filterDelay	delayTime	// actual filter update interval in minutes; usually filterDelay == filterMin, but filterDelay may back off exponentially
+)
+
+// formatting of structs
+type _ struct{}
+
+type _ struct { /* this comment should be visible */
+}
+
+type _ struct {
+	// this comment should be visible and properly indented
+}
+
+type _ struct {	// this comment must not change indentation
+	f			int
+	f, ff, fff, ffff	int
+}
+
+type _ struct {
+	string
+}
+
+type _ struct {
+	string	// comment
+}
+
+type _ struct {
+	string "tag"
+}
+
+type _ struct {
+	string "tag"	// comment
+}
+
+type _ struct {
+	f int
+}
+
+type _ struct {
+	f int	// comment
+}
+
+type _ struct {
+	f int "tag"
+}
+
+type _ struct {
+	f int "tag"	// comment
+}
+
+type _ struct {
+	bool
+	a, b, c			int
+	int			"tag"
+	ES				// comment
+	float			"tag"	// comment
+	f			int	// comment
+	f, ff, fff, ffff	int	// comment
+	g			float	"tag"
+	h			float	"tag"	// comment
+}
+
+type _ struct {
+	a, b,
+	c, d	int	// this line should be indented
+	u, v, w, x	float	// this line should be indented
+	p, q,
+	r, s	float	// this line should be indented
+}
+
+// difficult cases
+type _ struct {
+	bool		// comment
+	text	[]byte	// comment
+}
+
+// formatting of interfaces
+type EI interface{}
+
+type _ interface {
+	EI
+}
+
+type _ interface {
+	f()
+	fffff()
+}
+
+type _ interface {
+	EI
+	f()
+	fffffg()
+}
+
+type _ interface {	// this comment must not change indentation
+	EI				// here's a comment
+	f()				// no blank between identifier and ()
+	fffff()				// no blank between identifier and ()
+	gggggggggggg(x, y, z int)	// hurray
+}
+
+// formatting of variable declarations
+func _() {
+	type day struct {
+		n		int
+		short, long	string
+	}
+	var (
+		Sunday		= day{0, "SUN", "Sunday"}
+		Monday		= day{1, "MON", "Monday"}
+		Tuesday		= day{2, "TUE", "Tuesday"}
+		Wednesday	= day{3, "WED", "Wednesday"}
+		Thursday	= day{4, "THU", "Thursday"}
+		Friday		= day{5, "FRI", "Friday"}
+		Saturday	= day{6, "SAT", "Saturday"}
+	)
+}
+
+// formatting of multi-line variable declarations
+var a1, b1, c1 int	// all on one line
+
+var a2, b2,
+	c2 int	// this line should be indented
+
+var (
+	a3, b3,
+	c3, d3	int	// this line should be indented
+	a4, b4, c4	int	// this line should be indented
+)
+
+// Test case from issue 3304: multi-line declarations must end
+// a formatting section and not influence indentation of the
+// next line.
+var (
+	minRefreshTimeSec	= flag.Int64("min_refresh_time_sec", 604800,
+		"minimum time window between two refreshes for a given user.")
+	x	= flag.Int64("refresh_user_rollout_percent", 100,
+		"temporary flag to ramp up the refresh user rpc")
+	aVeryLongVariableName	= stats.GetVarInt("refresh-user-count")
+)
+
+func _() {
+	var privateKey2 = &Block{Type:	"RSA PRIVATE KEY",
+		Headers:	map[string]string{},
+		Bytes: []uint8{0x30, 0x82, 0x1, 0x3a, 0x2, 0x1, 0x0, 0x2,
+			0x41, 0x0, 0xb2, 0x99, 0xf, 0x49, 0xc4, 0x7d, 0xfa, 0x8c,
+			0xd4, 0x0, 0xae, 0x6a, 0x4d, 0x1b, 0x8a, 0x3b, 0x6a, 0x13,
+			0x64, 0x2b, 0x23, 0xf2, 0x8b, 0x0, 0x3b, 0xfb, 0x97, 0x79,
+		},
+	}
+}
+
+func _() {
+	var Universe = Scope{
+		Names: map[string]*Ident{
+			// basic types
+			"bool":		nil,
+			"byte":		nil,
+			"int8":		nil,
+			"int16":	nil,
+			"int32":	nil,
+			"int64":	nil,
+			"uint8":	nil,
+			"uint16":	nil,
+			"uint32":	nil,
+			"uint64":	nil,
+			"float32":	nil,
+			"float64":	nil,
+			"string":	nil,
+
+			// convenience types
+			"int":		nil,
+			"uint":		nil,
+			"uintptr":	nil,
+			"float":	nil,
+
+			// constants
+			"false":	nil,
+			"true":		nil,
+			"iota":		nil,
+			"nil":		nil,
+
+			// functions
+			"cap":		nil,
+			"len":		nil,
+			"new":		nil,
+			"make":		nil,
+			"panic":	nil,
+			"panicln":	nil,
+			"print":	nil,
+			"println":	nil,
+		},
+	}
+}
+
+// alignment of map composite entries
+var _ = map[int]int{
+	// small key sizes: always align even if size ratios are large
+	a:			a,
+	abcdefghabcdefgh:	a,
+	ab:			a,
+	abc:			a,
+	abcdefgabcdefg:		a,
+	abcd:			a,
+	abcde:			a,
+	abcdef:			a,
+
+	// mixed key sizes: align when key sizes change within accepted ratio
+	abcdefgh:		a,
+	abcdefghabcdefg:	a,
+	abcdefghij:		a,
+	abcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghij:	a,	// outlier - do not align with previous line
+	abcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghij:		a,	// align with previous line
+
+	ab:	a,	// do not align with previous line
+	abcde:	a,	// align with previous line
+}
+
+// alignment of map composite entries: test cases from issue 3965
+// aligned
+var _ = T1{
+	a:			x,
+	b:			y,
+	cccccccccccccccccccc:	z,
+}
+
+// not aligned
+var _ = T2{
+	a:	x,
+	b:	y,
+	ccccccccccccccccccccc:	z,
+}
+
+// aligned
+var _ = T3{
+	aaaaaaaaaaaaaaaaaaaa:	x,
+	b:			y,
+	c:			z,
+}
+
+// not aligned
+var _ = T4{
+	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa:	x,
+	b:	y,
+	c:	z,
+}
+
+func _() {
+	var _ = T{
+		a,	// must introduce trailing comma
+	}
+}
+
+// formatting of function results
+func _() func()				{}
+func _() func(int)			{ return nil }
+func _() func(int) int			{ return nil }
+func _() func(int) func(int) func()	{ return nil }
+
+// formatting of consecutive single-line functions
+func _()	{}
+func _()	{}
+func _()	{}
+
+func _()	{}	// an empty line before this function
+func _()	{}
+func _()	{}
+
+func _()		{ f(1, 2, 3) }
+func _(x int) int	{ y := x; return y + 1 }
+func _() int		{ type T struct{}; var x T; return x }
+
+// these must remain multi-line since they are multi-line in the source
+func _() {
+	f(1, 2, 3)
+}
+func _(x int) int {
+	y := x
+	return y + 1
+}
+func _() int {
+	type T struct{}
+	var x T
+	return x
+}
+
+// making function declarations safe for new semicolon rules
+func _()	{ /* single-line function because of "short-ish" comment */ }
+func _() { /* multi-line function because of "long-ish" comment - much more comment text is following here */ /* and more */
+}
+
+func _() {
+	/* multi-line func because block is on multiple lines */
+}
+
+// ellipsis parameters
+func _(...int)
+func _(...*int)
+func _(...[]int)
+func _(...struct{})
+func _(bool, ...interface{})
+func _(bool, ...func())
+func _(bool, ...func(...int))
+func _(bool, ...map[string]int)
+func _(bool, ...chan int)
+
+func _(b bool, x ...int)
+func _(b bool, x ...*int)
+func _(b bool, x ...[]int)
+func _(b bool, x ...struct{})
+func _(x ...interface{})
+func _(x ...func())
+func _(x ...func(...int))
+func _(x ...map[string]int)
+func _(x ...chan int)
+
+// these parameter lists must remain multi-line since they are multi-line in the source
+func _(bool,
+	int) {
+}
+func _(x bool,
+	y int) {
+}
+func _(x,
+	y bool) {
+}
+func _(bool,	// comment
+	int) {
+}
+func _(x bool,	// comment
+	y int) {
+}
+func _(x,	// comment
+	y bool) {
+}
+func _(bool,	// comment
+	// comment
+	int) {
+}
+func _(x bool,	// comment
+	// comment
+	y int) {
+}
+func _(x,	// comment
+	// comment
+	y bool) {
+}
+func _(bool,
+	// comment
+	int) {
+}
+func _(x bool,
+	// comment
+	y int) {
+}
+func _(x,
+	// comment
+	y bool) {
+}
+func _(x,	// comment
+	y,	// comment
+	z bool) {
+}
+func _(x,	// comment
+	y,	// comment
+	z bool) {
+}
+func _(x int,	// comment
+	y float,	// comment
+	z bool) {
+}
+
+// properly indent multi-line signatures
+func ManageStatus(in <-chan *Status, req <-chan Request,
+	stat chan<- *TargetInfo,
+	TargetHistorySize int) {
+}
+
+func MultiLineSignature0(
+	a, b, c int,
+) {
+}
+
+func MultiLineSignature1(
+	a, b, c int,
+	u, v, w float,
+) {
+}
+
+func MultiLineSignature2(
+	a, b,
+	c int,
+) {
+}
+
+func MultiLineSignature3(
+	a, b,
+	c int, u, v,
+	w float,
+	x ...int) {
+}
+
+func MultiLineSignature4(
+	a, b, c int,
+	u, v,
+	w float,
+	x ...int) {
+}
+
+func MultiLineSignature5(
+	a, b, c int,
+	u, v, w float,
+	p, q,
+	r string,
+	x ...int) {
+}
+
+// make sure it also works for methods in interfaces
+type _ interface {
+	MultiLineSignature0(
+		a, b, c int,
+	)
+
+	MultiLineSignature1(
+		a, b, c int,
+		u, v, w float,
+	)
+
+	MultiLineSignature2(
+		a, b,
+		c int,
+	)
+
+	MultiLineSignature3(
+		a, b,
+		c int, u, v,
+		w float,
+		x ...int)
+
+	MultiLineSignature4(
+		a, b, c int,
+		u, v,
+		w float,
+		x ...int)
+
+	MultiLineSignature5(
+		a, b, c int,
+		u, v, w float,
+		p, q,
+		r string,
+		x ...int)
+}
+
+// omit superfluous parentheses in parameter lists
+func _(int)
+func _(int)
+func _(x int)
+func _(x int)
+func _(x, y int)
+func _(x, y int)
+
+func _() int
+func _() int
+func _() int
+
+func _() (x int)
+func _() (x int)
+func _() (x int)
+
+// special cases: some channel types require parentheses
+func _(x chan (<-chan int))
+func _(x chan (<-chan int))
+func _(x chan (<-chan int))
+
+func _(x chan<- (chan int))
+func _(x chan<- (chan int))
+func _(x chan<- (chan int))
+
+// don't introduce comma after last parameter if the closing ) is on the same line
+// even if the parameter type itself is multi-line (test cases from issue 4533)
+func _(...interface{})
+func _(...interface {
+	m()
+	n()
+})	// no extra comma between } and )
+
+func (t *T) _(...interface{})
+func (t *T) _(...interface {
+	m()
+	n()
+})	// no extra comma between } and )
+
+func _(interface{})
+func _(interface {
+	m()
+})	// no extra comma between } and )
+
+func _(struct{})
+func _(struct {
+	x	int
+	y	int
+})	// no extra comma between } and )
diff --git a/src/go/printer/testdata/declarations.input b/src/go/printer/testdata/declarations.input
new file mode 100644
index 000000000..d9951d386
--- /dev/null
+++ b/src/go/printer/testdata/declarations.input
@@ -0,0 +1,967 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package imports
+
+import "io"
+
+import (
+	_ "io"
+)
+
+import _ "io"
+
+import (
+	"io"
+	"io"
+	"io"
+)
+
+import (
+	"io"
+	aLongRename "io"
+
+	b "io"
+)
+
+import (
+       "unrenamed"
+       renamed "renameMe"
+       . "io"
+       _ "io"
+       "io"
+       . "os"
+)
+
+// no newlines between consecutive single imports, but
+// respect extra line breaks in the source (at most one empty line)
+import _ "io"
+import _ "io"
+import _ "io"
+
+import _ "os"
+import _ "os"
+import _ "os"
+
+
+import _ "fmt"
+import _ "fmt"
+import _ "fmt"
+
+import "foo"  // a comment
+import "bar"  // a comment
+
+import (
+	_ "foo"
+	// a comment
+	"bar"
+	"foo"  // a comment
+	"bar"  // a comment
+)
+
+// comments + renames
+import (
+       "unrenamed" // a comment
+       renamed "renameMe"
+       . "io" /* a comment */
+       _ "io/ioutil" // a comment
+       "io" // testing alignment
+       . "os"
+       // a comment
+)
+
+// a case that caused problems in the past (comment placement)
+import (
+	. "fmt"
+	"io"
+	"malloc"	// for the malloc count test only
+	"math"
+	"strings"
+	"testing"
+)
+
+// more import examples
+import (
+	"xxx"
+	"much_longer_name" // comment
+	"short_name" // comment
+)
+
+import (
+	_ "xxx"
+	"much_longer_name" // comment
+)
+
+import (
+	mymath "math"
+	"/foo/bar/long_package_path" // a comment
+)
+
+import (
+	"package_a" // comment
+	"package_b"
+	my_better_c "package_c" // comment
+	"package_d" // comment
+	my_e "package_e" // comment
+
+	"package_a"    // comment
+	"package_bb"
+	"package_ccc"  // comment
+	"package_dddd" // comment
+)
+
+// at least one empty line between declarations of different kind
+import _ "io"
+var _ int
+
+// at least one empty line between declarations of the same kind
+// if there is associated documentation (was issue 2570)
+type T1 struct{}
+// T2 comment
+type T2 struct {
+} // should be a two-line struct
+
+
+// T3 comment
+type T2 struct {
+
+
+} // should be a two-line struct
+
+
+// printing of constant literals
+const (
+	_ = "foobar"
+	_ = "a۰۱۸"
+	_ = "foo६४"
+	_ = "bar９８７６"
+	_ = 0
+	_ = 1
+	_ = 123456789012345678890
+	_ = 01234567
+	_ = 0xcafebabe
+	_ = 0.
+	_ = .0
+	_ = 3.14159265
+	_ = 1e0
+	_ = 1e+100
+	_ = 1e-100
+	_ = 2.71828e-1000
+	_ = 0i
+	_ = 1i
+	_ = 012345678901234567889i
+	_ = 123456789012345678890i
+	_ = 0.i
+	_ = .0i
+	_ = 3.14159265i
+	_ = 1e0i
+	_ = 1e+100i
+	_ = 1e-100i
+	_ = 2.71828e-1000i
+	_ = 'a'
+	_ = '\000'
+	_ = '\xFF'
+	_ = '\uff16'
+	_ = '\U0000ff16'
+	_ = `foobar`
+	_ = `foo
+---
+---
+bar`
+)
+
+
+func _() {
+	type _ int
+	type _ *int
+	type _ []int
+	type _ map[string]int
+	type _ chan int
+	type _ func() int
+
+	var _ int
+	var _ *int
+	var _ []int
+	var _ map[string]int
+	var _ chan int
+	var _ func() int
+
+	type _ struct{}
+	type _ *struct{}
+	type _ []struct{}
+	type _ map[string]struct{}
+	type _ chan struct{}
+	type _ func() struct{}
+
+	type _ interface{}
+	type _ *interface{}
+	type _ []interface{}
+	type _ map[string]interface{}
+	type _ chan interface{}
+	type _ func() interface{}
+
+	var _ struct{}
+	var _ *struct{}
+	var _ []struct{}
+	var _ map[string]struct{}
+	var _ chan struct{}
+	var _ func() struct{}
+
+	var _ interface{}
+	var _ *interface{}
+	var _ []interface{}
+	var _ map[string]interface{}
+	var _ chan interface{}
+	var _ func() interface{}
+}
+
+
+// don't lose blank lines in grouped declarations
+const (
+	_ int = 0
+	_ float = 1
+
+	_ string = "foo"
+
+	_ = iota
+	_
+	
+	// a comment
+	_
+
+	_
+)
+
+
+type (
+	_ int
+	_ struct {}
+	
+	_ interface{}
+	
+	// a comment
+	_ map[string]int
+)
+
+
+var (
+	_ int = 0
+	_ float = 1
+
+	_ string = "foo"
+
+	_ bool
+	
+	// a comment
+	_ bool
+)
+
+
+// don't lose blank lines in this struct
+type _ struct {
+	String struct {
+		Str, Len int
+	}
+	Slice struct {
+		Array, Len, Cap int
+	}
+	Eface struct {
+		Typ, Ptr int
+	}
+
+	UncommonType struct {
+		Name, PkgPath int
+	}
+	CommonType struct {
+		Size, Hash, Alg, Align, FieldAlign, String, UncommonType int
+	}
+	Type struct {
+		Typ, Ptr int
+	}
+	StructField struct {
+		Name, PkgPath, Typ, Tag, Offset int
+	}
+	StructType struct {
+		Fields int
+	}
+	PtrType struct {
+		Elem int
+	}
+	SliceType struct {
+		Elem int
+	}
+	ArrayType struct {
+		Elem, Len int
+	}
+
+	Stktop struct {
+		Stackguard, Stackbase, Gobuf int
+	}
+	Gobuf struct {
+		Sp, Pc, G int
+	}
+	G struct {
+		Stackbase, Sched, Status, Alllink int
+	}
+}
+
+
+// no blank lines in empty structs and interfaces, but leave 1- or 2-line layout alone
+type _ struct{            }
+type _ struct {
+
+}
+
+type _ interface{            }
+type _ interface {
+
+}
+
+
+// no tabs for single or ungrouped decls
+func _() {
+	const xxxxxx = 0
+	type x int
+	var xxx int
+	var yyyy float = 3.14
+	var zzzzz = "bar"
+
+	const (
+		xxxxxx = 0
+	)
+	type (
+		x int
+	)
+	var (
+		xxx int
+	)
+	var (
+		yyyy float = 3.14
+	)
+	var (
+		zzzzz = "bar"
+	)
+}
+
+// tabs for multiple or grouped decls
+func _() {
+	// no entry has a type
+	const (
+		zzzzzz = 1
+		z = 2
+		zzz = 3
+	)
+	// some entries have a type
+	const (
+		xxxxxx = 1
+		x = 2
+		xxx = 3
+		yyyyyyyy float = iota
+		yyyy = "bar"
+		yyy
+		yy = 2
+	)
+}
+
+func _() {
+	// no entry has a type
+	var (
+		zzzzzz = 1
+		z = 2
+		zzz = 3
+	)
+	// no entry has a value
+	var (
+		_ int
+		_ float
+		_ string
+
+		_ int  // comment
+		_ float  // comment
+		_ string  // comment
+	)
+	// some entries have a type
+	var (
+		xxxxxx int
+		x float
+		xxx string
+		yyyyyyyy int = 1234
+		y float = 3.14
+		yyyy = "bar"
+		yyy string = "foo"
+	)
+	// mixed entries - all comments should be aligned
+	var (
+		a, b, c int
+		x = 10
+		d int  // comment
+		y = 20  // comment
+		f, ff, fff, ffff int = 0, 1, 2, 3  // comment
+	)
+	// respect original line breaks
+	var _ = []T {
+		T{0x20,	"Telugu"},
+	}
+	var _ = []T {
+		// respect original line breaks
+		T{0x20,	"Telugu"},
+	}
+}
+
+// use the formatted output rather than the input to decide when to align
+// (was issue 4505)
+const (
+	short = 2 * (
+	1 + 2)
+	aMuchLongerName = 3
+)
+
+var (
+	short = X{
+	}
+	aMuchLongerName = X{}
+
+	x1 = X{} // foo
+	x2 = X{
+	} // foo
+)
+
+func _() {
+	type (
+		xxxxxx int
+		x float
+		xxx string
+		xxxxx []x
+		xx struct{}
+		xxxxxxx struct {
+			_, _ int
+			_ float
+		}
+		xxxx chan<- string
+	)
+}
+
+// alignment of "=" in consecutive lines (extended example from issue 1414)
+const (
+	umax uint                  = ^uint(0) // maximum value for a uint
+	bpu  = 1 << (5 + umax>>63)            // bits per uint
+	foo
+	bar  = -1
+)
+
+// typical enum
+const (
+	a MyType = iota
+	abcd
+	b
+	c
+	def
+)
+
+// excerpt from godoc.go
+var (
+	goroot = flag.String("goroot", runtime.GOROOT(), "Go root directory")
+	testDir = flag.String("testdir", "", "Go root subdirectory - for testing only (faster startups)")
+	pkgPath = flag.String("path", "", "additional package directories (colon-separated)")
+	filter = flag.String("filter", "", "filter file containing permitted package directory paths")
+	filterMin = flag.Int("filter_minutes", 0, "filter file update interval in minutes; disabled if <= 0")
+	filterDelay delayTime // actual filter update interval in minutes; usually filterDelay == filterMin, but filterDelay may back off exponentially
+)
+
+
+// formatting of structs
+type _ struct{}
+
+type _ struct{ /* this comment should be visible */ }
+
+type _ struct{
+	// this comment should be visible and properly indented
+}
+
+type _ struct {  // this comment must not change indentation
+	f int
+	f, ff, fff, ffff int
+}
+
+type _ struct {
+	string
+}
+
+type _ struct {
+	string  // comment
+}
+
+type _ struct {
+	string "tag"
+}
+
+type _ struct {
+	string "tag"  // comment
+}
+
+type _ struct {
+	f int
+}
+
+type _ struct {
+	f int  // comment
+}
+
+type _ struct {
+	f int "tag"
+}
+
+type _ struct {
+	f int "tag"  // comment
+}
+
+type _ struct {
+	bool
+	a, b, c int
+	int "tag"
+	ES // comment
+	float "tag"  // comment
+	f int  // comment
+	f, ff, fff, ffff int  // comment
+	g float "tag"
+	h float "tag"  // comment
+}
+
+type _ struct { a, b,
+c, d int  // this line should be indented
+u, v, w, x float // this line should be indented
+p, q,
+r, s float // this line should be indented
+}
+
+
+// difficult cases
+type _ struct {
+	bool  // comment
+	text []byte  // comment
+}
+
+
+// formatting of interfaces
+type EI interface{}
+
+type _ interface {
+	EI
+}
+
+type _ interface {
+	f()
+	fffff()
+}
+
+type _ interface {
+	EI
+	f()
+	fffffg()
+}
+
+type _ interface {  // this comment must not change indentation
+	EI  // here's a comment
+	f()  // no blank between identifier and ()
+	fffff()  // no blank between identifier and ()
+	gggggggggggg(x, y, z int) ()  // hurray
+}
+
+
+// formatting of variable declarations
+func _() {
+	type day struct { n int; short, long string }
+	var (
+		Sunday = day{ 0, "SUN", "Sunday" }
+		Monday = day{ 1, "MON", "Monday" }
+		Tuesday = day{ 2, "TUE", "Tuesday" }
+		Wednesday = day{ 3, "WED", "Wednesday" }
+		Thursday = day{ 4, "THU", "Thursday" }
+		Friday = day{ 5, "FRI", "Friday" }
+		Saturday = day{ 6, "SAT", "Saturday" }
+	)
+}
+
+
+// formatting of multi-line variable declarations
+var a1, b1, c1 int  // all on one line
+
+var a2, b2,
+c2 int  // this line should be indented
+
+var (a3, b3,
+c3, d3 int  // this line should be indented
+a4, b4, c4 int  // this line should be indented
+)
+
+// Test case from issue 3304: multi-line declarations must end
+// a formatting section and not influence indentation of the
+// next line.
+var (
+	minRefreshTimeSec = flag.Int64("min_refresh_time_sec", 604800,
+		"minimum time window between two refreshes for a given user.")
+	x = flag.Int64("refresh_user_rollout_percent", 100,
+		"temporary flag to ramp up the refresh user rpc")
+	aVeryLongVariableName = stats.GetVarInt("refresh-user-count")
+)
+
+func _() {
+	var privateKey2 = &Block{Type: "RSA PRIVATE KEY",
+					Headers: map[string]string{},
+					Bytes: []uint8{0x30, 0x82, 0x1, 0x3a, 0x2, 0x1, 0x0, 0x2,
+			0x41, 0x0, 0xb2, 0x99, 0xf, 0x49, 0xc4, 0x7d, 0xfa, 0x8c,
+			0xd4, 0x0, 0xae, 0x6a, 0x4d, 0x1b, 0x8a, 0x3b, 0x6a, 0x13,
+			0x64, 0x2b, 0x23, 0xf2, 0x8b, 0x0, 0x3b, 0xfb, 0x97, 0x79,
+		},
+	}
+}
+
+
+func _() {
+	var Universe = Scope {
+		Names: map[string]*Ident {
+			// basic types
+			"bool": nil,
+			"byte": nil,
+			"int8": nil,
+			"int16": nil,
+			"int32": nil,
+			"int64": nil,
+			"uint8": nil,
+			"uint16": nil,
+			"uint32": nil,
+			"uint64": nil,
+			"float32": nil,
+			"float64": nil,
+			"string": nil,
+
+			// convenience types
+			"int": nil,
+			"uint": nil,
+			"uintptr": nil,
+			"float": nil,
+
+			// constants
+			"false": nil,
+			"true": nil,
+			"iota": nil,
+			"nil": nil,
+
+			// functions
+			"cap": nil,
+			"len": nil,
+			"new": nil,
+			"make": nil,
+			"panic": nil,
+			"panicln": nil,
+			"print": nil,
+			"println": nil,
+		},
+	}
+}
+
+
+// alignment of map composite entries
+var _ = map[int]int{
+	// small key sizes: always align even if size ratios are large
+	a: a,
+	abcdefghabcdefgh: a,
+	ab: a,
+	abc: a,
+	abcdefgabcdefg: a,
+	abcd: a,
+	abcde: a,
+	abcdef: a,
+
+	// mixed key sizes: align when key sizes change within accepted ratio
+	abcdefgh: a,
+	abcdefghabcdefg: a,
+	abcdefghij: a,
+	abcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghij: a, // outlier - do not align with previous line
+	abcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghij: a, // align with previous line
+
+	ab: a, // do not align with previous line
+	abcde: a, // align with previous line
+}
+
+// alignment of map composite entries: test cases from issue 3965
+// aligned
+var _ = T1{
+	a:                    x,
+	b:                    y,
+	cccccccccccccccccccc: z,
+}
+
+// not aligned
+var _ = T2{
+	a: x,
+	b: y,
+	ccccccccccccccccccccc: z,
+}
+
+// aligned
+var _ = T3{
+	aaaaaaaaaaaaaaaaaaaa: x,
+	b:                    y,
+	c:                    z,
+}
+
+// not aligned
+var _ = T4{
+	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa: x,
+	b:                                       y,
+	c:                                       z,
+}
+
+
+func _() {
+	var _ = T{
+		a,	// must introduce trailing comma
+	}
+}
+
+
+// formatting of function results
+func _() func() {}
+func _() func(int) { return nil }
+func _() func(int) int { return nil }
+func _() func(int) func(int) func() { return nil }
+
+
+// formatting of consecutive single-line functions
+func _() {}
+func _() {}
+func _() {}
+
+func _() {}  // an empty line before this function
+func _() {}
+func _() {}
+
+func _() { f(1, 2, 3) }
+func _(x int) int { y := x; return y+1 }
+func _() int { type T struct{}; var x T; return x }
+
+// these must remain multi-line since they are multi-line in the source
+func _() {
+	f(1, 2, 3)
+}
+func _(x int) int {
+	y := x; return y+1
+}
+func _() int {
+	type T struct{}; var x T; return x
+}
+
+
+// making function declarations safe for new semicolon rules
+func _() { /* single-line function because of "short-ish" comment */ }
+func _() { /* multi-line function because of "long-ish" comment - much more comment text is following here */ /* and more */ }
+
+func _() {
+/* multi-line func because block is on multiple lines */ }
+
+
+// ellipsis parameters
+func _(...int)
+func _(...*int)
+func _(...[]int)
+func _(...struct{})
+func _(bool, ...interface{})
+func _(bool, ...func())
+func _(bool, ...func(...int))
+func _(bool, ...map[string]int)
+func _(bool, ...chan int)
+
+func _(b bool, x ...int)
+func _(b bool, x ...*int)
+func _(b bool, x ...[]int)
+func _(b bool, x ...struct{})
+func _(x ...interface{})
+func _(x ...func())
+func _(x ...func(...int))
+func _(x ...map[string]int)
+func _(x ...chan int)
+
+
+// these parameter lists must remain multi-line since they are multi-line in the source
+func _(bool,
+int) {
+}
+func _(x bool,
+y int) {
+}
+func _(x,
+y bool) {
+}
+func _(bool, // comment
+int) {
+}
+func _(x bool, // comment
+y int) {
+}
+func _(x, // comment
+y bool) {
+}
+func _(bool, // comment
+// comment
+int) {
+}
+func _(x bool, // comment
+// comment
+y int) {
+}
+func _(x, // comment
+// comment
+y bool) {
+}
+func _(bool,
+// comment
+int) {
+}
+func _(x bool,
+// comment
+y int) {
+}
+func _(x,
+// comment
+y bool) {
+}
+func _(x, // comment
+y,// comment
+z bool) {
+}
+func _(x, // comment
+	y,// comment
+	z bool) {
+}
+func _(x int,	// comment
+	y float,	// comment
+	z bool) {
+}
+
+
+// properly indent multi-line signatures
+func ManageStatus(in <-chan *Status, req <-chan Request,
+stat chan<- *TargetInfo,
+TargetHistorySize int) {
+}
+
+func MultiLineSignature0(
+a, b, c int,
+) {}
+
+func MultiLineSignature1(
+a, b, c int,
+u, v, w float,
+) {}
+
+func MultiLineSignature2(
+a, b,
+c int,
+) {}
+
+func MultiLineSignature3(
+a, b,
+c int, u, v,
+w float,
+		x ...int) {}
+
+func MultiLineSignature4(
+a, b, c int,
+u, v,
+w float,
+		x ...int) {}
+
+func MultiLineSignature5(
+a, b, c int,
+u, v, w float,
+p, q,
+r string,
+		x ...int) {}
+
+// make sure it also works for methods in interfaces
+type _ interface {
+MultiLineSignature0(
+a, b, c int,
+)
+
+MultiLineSignature1(
+a, b, c int,
+u, v, w float,
+)
+
+MultiLineSignature2(
+a, b,
+c int,
+)
+
+MultiLineSignature3(
+a, b,
+c int, u, v,
+w float,
+		x ...int)
+
+MultiLineSignature4(
+a, b, c int,
+u, v,
+w float,
+		x ...int)
+
+MultiLineSignature5(
+a, b, c int,
+u, v, w float,
+p, q,
+r string,
+		x ...int)
+}
+
+// omit superfluous parentheses in parameter lists
+func _((int))
+func _((((((int))))))
+func _(x (int))
+func _(x (((((int))))))
+func _(x, y (int))
+func _(x, y (((((int))))))
+
+func _() (int)
+func _() ((int))
+func _() ((((((int))))))
+
+func _() (x int)
+func _() (x (int))
+func _() (x (((((int))))))
+
+// special cases: some channel types require parentheses
+func _(x chan(<-chan int))
+func _(x (chan(<-chan int)))
+func _(x ((((chan(<-chan int))))))
+
+func _(x chan<-(chan int))
+func _(x (chan<-(chan int)))
+func _(x ((((chan<-(chan int))))))
+
+// don't introduce comma after last parameter if the closing ) is on the same line
+// even if the parameter type itself is multi-line (test cases from issue 4533)
+func _(...interface{})
+func _(...interface {
+	m()
+	n()
+}) // no extra comma between } and )
+
+func (t *T) _(...interface{})
+func (t *T) _(...interface {
+	m()
+	n()
+}) // no extra comma between } and )
+
+func _(interface{})
+func _(interface {
+	m()
+}) // no extra comma between } and )
+
+func _(struct{})
+func _(struct {
+	x int
+	y int
+}) // no extra comma between } and )
diff --git a/src/go/printer/testdata/empty.golden b/src/go/printer/testdata/empty.golden
new file mode 100644
index 000000000..a055f4758
--- /dev/null
+++ b/src/go/printer/testdata/empty.golden
@@ -0,0 +1,5 @@
+// a comment at the beginning of the file
+
+package empty
+
+// a comment at the end of the file
diff --git a/src/go/printer/testdata/empty.input b/src/go/printer/testdata/empty.input
new file mode 100644
index 000000000..a055f4758
--- /dev/null
+++ b/src/go/printer/testdata/empty.input
@@ -0,0 +1,5 @@
+// a comment at the beginning of the file
+
+package empty
+
+// a comment at the end of the file
diff --git a/src/go/printer/testdata/expressions.golden b/src/go/printer/testdata/expressions.golden
new file mode 100644
index 000000000..fbe8275b3
--- /dev/null
+++ b/src/go/printer/testdata/expressions.golden
@@ -0,0 +1,681 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package expressions
+
+type T struct {
+	x, y, z int
+}
+
+var (
+	a, b, c, d, e						int
+	under_bar						int
+	longIdentifier1, longIdentifier2, longIdentifier3	int
+	t0, t1, t2						T
+	s							string
+	p							*int
+)
+
+func _() {
+	// no spaces around simple or parenthesized expressions
+	_ = (a + 0)
+	_ = a + b
+	_ = a + b + c
+	_ = a + b - c
+	_ = a - b - c
+	_ = a + (b * c)
+	_ = a + (b / c)
+	_ = a - (b % c)
+	_ = 1 + a
+	_ = a + 1
+	_ = a + b + 1
+	_ = s[a]
+	_ = s[a:]
+	_ = s[:b]
+	_ = s[1:2]
+	_ = s[a:b]
+	_ = s[0:len(s)]
+	_ = s[0] << 1
+	_ = (s[0] << 1) & 0xf
+	_ = s[0]<<2 | s[1]>>4
+	_ = "foo" + s
+	_ = s + "foo"
+	_ = 'a' + 'b'
+	_ = len(s) / 2
+	_ = len(t0.x) / a
+
+	// spaces around expressions of different precedence or expressions containing spaces
+	_ = a + -b
+	_ = a - ^b
+	_ = a / *p
+	_ = a + b*c
+	_ = 1 + b*c
+	_ = a + 2*c
+	_ = a + c*2
+	_ = 1 + 2*3
+	_ = s[1 : 2*3]
+	_ = s[a : b-c]
+	_ = s[0:]
+	_ = s[a+b]
+	_ = s[:b-c]
+	_ = s[a+b:]
+	_ = a[a<<b+1]
+	_ = a[a<<b+1:]
+	_ = s[a+b : len(s)]
+	_ = s[len(s):-a]
+	_ = s[a : len(s)+1]
+	_ = s[a:len(s)+1] + s
+
+	// spaces around operators with equal or lower precedence than comparisons
+	_ = a == b
+	_ = a != b
+	_ = a > b
+	_ = a >= b
+	_ = a < b
+	_ = a <= b
+	_ = a < b && c > d
+	_ = a < b || c > d
+
+	// spaces around "long" operands
+	_ = a + longIdentifier1
+	_ = longIdentifier1 + a
+	_ = longIdentifier1 + longIdentifier2*longIdentifier3
+	_ = s + "a longer string"
+
+	// some selected cases
+	_ = a + t0.x
+	_ = a + t0.x + t1.x*t2.x
+	_ = a + b + c + d + e + 2*3
+	_ = a + b + c + 2*3 + d + e
+	_ = (a + b + c) * 2
+	_ = a - b + c - d + (a + b + c) + d&e
+	_ = under_bar - 1
+	_ = Open(dpath+"/file", O_WRONLY|O_CREAT, 0666)
+	_ = int(c0&_Mask4)<<18 | int(c1&_Maskx)<<12 | int(c2&_Maskx)<<6 | int(c3&_Maskx)
+
+	// the parser does not restrict expressions that may appear as statements
+	true
+	42
+	"foo"
+	x
+	(x)
+	a + b
+	a + b + c
+	a + (b * c)
+	a + (b / c)
+	1 + a
+	a + 1
+	s[a]
+	x << 1
+	(s[0] << 1) & 0xf
+	"foo" + s
+	x == y
+	x < y || z > 42
+}
+
+// slice expressions with cap
+func _() {
+	_ = x[a:b:c]
+	_ = x[a:b : c+d]
+	_ = x[a : b+d : c]
+	_ = x[a : b+d : c+d]
+	_ = x[a+d : b:c]
+	_ = x[a+d : b : c+d]
+	_ = x[a+d : b+d : c]
+	_ = x[a+d : b+d : c+d]
+
+	_ = x[:b:c]
+	_ = x[:b : c+d]
+	_ = x[:b+d : c]
+	_ = x[:b+d : c+d]
+}
+
+func _() {
+	_ = a + b
+	_ = a + b + c
+	_ = a + b*c
+	_ = a + (b * c)
+	_ = (a + b) * c
+	_ = a + (b * c * d)
+	_ = a + (b*c + d)
+
+	_ = 1 << x
+	_ = -1 << x
+	_ = 1<<x - 1
+	_ = -1<<x - 1
+
+	_ = f(a + b)
+	_ = f(a + b + c)
+	_ = f(a + b*c)
+	_ = f(a + (b * c))
+	_ = f(1<<x-1, 1<<x-2)
+
+	_ = 1<<d.logWindowSize - 1
+
+	buf = make(x, 2*cap(b.buf)+n)
+
+	dst[i*3+2] = dbuf[0] << 2
+	dst[i*3+2] = dbuf[0]<<2 | dbuf[1]>>4
+
+	b.buf = b.buf[0 : b.off+m+n]
+	b.buf = b.buf[0 : b.off+m*n]
+	f(b.buf[0 : b.off+m+n])
+
+	signed += ' ' * 8
+	tw.octal(header[148:155], chksum)
+
+	_ = x > 0 && i >= 0
+
+	x1, x0 := x>>w2, x&m2
+	z0 = t1<<w2 + t0
+	z1 = (t1 + t0>>w2) >> w2
+	q1, r1 := x1/d1, x1%d1
+	r1 = r1*b2 | x0>>w2
+	x1 = (x1 << z) | (x0 >> (uint(w) - z))
+	x1 = x1<<z | x0>>(uint(w)-z)
+
+	_ = buf[0 : len(buf)+1]
+	_ = buf[0 : n+1]
+
+	a, b = b, a
+	a = b + c
+	a = b*c + d
+	_ = a*b + c
+	_ = a - b - c
+	_ = a - (b - c)
+	_ = a - b*c
+	_ = a - (b * c)
+	_ = a * b / c
+	_ = a / *b
+	_ = x[a|^b]
+	_ = x[a / *b]
+	_ = a & ^b
+	_ = a + +b
+	_ = a - -b
+	_ = x[a*-b]
+	_ = x[a + +b]
+	_ = x ^ y ^ z
+	_ = b[a>>24] ^ b[(a>>16)&0xFF] ^ b[(a>>8)&0xFF] ^ b[a&0xFF]
+	_ = len(longVariableName) * 2
+
+	_ = token(matchType + xlength<<lengthShift + xoffset)
+}
+
+func f(x int, args ...int) {
+	f(0, args...)
+	f(1, args)
+	f(2, args[0])
+
+	// make sure syntactically legal code remains syntactically legal
+	f(3, 42 ...)	// a blank must remain between 42 and ...
+	f(4, 42....)
+	f(5, 42....)
+	f(6, 42.0...)
+	f(7, 42.0...)
+	f(8, .42...)
+	f(9, .42...)
+	f(10, 42e0...)
+	f(11, 42e0...)
+
+	_ = 42 .x	// a blank must remain between 42 and .x
+	_ = 42..x
+	_ = 42..x
+	_ = 42.0.x
+	_ = 42.0.x
+	_ = .42.x
+	_ = .42.x
+	_ = 42e0.x
+	_ = 42e0.x
+
+	// a blank must remain between the binary operator and the 2nd operand
+	_ = x / *y
+	_ = x < -1
+	_ = x < <-1
+	_ = x + +1
+	_ = x - -1
+	_ = x & &x
+	_ = x & ^x
+
+	_ = f(x / *y, x < -1, x < <-1, x + +1, x - -1, x & &x, x & ^x)
+}
+
+func _() {
+	_ = T{}
+	_ = struct{}{}
+	_ = [10]T{}
+	_ = [...]T{}
+	_ = []T{}
+	_ = map[int]T{}
+}
+
+// one-line structs/interfaces in composite literals (up to a threshold)
+func _() {
+	_ = struct{}{}
+	_ = struct{ x int }{0}
+	_ = struct{ x, y, z int }{0, 1, 2}
+	_ = struct{ int }{0}
+	_ = struct{ s struct{ int } }{struct{ int }{0}}
+}
+
+func _() {
+	// do not modify literals
+	_ = "tab1	tab2	tab3	end"	// string contains 3 tabs
+	_ = "tab1 tab2 tab3 end"	// same string with 3 blanks - may be unaligned because editors see tabs in strings
+	_ = ""				// this comment should be aligned with the one on the previous line
+	_ = ``
+	_ = `
+`
+	_ = `foo
+		bar`
+	_ = `three spaces before the end of the line starting here:   
+they must not be removed`
+}
+
+func _() {
+	// smart handling of indentation for multi-line raw strings
+	var _ = ``
+	var _ = `foo`
+	var _ = `foo
+bar`
+
+	var _ = ``
+	var _ = `foo`
+	var _ =
+	// the next line should remain indented
+	`foo
+bar`
+
+	var _ =	// comment
+	``
+	var _ =	// comment
+	`foo`
+	var _ =	// comment
+	// the next line should remain indented
+	`foo
+bar`
+
+	var _ = /* comment */ ``
+	var _ = /* comment */ `foo`
+	var _ = /* comment */ `foo
+bar`
+
+	var _ =	/* comment */
+	``
+	var _ =	/* comment */
+	`foo`
+	var _ =	/* comment */
+	// the next line should remain indented
+	`foo
+bar`
+
+	var board = []int(
+		`...........
+...........
+....●●●....
+....●●●....
+..●●●●●●●..
+..●●●○●●●..
+..●●●●●●●..
+....●●●....
+....●●●....
+...........
+...........
+`)
+
+	var state = S{
+		"foo",
+		// the next line should remain indented
+		`...........
+...........
+....●●●....
+....●●●....
+..●●●●●●●..
+..●●●○●●●..
+..●●●●●●●..
+....●●●....
+....●●●....
+...........
+...........
+`,
+		"bar",
+	}
+}
+
+func _() {
+	// one-line function literals (body is on a single line)
+	_ = func() {}
+	_ = func() int { return 0 }
+	_ = func(x, y int) bool { m := (x + y) / 2; return m < 0 }
+
+	// multi-line function literals (body is not on one line)
+	_ = func() {
+	}
+	_ = func() int {
+		return 0
+	}
+	_ = func(x, y int) bool {
+		m := (x + y) / 2
+		return x < y
+	}
+
+	f(func() {
+	})
+	f(func() int {
+		return 0
+	})
+	f(func(x, y int) bool {
+		m := (x + y) / 2
+		return x < y
+	})
+}
+
+func _() {
+	_ = [][]int{
+		[]int{1},
+		[]int{1, 2},
+		[]int{1, 2, 3},
+	}
+	_ = [][]int{
+		{1},
+		[]int{1, 2},
+		[]int{1, 2, 3},
+	}
+	_ = [][]int{
+		{1},
+		{1, 2},
+		{1, 2, 3},
+	}
+	_ = [][]int{{1}, {1, 2}, {1, 2, 3}}
+}
+
+// various multi-line expressions
+func _() {
+	// do not add extra indentation to multi-line string lists
+	_ = "foo" + "bar"
+	_ = "foo" +
+		"bar" +
+		"bah"
+	_ = []string{
+		"abc" +
+			"def",
+		"foo" +
+			"bar",
+	}
+}
+
+const _ = F1 +
+	`string = "%s";` +
+	`ptr = *;` +
+	`datafmt.T2 = s ["-" p "-"];`
+
+const _ = `datafmt "datafmt";` +
+	`default = "%v";` +
+	`array = *;` +
+	`datafmt.T3 = s  {" " a a / ","};`
+
+const _ = `datafmt "datafmt";` +
+	`default = "%v";` +
+	`array = *;` +
+	`datafmt.T3 = s  {" " a a / ","};`
+
+func _() {
+	_ = F1 +
+		`string = "%s";` +
+		`ptr = *;` +
+		`datafmt.T2 = s ["-" p "-"];`
+
+	_ =
+		`datafmt "datafmt";` +
+			`default = "%v";` +
+			`array = *;` +
+			`datafmt.T3 = s  {" " a a / ","};`
+
+	_ = `datafmt "datafmt";` +
+		`default = "%v";` +
+		`array = *;` +
+		`datafmt.T3 = s  {" " a a / ","};`
+}
+
+func _() {
+	// respect source lines in multi-line expressions
+	_ = a +
+		b +
+		c
+	_ = a < b ||
+		b < a
+	_ = "933262154439441526816992388562667004907159682643816214685929" +
+		"638952175999932299156089414639761565182862536979208272237582" +
+		"51185210916864000000000000000000000000"	// 100!
+	_ = "170141183460469231731687303715884105727"	// prime
+}
+
+// Alignment after overlong lines
+const (
+	_	= "991"
+	_	= "2432902008176640000"	// 20!
+	_	= "933262154439441526816992388562667004907159682643816214685929" +
+		"638952175999932299156089414639761565182862536979208272237582" +
+		"51185210916864000000000000000000000000"	// 100!
+	_	= "170141183460469231731687303715884105727"	// prime
+)
+
+// Correct placement of operators and comments in multi-line expressions
+func _() {
+	_ = a +	// comment
+		b +	// comment
+		c
+	_ = "a" +
+		"b" +	// comment
+		"c"
+	_ = "ba0408" + "7265717569726564"	// field 71, encoding 2, string "required"
+}
+
+// Correct placement of terminating comma/closing parentheses in multi-line calls.
+func _() {
+	f(1,
+		2,
+		3)
+	f(1,
+		2,
+		3,
+	)
+	f(1,
+		2,
+		3)	// comment
+	f(1,
+		2,
+		3,	// comment
+	)
+	f(1,
+		2,
+		3)	// comment
+	f(1,
+		2,
+		3,	// comment
+	)
+}
+
+// Align comments in multi-line lists of single-line expressions.
+var txpix = [NCOL]draw.Color{
+	draw.Yellow,		// yellow
+	draw.Cyan,		// cyan
+	draw.Green,		// lime green
+	draw.GreyBlue,		// slate
+	draw.Red,		/* red */
+	draw.GreyGreen,		/* olive green */
+	draw.Blue,		/* blue */
+	draw.Color(0xFF55AAFF),	/* pink */
+	draw.Color(0xFFAAFFFF),	/* lavender */
+	draw.Color(0xBB005DFF),	/* maroon */
+}
+
+func same(t, u *Time) bool {
+	// respect source lines in multi-line expressions
+	return t.Year == u.Year &&
+		t.Month == u.Month &&
+		t.Day == u.Day &&
+		t.Hour == u.Hour &&
+		t.Minute == u.Minute &&
+		t.Second == u.Second &&
+		t.Weekday == u.Weekday &&
+		t.ZoneOffset == u.ZoneOffset &&
+		t.Zone == u.Zone
+}
+
+func (p *parser) charClass() {
+	// respect source lines in multi-line expressions
+	if cc.negate && len(cc.ranges) == 2 &&
+		cc.ranges[0] == '\n' && cc.ranges[1] == '\n' {
+		nl := new(_NotNl)
+		p.re.add(nl)
+	}
+}
+
+func addState(s []state, inst instr, match []int) {
+	// handle comments correctly in multi-line expressions
+	for i := 0; i < l; i++ {
+		if s[i].inst.index() == index &&	// same instruction
+			s[i].match[0] < pos {	// earlier match already going; leftmost wins
+			return s
+		}
+	}
+}
+
+func (self *T) foo(x int) *T	{ return self }
+
+func _()	{ module.Func1().Func2() }
+
+func _() {
+	_ = new(T).
+		foo(1).
+		foo(2).
+		foo(3)
+
+	_ = new(T).
+		foo(1).
+		foo(2).	// inline comments
+		foo(3)
+
+	_ = new(T).foo(1).foo(2).foo(3)
+
+	// handle multiline argument list correctly
+	_ = new(T).
+		foo(
+		1).
+		foo(2)
+
+	_ = new(T).foo(
+		1).foo(2)
+
+	_ = Array[3+
+		4]
+
+	_ = Method(1, 2,
+		3)
+
+	_ = new(T).
+		foo().
+		bar().(*Type)
+
+	_ = new(T).
+		foo().
+		bar().(*Type).
+		baz()
+
+	_ = new(T).
+		foo().
+		bar()["idx"]
+
+	_ = new(T).
+		foo().
+		bar()["idx"].
+		baz()
+
+	_ = new(T).
+		foo().
+		bar()[1:2]
+
+	_ = new(T).
+		foo().
+		bar()[1:2].
+		baz()
+
+	_ = new(T).
+		Field.
+		Array[3+
+		4].
+		Table["foo"].
+		Blob.(*Type).
+		Slices[1:4].
+		Method(1, 2,
+		3).
+		Thingy
+
+	_ = a.b.c
+	_ = a.
+		b.
+		c
+	_ = a.b().c
+	_ = a.
+		b().
+		c
+	_ = a.b[0].c
+	_ = a.
+		b[0].
+		c
+	_ = a.b[0:].c
+	_ = a.
+		b[0:].
+		c
+	_ = a.b.(T).c
+	_ = a.
+		b.(T).
+		c
+}
+
+// Don't introduce extra newlines in strangely formatted expression lists.
+func f() {
+	// os.Open parameters should remain on two lines
+	if writer, err = os.Open(outfile, s.O_WRONLY|os.O_CREATE|
+		os.O_TRUNC, 0666); err != nil {
+		log.Fatal(err)
+	}
+}
+
+// Handle multi-line argument lists ending in ... correctly.
+// Was issue 3130.
+func _() {
+	_ = append(s, a...)
+	_ = append(
+		s, a...)
+	_ = append(s,
+		a...)
+	_ = append(
+		s,
+		a...)
+	_ = append(s, a...,
+	)
+	_ = append(s,
+		a...,
+	)
+	_ = append(
+		s,
+		a...,
+	)
+}
+
+// Literal function types in conversions must be parenthesized;
+// for now go/parser accepts the unparenthesized form where it
+// is non-ambiguous.
+func _() {
+	// these conversions should be rewritten to look
+	// the same as the parenthesized conversions below
+	_ = (func())(nil)
+	_ = (func(x int) float)(nil)
+	_ = (func() func() func())(nil)
+
+	_ = (func())(nil)
+	_ = (func(x int) float)(nil)
+	_ = (func() func() func())(nil)
+}
diff --git a/src/go/printer/testdata/expressions.input b/src/go/printer/testdata/expressions.input
new file mode 100644
index 000000000..f4d20fa0f
--- /dev/null
+++ b/src/go/printer/testdata/expressions.input
@@ -0,0 +1,710 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package expressions
+
+type T struct {
+	x, y, z int
+}
+
+var (
+	a, b, c, d, e int
+	under_bar int
+	longIdentifier1, longIdentifier2, longIdentifier3 int
+	t0, t1, t2 T
+	s string
+	p *int
+)
+
+
+func _() {
+	// no spaces around simple or parenthesized expressions
+	_ = (a+0)
+	_ = a+b
+	_ = a+b+c
+	_ = a+b-c
+	_ = a-b-c
+	_ = a+(b*c)
+	_ = a+(b/c)
+	_ = a-(b%c)
+	_ = 1+a
+	_ = a+1
+	_ = a+b+1
+	_ = s[a]
+	_ = s[a:]
+	_ = s[:b]
+	_ = s[1:2]
+	_ = s[a:b]
+	_ = s[0:len(s)]
+	_ = s[0]<<1
+	_ = (s[0]<<1)&0xf
+	_ = s[0] << 2 | s[1] >> 4
+	_ = "foo"+s
+	_ = s+"foo"
+	_ = 'a'+'b'
+	_ = len(s)/2
+	_ = len(t0.x)/a
+
+	// spaces around expressions of different precedence or expressions containing spaces
+	_ = a + -b
+	_ = a - ^b
+	_ = a / *p
+	_ = a + b*c
+	_ = 1 + b*c
+	_ = a + 2*c
+	_ = a + c*2
+	_ = 1 + 2*3
+	_ = s[1 : 2*3]
+	_ = s[a : b-c]
+	_ = s[0:]
+	_ = s[a+b]
+	_ = s[: b-c]
+	_ = s[a+b :]
+	_ = a[a<<b+1]
+	_ = a[a<<b+1 :]
+	_ = s[a+b : len(s)]
+	_ = s[len(s) : -a]
+	_ = s[a : len(s)+1]
+	_ = s[a : len(s)+1]+s
+
+	// spaces around operators with equal or lower precedence than comparisons
+	_ = a == b
+	_ = a != b
+	_ = a > b
+	_ = a >= b
+	_ = a < b
+	_ = a <= b
+	_ = a < b && c > d
+	_ = a < b || c > d
+
+	// spaces around "long" operands
+	_ = a + longIdentifier1
+	_ = longIdentifier1 + a
+	_ = longIdentifier1 + longIdentifier2 * longIdentifier3
+	_ = s + "a longer string"
+
+	// some selected cases
+	_ = a + t0.x
+	_ = a + t0.x + t1.x * t2.x
+	_ = a + b + c + d + e + 2*3
+	_ = a + b + c + 2*3 + d + e
+	_ = (a+b+c)*2
+	_ = a - b + c - d + (a+b+c) + d&e
+	_ = under_bar-1
+	_ = Open(dpath + "/file", O_WRONLY | O_CREAT, 0666)
+	_ = int(c0&_Mask4)<<18 | int(c1&_Maskx)<<12 | int(c2&_Maskx)<<6 | int(c3&_Maskx)
+
+	// the parser does not restrict expressions that may appear as statements
+	true
+	42
+	"foo"
+	x
+	(x)
+	a+b
+	a+b+c
+	a+(b*c)
+	a+(b/c)
+	1+a
+	a+1
+	s[a]
+	x<<1
+	(s[0]<<1)&0xf
+	"foo"+s
+	x == y
+	x < y || z > 42
+}
+
+
+// slice expressions with cap
+func _() {
+	_ = x[a:b:c]
+	_ = x[a:b:c+d]
+	_ = x[a:b+d:c]
+	_ = x[a:b+d:c+d]
+	_ = x[a+d:b:c]
+	_ = x[a+d:b:c+d]
+	_ = x[a+d:b+d:c]
+	_ = x[a+d:b+d:c+d]
+
+	_ = x[:b:c]
+	_ = x[:b:c+d]
+	_ = x[:b+d:c]
+	_ = x[:b+d:c+d]
+}
+
+func _() {
+	_ = a+b
+	_ = a+b+c
+	_ = a+b*c
+	_ = a+(b*c)
+	_ = (a+b)*c
+	_ = a+(b*c*d)
+	_ = a+(b*c+d)
+
+	_ = 1<<x
+	_ = -1<<x
+	_ = 1<<x-1
+	_ = -1<<x-1
+
+	_ = f(a+b)
+	_ = f(a+b+c)
+	_ = f(a+b*c)
+	_ = f(a+(b*c))
+	_ = f(1<<x-1, 1<<x-2)
+
+	_ = 1<<d.logWindowSize-1
+
+	buf = make(x, 2*cap(b.buf) + n)
+
+	dst[i*3+2] = dbuf[0]<<2
+	dst[i*3+2] = dbuf[0]<<2 | dbuf[1]>>4
+
+	b.buf = b.buf[0:b.off+m+n]
+	b.buf = b.buf[0:b.off+m*n]
+	f(b.buf[0:b.off+m+n])
+
+	signed += ' '*8
+	tw.octal(header[148:155], chksum)
+
+	_ = x > 0 && i >= 0
+
+	x1, x0 := x>>w2, x&m2
+	z0 = t1<<w2+t0
+	z1 = (t1+t0>>w2)>>w2
+	q1, r1 := x1/d1, x1%d1
+	r1 = r1*b2 | x0>>w2
+	x1 = (x1<<z)|(x0>>(uint(w)-z))
+	x1 = x1<<z | x0>>(uint(w)-z)
+
+	_ = buf[0:len(buf)+1]
+	_ = buf[0:n+1]
+
+	a,b = b,a
+	a = b+c
+	a = b*c+d
+	_ = a*b+c
+	_ = a-b-c
+	_ = a-(b-c)
+	_ = a-b*c
+	_ = a-(b*c)
+	_ = a*b/c
+	_ = a/ *b
+	_ = x[a|^b]
+	_ = x[a/ *b]
+	_ = a& ^b
+	_ = a+ +b
+	_ = a- -b
+	_ = x[a*-b]
+	_ = x[a+ +b]
+	_ = x^y^z
+	_ = b[a>>24] ^ b[(a>>16)&0xFF] ^ b[(a>>8)&0xFF] ^ b[a&0xFF]
+	_ = len(longVariableName)*2
+
+	_ = token(matchType + xlength<<lengthShift + xoffset)
+}
+
+
+func f(x int, args ...int) {
+	f(0, args...)
+	f(1, args)
+	f(2, args[0])
+
+	// make sure syntactically legal code remains syntactically legal
+	f(3, 42 ...) // a blank must remain between 42 and ...
+	f(4, 42. ...)
+	f(5, 42....)
+	f(6, 42.0 ...)
+	f(7, 42.0...)
+	f(8, .42 ...)
+	f(9, .42...)
+	f(10, 42e0 ...)
+	f(11, 42e0...)
+
+	_ = 42 .x // a blank must remain between 42 and .x
+	_ = 42. .x
+	_ = 42..x
+	_ = 42.0 .x
+	_ = 42.0.x
+	_ = .42 .x
+	_ = .42.x
+	_ = 42e0 .x
+	_ = 42e0.x
+
+	// a blank must remain between the binary operator and the 2nd operand
+	_ = x/ *y
+	_ = x< -1
+	_ = x< <-1
+	_ = x+ +1
+	_ = x- -1
+	_ = x& &x
+	_ = x& ^x
+
+	_ = f(x/ *y, x< -1, x< <-1, x+ +1, x- -1, x& &x, x& ^x)
+}
+
+
+func _() {
+	_ = T{}
+	_ = struct{}{}
+	_ = [10]T{}
+	_ = [...]T{}
+	_ = []T{}
+	_ = map[int]T{}
+}
+
+
+// one-line structs/interfaces in composite literals (up to a threshold)
+func _() {
+	_ = struct{}{}
+	_ = struct{ x int }{0}
+	_ = struct{ x, y, z int }{0, 1, 2}
+	_ = struct{ int }{0}
+	_ = struct{ s struct { int } }{struct{ int}{0} }
+}
+
+
+func _() {
+	// do not modify literals
+	_ = "tab1	tab2	tab3	end"  // string contains 3 tabs
+	_ = "tab1 tab2 tab3 end"  // same string with 3 blanks - may be unaligned because editors see tabs in strings
+	_ = ""  // this comment should be aligned with the one on the previous line
+	_ = ``
+	_ = `
+`
+_ = `foo
+		bar`
+	_ = `three spaces before the end of the line starting here:   
+they must not be removed`
+}
+
+
+func _() {
+	// smart handling of indentation for multi-line raw strings
+	var _ = ``
+	var _ = `foo`
+	var _ = `foo
+bar`
+
+
+var _ =
+	``
+var _ =
+	`foo`
+var _ =
+	// the next line should remain indented
+	`foo
+bar`
+
+
+	var _ = // comment
+		``
+	var _ = // comment
+		`foo`
+	var _ = // comment
+		// the next line should remain indented
+		`foo
+bar`
+
+
+var _ = /* comment */ ``
+var _ = /* comment */ `foo`
+var _ = /* comment */ `foo
+bar`
+
+
+	var _ = /* comment */
+		``
+	var _ = /* comment */
+		`foo`
+	var _ = /* comment */
+		// the next line should remain indented
+		`foo
+bar`
+
+
+var board = []int(
+	`...........
+...........
+....●●●....
+....●●●....
+..●●●●●●●..
+..●●●○●●●..
+..●●●●●●●..
+....●●●....
+....●●●....
+...........
+...........
+`)
+
+
+	var state = S{
+		"foo",
+		// the next line should remain indented
+		`...........
+...........
+....●●●....
+....●●●....
+..●●●●●●●..
+..●●●○●●●..
+..●●●●●●●..
+....●●●....
+....●●●....
+...........
+...........
+`,
+		"bar",
+	}
+}
+
+
+func _() {
+	// one-line function literals (body is on a single line)
+	_ = func() {}
+	_ = func() int { return 0 }
+	_ = func(x, y int) bool { m := (x+y)/2; return m < 0 }
+
+	// multi-line function literals (body is not on one line)
+	_ = func() {
+	}
+	_ = func() int {
+		return 0
+	}
+	_ = func(x, y int) bool {
+		m := (x+y)/2; return x < y }
+
+	f(func() {
+	})
+	f(func() int {
+		return 0
+	})
+	f(func(x, y int) bool {
+		m := (x+y)/2; return x < y })
+}
+
+
+func _() {
+	_ = [][]int {
+		[]int{1},
+		[]int{1, 2},
+		[]int{1, 2, 3},
+	}
+	_ = [][]int {
+		{1},
+		[]int{1, 2},
+		[]int{1, 2, 3},
+	}
+	_ = [][]int {
+		{1},
+		{1, 2},
+		{1, 2, 3},
+	}
+	_ = [][]int {{1}, {1, 2}, {1, 2, 3}}
+}
+
+
+// various multi-line expressions
+func _() {
+	// do not add extra indentation to multi-line string lists
+	_ = "foo" + "bar"
+	_ = "foo" +
+	"bar" +
+	"bah"
+	_ = []string {
+		"abc" +
+		"def",
+		"foo" +
+		"bar",
+	}
+}
+
+
+const _ = F1 +
+	`string = "%s";` +
+	`ptr = *;` +
+	`datafmt.T2 = s ["-" p "-"];`
+
+
+const _ =
+	`datafmt "datafmt";` +
+	`default = "%v";` +
+	`array = *;` +
+	`datafmt.T3 = s  {" " a a / ","};`
+
+
+const _ = `datafmt "datafmt";` +
+`default = "%v";` +
+`array = *;` +
+`datafmt.T3 = s  {" " a a / ","};`
+
+
+func _() {
+	_ = F1 +
+		`string = "%s";` +
+		`ptr = *;` +
+		`datafmt.T2 = s ["-" p "-"];`
+
+	_ =
+		`datafmt "datafmt";` +
+		`default = "%v";` +
+		`array = *;` +
+		`datafmt.T3 = s  {" " a a / ","};`
+
+	_ = `datafmt "datafmt";` +
+	`default = "%v";` +
+	`array = *;` +
+	`datafmt.T3 = s  {" " a a / ","};`
+}
+
+
+func _() {
+	// respect source lines in multi-line expressions
+	_ = a+
+	b+
+	c
+	_ = a < b ||
+		b < a
+	_ = "933262154439441526816992388562667004907159682643816214685929" +
+	"638952175999932299156089414639761565182862536979208272237582" +
+	"51185210916864000000000000000000000000"  // 100!
+	_ = "170141183460469231731687303715884105727"  // prime
+}
+
+
+// Alignment after overlong lines
+const (
+	_ = "991"
+	_ = "2432902008176640000"  // 20!
+	_ = "933262154439441526816992388562667004907159682643816214685929" +
+	"638952175999932299156089414639761565182862536979208272237582" +
+	"51185210916864000000000000000000000000"  // 100!
+	_ = "170141183460469231731687303715884105727"  // prime
+)
+
+
+// Correct placement of operators and comments in multi-line expressions
+func _() {
+	_ = a +  // comment
+		b +  // comment
+		c
+	_ = "a"	+
+		"b" +	// comment
+		"c"
+	_ = "ba0408" + "7265717569726564"     // field 71, encoding 2, string "required"
+}
+
+
+// Correct placement of terminating comma/closing parentheses in multi-line calls.
+func _() {
+	f(1,
+		2,
+		3)
+	f(1,
+		2,
+		3,
+	)
+	f(1,
+		2,
+		3)  // comment
+	f(1,
+		2,
+		3,  // comment
+	)
+	f(1,
+		2,
+		3)// comment
+	f(1,
+		2,
+		3,// comment
+	)
+}
+
+
+// Align comments in multi-line lists of single-line expressions.
+var txpix = [NCOL]draw.Color{
+	draw.Yellow, // yellow
+	draw.Cyan, // cyan
+	draw.Green, // lime green
+	draw.GreyBlue, // slate
+	draw.Red, /* red */
+	draw.GreyGreen, /* olive green */
+	draw.Blue, /* blue */
+	draw.Color(0xFF55AAFF), /* pink */
+	draw.Color(0xFFAAFFFF), /* lavender */
+	draw.Color(0xBB005DFF), /* maroon */
+}
+
+
+func same(t, u *Time) bool {
+	// respect source lines in multi-line expressions
+	return t.Year == u.Year &&
+		t.Month == u.Month &&
+		t.Day == u.Day &&
+		t.Hour == u.Hour &&
+		t.Minute == u.Minute &&
+		t.Second == u.Second &&
+		t.Weekday == u.Weekday &&
+		t.ZoneOffset == u.ZoneOffset &&
+		t.Zone == u.Zone
+}
+
+
+func (p *parser) charClass() {
+	// respect source lines in multi-line expressions
+	if cc.negate && len(cc.ranges) == 2 &&
+		cc.ranges[0] == '\n' && cc.ranges[1] == '\n' {
+		nl := new(_NotNl)
+		p.re.add(nl)
+	}
+}
+
+
+func addState(s []state, inst instr, match []int) {
+	// handle comments correctly in multi-line expressions
+	for i := 0; i < l; i++ {
+		if s[i].inst.index() == index && // same instruction
+		   s[i].match[0] < pos {	// earlier match already going; leftmost wins
+		   	return s
+		 }
+	}
+}
+
+func (self *T) foo(x int) *T { return self }
+
+func _() { module.Func1().Func2() }
+
+func _() {
+	_ = new(T).
+		foo(1).
+			foo(2).
+		foo(3)
+
+	_ = new(T).
+	foo(1).
+	foo(2). // inline comments
+	foo(3)
+
+	_ = new(T).foo(1).foo(2).foo(3)
+
+	// handle multiline argument list correctly
+	_ = new(T).
+	foo(
+		1).
+		foo(2)
+
+	_ = new(T).foo(
+		1).foo(2)
+
+	_ = Array[3 +
+4]
+
+	_ = Method(1, 2,
+		3)
+
+	_ = new(T).
+   foo().
+   bar() . (*Type)
+
+	_ = new(T).
+foo().
+bar().(*Type).
+baz()
+
+	_ = new(T).
+	foo().
+	bar()["idx"]
+
+	_ = new(T).
+	foo().
+	bar()["idx"]	.
+	baz()
+
+	_ = new(T).
+	foo().
+	bar()[1:2]
+
+	_ = new(T).
+	foo().
+	bar()[1:2].
+	baz()
+
+	_ = new(T).
+		Field.
+		Array[3+
+       		4].
+		Table ["foo"].
+		Blob. (*Type).
+	Slices[1:4].
+	Method(1, 2,
+	3).
+		Thingy
+
+	_ = a.b.c
+	_ = a.
+	b.
+	c
+	_ = a.b().c
+	_ = a.
+	b().
+	c
+	_ = a.b[0].c
+	_ = a.
+	b[0].
+	c
+	_ = a.b[0:].c
+	_ = a.
+	b[0:].
+	c
+	_ = a.b.(T).c
+	_ = a.
+	b.
+	(T).
+	c
+}
+
+
+// Don't introduce extra newlines in strangely formatted expression lists.
+func f() {
+	// os.Open parameters should remain on two lines
+	if writer, err = os.Open(outfile, s.O_WRONLY|os.O_CREATE|
+		os.O_TRUNC, 0666); err != nil {
+	    log.Fatal(err)
+	}
+}
+
+// Handle multi-line argument lists ending in ... correctly.
+// Was issue 3130.
+func _() {
+	_ = append(s, a...)
+	_ = append(
+		s, a...)
+	_ = append(s,
+		a...)
+	_ = append(
+		s,
+		a...)
+	_ = append(s, a...,
+	)
+	_ = append(s,
+		a...,
+	)
+	_ = append(
+		s,
+		a...,
+	)
+}
+
+// Literal function types in conversions must be parenthesized;
+// for now go/parser accepts the unparenthesized form where it
+// is non-ambiguous.
+func _() {
+	// these conversions should be rewritten to look
+	// the same as the parenthesized conversions below
+	_ = func()()(nil)
+	_ = func(x int)(float)(nil)
+	_ = func() func() func()()(nil)
+
+	_ = (func()())(nil)
+	_ = (func(x int)(float))(nil)
+	_ = (func() func() func()())(nil)
+}
diff --git a/src/go/printer/testdata/expressions.raw b/src/go/printer/testdata/expressions.raw
new file mode 100644
index 000000000..97bc81dad
--- /dev/null
+++ b/src/go/printer/testdata/expressions.raw
@@ -0,0 +1,681 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package expressions
+
+type T struct {
+	x, y, z int
+}
+
+var (
+	a, b, c, d, e	int
+	under_bar	int
+	longIdentifier1, longIdentifier2, longIdentifier3	int
+	t0, t1, t2	T
+	s	string
+	p	*int
+)
+
+func _() {
+	// no spaces around simple or parenthesized expressions
+	_ = (a + 0)
+	_ = a + b
+	_ = a + b + c
+	_ = a + b - c
+	_ = a - b - c
+	_ = a + (b * c)
+	_ = a + (b / c)
+	_ = a - (b % c)
+	_ = 1 + a
+	_ = a + 1
+	_ = a + b + 1
+	_ = s[a]
+	_ = s[a:]
+	_ = s[:b]
+	_ = s[1:2]
+	_ = s[a:b]
+	_ = s[0:len(s)]
+	_ = s[0] << 1
+	_ = (s[0] << 1) & 0xf
+	_ = s[0]<<2 | s[1]>>4
+	_ = "foo" + s
+	_ = s + "foo"
+	_ = 'a' + 'b'
+	_ = len(s) / 2
+	_ = len(t0.x) / a
+
+	// spaces around expressions of different precedence or expressions containing spaces
+	_ = a + -b
+	_ = a - ^b
+	_ = a / *p
+	_ = a + b*c
+	_ = 1 + b*c
+	_ = a + 2*c
+	_ = a + c*2
+	_ = 1 + 2*3
+	_ = s[1 : 2*3]
+	_ = s[a : b-c]
+	_ = s[0:]
+	_ = s[a+b]
+	_ = s[:b-c]
+	_ = s[a+b:]
+	_ = a[a<<b+1]
+	_ = a[a<<b+1:]
+	_ = s[a+b : len(s)]
+	_ = s[len(s):-a]
+	_ = s[a : len(s)+1]
+	_ = s[a:len(s)+1] + s
+
+	// spaces around operators with equal or lower precedence than comparisons
+	_ = a == b
+	_ = a != b
+	_ = a > b
+	_ = a >= b
+	_ = a < b
+	_ = a <= b
+	_ = a < b && c > d
+	_ = a < b || c > d
+
+	// spaces around "long" operands
+	_ = a + longIdentifier1
+	_ = longIdentifier1 + a
+	_ = longIdentifier1 + longIdentifier2*longIdentifier3
+	_ = s + "a longer string"
+
+	// some selected cases
+	_ = a + t0.x
+	_ = a + t0.x + t1.x*t2.x
+	_ = a + b + c + d + e + 2*3
+	_ = a + b + c + 2*3 + d + e
+	_ = (a + b + c) * 2
+	_ = a - b + c - d + (a + b + c) + d&e
+	_ = under_bar - 1
+	_ = Open(dpath+"/file", O_WRONLY|O_CREAT, 0666)
+	_ = int(c0&_Mask4)<<18 | int(c1&_Maskx)<<12 | int(c2&_Maskx)<<6 | int(c3&_Maskx)
+
+	// the parser does not restrict expressions that may appear as statements
+	true
+	42
+	"foo"
+	x
+	(x)
+	a + b
+	a + b + c
+	a + (b * c)
+	a + (b / c)
+	1 + a
+	a + 1
+	s[a]
+	x << 1
+	(s[0] << 1) & 0xf
+	"foo" + s
+	x == y
+	x < y || z > 42
+}
+
+// slice expressions with cap
+func _() {
+	_ = x[a:b:c]
+	_ = x[a:b : c+d]
+	_ = x[a : b+d : c]
+	_ = x[a : b+d : c+d]
+	_ = x[a+d : b:c]
+	_ = x[a+d : b : c+d]
+	_ = x[a+d : b+d : c]
+	_ = x[a+d : b+d : c+d]
+
+	_ = x[:b:c]
+	_ = x[:b : c+d]
+	_ = x[:b+d : c]
+	_ = x[:b+d : c+d]
+}
+
+func _() {
+	_ = a + b
+	_ = a + b + c
+	_ = a + b*c
+	_ = a + (b * c)
+	_ = (a + b) * c
+	_ = a + (b * c * d)
+	_ = a + (b*c + d)
+
+	_ = 1 << x
+	_ = -1 << x
+	_ = 1<<x - 1
+	_ = -1<<x - 1
+
+	_ = f(a + b)
+	_ = f(a + b + c)
+	_ = f(a + b*c)
+	_ = f(a + (b * c))
+	_ = f(1<<x-1, 1<<x-2)
+
+	_ = 1<<d.logWindowSize - 1
+
+	buf = make(x, 2*cap(b.buf)+n)
+
+	dst[i*3+2] = dbuf[0] << 2
+	dst[i*3+2] = dbuf[0]<<2 | dbuf[1]>>4
+
+	b.buf = b.buf[0 : b.off+m+n]
+	b.buf = b.buf[0 : b.off+m*n]
+	f(b.buf[0 : b.off+m+n])
+
+	signed += ' ' * 8
+	tw.octal(header[148:155], chksum)
+
+	_ = x > 0 && i >= 0
+
+	x1, x0 := x>>w2, x&m2
+	z0 = t1<<w2 + t0
+	z1 = (t1 + t0>>w2) >> w2
+	q1, r1 := x1/d1, x1%d1
+	r1 = r1*b2 | x0>>w2
+	x1 = (x1 << z) | (x0 >> (uint(w) - z))
+	x1 = x1<<z | x0>>(uint(w)-z)
+
+	_ = buf[0 : len(buf)+1]
+	_ = buf[0 : n+1]
+
+	a, b = b, a
+	a = b + c
+	a = b*c + d
+	_ = a*b + c
+	_ = a - b - c
+	_ = a - (b - c)
+	_ = a - b*c
+	_ = a - (b * c)
+	_ = a * b / c
+	_ = a / *b
+	_ = x[a|^b]
+	_ = x[a / *b]
+	_ = a & ^b
+	_ = a + +b
+	_ = a - -b
+	_ = x[a*-b]
+	_ = x[a + +b]
+	_ = x ^ y ^ z
+	_ = b[a>>24] ^ b[(a>>16)&0xFF] ^ b[(a>>8)&0xFF] ^ b[a&0xFF]
+	_ = len(longVariableName) * 2
+
+	_ = token(matchType + xlength<<lengthShift + xoffset)
+}
+
+func f(x int, args ...int) {
+	f(0, args...)
+	f(1, args)
+	f(2, args[0])
+
+	// make sure syntactically legal code remains syntactically legal
+	f(3, 42 ...)	// a blank must remain between 42 and ...
+	f(4, 42....)
+	f(5, 42....)
+	f(6, 42.0...)
+	f(7, 42.0...)
+	f(8, .42...)
+	f(9, .42...)
+	f(10, 42e0...)
+	f(11, 42e0...)
+
+	_ = 42 .x	// a blank must remain between 42 and .x
+	_ = 42..x
+	_ = 42..x
+	_ = 42.0.x
+	_ = 42.0.x
+	_ = .42.x
+	_ = .42.x
+	_ = 42e0.x
+	_ = 42e0.x
+
+	// a blank must remain between the binary operator and the 2nd operand
+	_ = x / *y
+	_ = x < -1
+	_ = x < <-1
+	_ = x + +1
+	_ = x - -1
+	_ = x & &x
+	_ = x & ^x
+
+	_ = f(x / *y, x < -1, x < <-1, x + +1, x - -1, x & &x, x & ^x)
+}
+
+func _() {
+	_ = T{}
+	_ = struct{}{}
+	_ = [10]T{}
+	_ = [...]T{}
+	_ = []T{}
+	_ = map[int]T{}
+}
+
+// one-line structs/interfaces in composite literals (up to a threshold)
+func _() {
+	_ = struct{}{}
+	_ = struct{ x int }{0}
+	_ = struct{ x, y, z int }{0, 1, 2}
+	_ = struct{ int }{0}
+	_ = struct{ s struct{ int } }{struct{ int }{0}}
+}
+
+func _() {
+	// do not modify literals
+	_ = "tab1	tab2	tab3	end"	// string contains 3 tabs
+	_ = "tab1 tab2 tab3 end"	// same string with 3 blanks - may be unaligned because editors see tabs in strings
+	_ = ""	// this comment should be aligned with the one on the previous line
+	_ = ``
+	_ = `
+`
+	_ = `foo
+		bar`
+	_ = `three spaces before the end of the line starting here:   
+they must not be removed`
+}
+
+func _() {
+	// smart handling of indentation for multi-line raw strings
+	var _ = ``
+	var _ = `foo`
+	var _ = `foo
+bar`
+
+	var _ = ``
+	var _ = `foo`
+	var _ =
+	// the next line should remain indented
+	`foo
+bar`
+
+	var _ =	// comment
+	``
+	var _ =	// comment
+	`foo`
+	var _ =	// comment
+	// the next line should remain indented
+	`foo
+bar`
+
+	var _ = /* comment */ ``
+	var _ = /* comment */ `foo`
+	var _ = /* comment */ `foo
+bar`
+
+	var _ =	/* comment */
+	``
+	var _ =	/* comment */
+	`foo`
+	var _ =	/* comment */
+	// the next line should remain indented
+	`foo
+bar`
+
+	var board = []int(
+		`...........
+...........
+....●●●....
+....●●●....
+..●●●●●●●..
+..●●●○●●●..
+..●●●●●●●..
+....●●●....
+....●●●....
+...........
+...........
+`)
+
+	var state = S{
+		"foo",
+		// the next line should remain indented
+		`...........
+...........
+....●●●....
+....●●●....
+..●●●●●●●..
+..●●●○●●●..
+..●●●●●●●..
+....●●●....
+....●●●....
+...........
+...........
+`,
+		"bar",
+	}
+}
+
+func _() {
+	// one-line function literals (body is on a single line)
+	_ = func() {}
+	_ = func() int { return 0 }
+	_ = func(x, y int) bool { m := (x + y) / 2; return m < 0 }
+
+	// multi-line function literals (body is not on one line)
+	_ = func() {
+	}
+	_ = func() int {
+		return 0
+	}
+	_ = func(x, y int) bool {
+		m := (x + y) / 2
+		return x < y
+	}
+
+	f(func() {
+	})
+	f(func() int {
+		return 0
+	})
+	f(func(x, y int) bool {
+		m := (x + y) / 2
+		return x < y
+	})
+}
+
+func _() {
+	_ = [][]int{
+		[]int{1},
+		[]int{1, 2},
+		[]int{1, 2, 3},
+	}
+	_ = [][]int{
+		{1},
+		[]int{1, 2},
+		[]int{1, 2, 3},
+	}
+	_ = [][]int{
+		{1},
+		{1, 2},
+		{1, 2, 3},
+	}
+	_ = [][]int{{1}, {1, 2}, {1, 2, 3}}
+}
+
+// various multi-line expressions
+func _() {
+	// do not add extra indentation to multi-line string lists
+	_ = "foo" + "bar"
+	_ = "foo" +
+		"bar" +
+		"bah"
+	_ = []string{
+		"abc" +
+			"def",
+		"foo" +
+			"bar",
+	}
+}
+
+const _ = F1 +
+	`string = "%s";` +
+	`ptr = *;` +
+	`datafmt.T2 = s ["-" p "-"];`
+
+const _ = `datafmt "datafmt";` +
+	`default = "%v";` +
+	`array = *;` +
+	`datafmt.T3 = s  {" " a a / ","};`
+
+const _ = `datafmt "datafmt";` +
+	`default = "%v";` +
+	`array = *;` +
+	`datafmt.T3 = s  {" " a a / ","};`
+
+func _() {
+	_ = F1 +
+		`string = "%s";` +
+		`ptr = *;` +
+		`datafmt.T2 = s ["-" p "-"];`
+
+	_ =
+		`datafmt "datafmt";` +
+			`default = "%v";` +
+			`array = *;` +
+			`datafmt.T3 = s  {" " a a / ","};`
+
+	_ = `datafmt "datafmt";` +
+		`default = "%v";` +
+		`array = *;` +
+		`datafmt.T3 = s  {" " a a / ","};`
+}
+
+func _() {
+	// respect source lines in multi-line expressions
+	_ = a +
+		b +
+		c
+	_ = a < b ||
+		b < a
+	_ = "933262154439441526816992388562667004907159682643816214685929" +
+		"638952175999932299156089414639761565182862536979208272237582" +
+		"51185210916864000000000000000000000000"	// 100!
+	_ = "170141183460469231731687303715884105727"	// prime
+}
+
+// Alignment after overlong lines
+const (
+	_	= "991"
+	_	= "2432902008176640000"		// 20!
+	_	= "933262154439441526816992388562667004907159682643816214685929" +
+		"638952175999932299156089414639761565182862536979208272237582" +
+		"51185210916864000000000000000000000000"	// 100!
+	_	= "170141183460469231731687303715884105727"		// prime
+)
+
+// Correct placement of operators and comments in multi-line expressions
+func _() {
+	_ = a +	// comment
+		b +	// comment
+		c
+	_ = "a" +
+		"b" +	// comment
+		"c"
+	_ = "ba0408" + "7265717569726564"	// field 71, encoding 2, string "required"
+}
+
+// Correct placement of terminating comma/closing parentheses in multi-line calls.
+func _() {
+	f(1,
+		2,
+		3)
+	f(1,
+		2,
+		3,
+	)
+	f(1,
+		2,
+		3)	// comment
+	f(1,
+		2,
+		3,	// comment
+	)
+	f(1,
+		2,
+		3)	// comment
+	f(1,
+		2,
+		3,	// comment
+	)
+}
+
+// Align comments in multi-line lists of single-line expressions.
+var txpix = [NCOL]draw.Color{
+	draw.Yellow,	// yellow
+	draw.Cyan,	// cyan
+	draw.Green,	// lime green
+	draw.GreyBlue,	// slate
+	draw.Red,	/* red */
+	draw.GreyGreen,	/* olive green */
+	draw.Blue,	/* blue */
+	draw.Color(0xFF55AAFF),	/* pink */
+	draw.Color(0xFFAAFFFF),	/* lavender */
+	draw.Color(0xBB005DFF),	/* maroon */
+}
+
+func same(t, u *Time) bool {
+	// respect source lines in multi-line expressions
+	return t.Year == u.Year &&
+		t.Month == u.Month &&
+		t.Day == u.Day &&
+		t.Hour == u.Hour &&
+		t.Minute == u.Minute &&
+		t.Second == u.Second &&
+		t.Weekday == u.Weekday &&
+		t.ZoneOffset == u.ZoneOffset &&
+		t.Zone == u.Zone
+}
+
+func (p *parser) charClass() {
+	// respect source lines in multi-line expressions
+	if cc.negate && len(cc.ranges) == 2 &&
+		cc.ranges[0] == '\n' && cc.ranges[1] == '\n' {
+		nl := new(_NotNl)
+		p.re.add(nl)
+	}
+}
+
+func addState(s []state, inst instr, match []int) {
+	// handle comments correctly in multi-line expressions
+	for i := 0; i < l; i++ {
+		if s[i].inst.index() == index &&	// same instruction
+			s[i].match[0] < pos {	// earlier match already going; leftmost wins
+			return s
+		}
+	}
+}
+
+func (self *T) foo(x int) *T	{ return self }
+
+func _()	{ module.Func1().Func2() }
+
+func _() {
+	_ = new(T).
+		foo(1).
+		foo(2).
+		foo(3)
+
+	_ = new(T).
+		foo(1).
+		foo(2).	// inline comments
+		foo(3)
+
+	_ = new(T).foo(1).foo(2).foo(3)
+
+	// handle multiline argument list correctly
+	_ = new(T).
+		foo(
+		1).
+		foo(2)
+
+	_ = new(T).foo(
+		1).foo(2)
+
+	_ = Array[3+
+		4]
+
+	_ = Method(1, 2,
+		3)
+
+	_ = new(T).
+		foo().
+		bar().(*Type)
+
+	_ = new(T).
+		foo().
+		bar().(*Type).
+		baz()
+
+	_ = new(T).
+		foo().
+		bar()["idx"]
+
+	_ = new(T).
+		foo().
+		bar()["idx"].
+		baz()
+
+	_ = new(T).
+		foo().
+		bar()[1:2]
+
+	_ = new(T).
+		foo().
+		bar()[1:2].
+		baz()
+
+	_ = new(T).
+		Field.
+		Array[3+
+		4].
+		Table["foo"].
+		Blob.(*Type).
+		Slices[1:4].
+		Method(1, 2,
+		3).
+		Thingy
+
+	_ = a.b.c
+	_ = a.
+		b.
+		c
+	_ = a.b().c
+	_ = a.
+		b().
+		c
+	_ = a.b[0].c
+	_ = a.
+		b[0].
+		c
+	_ = a.b[0:].c
+	_ = a.
+		b[0:].
+		c
+	_ = a.b.(T).c
+	_ = a.
+		b.(T).
+		c
+}
+
+// Don't introduce extra newlines in strangely formatted expression lists.
+func f() {
+	// os.Open parameters should remain on two lines
+	if writer, err = os.Open(outfile, s.O_WRONLY|os.O_CREATE|
+		os.O_TRUNC, 0666); err != nil {
+		log.Fatal(err)
+	}
+}
+
+// Handle multi-line argument lists ending in ... correctly.
+// Was issue 3130.
+func _() {
+	_ = append(s, a...)
+	_ = append(
+		s, a...)
+	_ = append(s,
+		a...)
+	_ = append(
+		s,
+		a...)
+	_ = append(s, a...,
+	)
+	_ = append(s,
+		a...,
+	)
+	_ = append(
+		s,
+		a...,
+	)
+}
+
+// Literal function types in conversions must be parenthesized;
+// for now go/parser accepts the unparenthesized form where it
+// is non-ambiguous.
+func _() {
+	// these conversions should be rewritten to look
+	// the same as the parenthesized conversions below
+	_ = (func())(nil)
+	_ = (func(x int) float)(nil)
+	_ = (func() func() func())(nil)
+
+	_ = (func())(nil)
+	_ = (func(x int) float)(nil)
+	_ = (func() func() func())(nil)
+}
diff --git a/src/go/printer/testdata/linebreaks.golden b/src/go/printer/testdata/linebreaks.golden
new file mode 100644
index 000000000..006cf1718
--- /dev/null
+++ b/src/go/printer/testdata/linebreaks.golden
@@ -0,0 +1,275 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package linebreaks
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+type writerTestEntry struct {
+	header		*Header
+	contents	string
+}
+
+type writerTest struct {
+	file	string	// filename of expected output
+	entries	[]*writerTestEntry
+}
+
+var writerTests = []*writerTest{
+	&writerTest{
+		file:	"testdata/writer.tar",
+		entries: []*writerTestEntry{
+			&writerTestEntry{
+				header: &Header{
+					Name:		"small.txt",
+					Mode:		0640,
+					Uid:		73025,
+					Gid:		5000,
+					Size:		5,
+					Mtime:		1246508266,
+					Typeflag:	'0',
+					Uname:		"dsymonds",
+					Gname:		"eng",
+				},
+				contents:	"Kilts",
+			},
+			&writerTestEntry{
+				header: &Header{
+					Name:		"small2.txt",
+					Mode:		0640,
+					Uid:		73025,
+					Gid:		5000,
+					Size:		11,
+					Mtime:		1245217492,
+					Typeflag:	'0',
+					Uname:		"dsymonds",
+					Gname:		"eng",
+				},
+				contents:	"Google.com\n",
+			},
+		},
+	},
+	// The truncated test file was produced using these commands:
+	//   dd if=/dev/zero bs=1048576 count=16384 > /tmp/16gig.txt
+	//   tar -b 1 -c -f- /tmp/16gig.txt | dd bs=512 count=8 > writer-big.tar
+	&writerTest{
+		file:	"testdata/writer-big.tar",
+		entries: []*writerTestEntry{
+			&writerTestEntry{
+				header: &Header{
+					Name:		"tmp/16gig.txt",
+					Mode:		0640,
+					Uid:		73025,
+					Gid:		5000,
+					Size:		16 << 30,
+					Mtime:		1254699560,
+					Typeflag:	'0',
+					Uname:		"dsymonds",
+					Gname:		"eng",
+				},
+				// no contents
+			},
+		},
+	},
+}
+
+type untarTest struct {
+	file	string
+	headers	[]*Header
+}
+
+var untarTests = []*untarTest{
+	&untarTest{
+		file:	"testdata/gnu.tar",
+		headers: []*Header{
+			&Header{
+				Name:		"small.txt",
+				Mode:		0640,
+				Uid:		73025,
+				Gid:		5000,
+				Size:		5,
+				Mtime:		1244428340,
+				Typeflag:	'0',
+				Uname:		"dsymonds",
+				Gname:		"eng",
+			},
+			&Header{
+				Name:		"small2.txt",
+				Mode:		0640,
+				Uid:		73025,
+				Gid:		5000,
+				Size:		11,
+				Mtime:		1244436044,
+				Typeflag:	'0',
+				Uname:		"dsymonds",
+				Gname:		"eng",
+			},
+		},
+	},
+	&untarTest{
+		file:	"testdata/star.tar",
+		headers: []*Header{
+			&Header{
+				Name:		"small.txt",
+				Mode:		0640,
+				Uid:		73025,
+				Gid:		5000,
+				Size:		5,
+				Mtime:		1244592783,
+				Typeflag:	'0',
+				Uname:		"dsymonds",
+				Gname:		"eng",
+				Atime:		1244592783,
+				Ctime:		1244592783,
+			},
+			&Header{
+				Name:		"small2.txt",
+				Mode:		0640,
+				Uid:		73025,
+				Gid:		5000,
+				Size:		11,
+				Mtime:		1244592783,
+				Typeflag:	'0',
+				Uname:		"dsymonds",
+				Gname:		"eng",
+				Atime:		1244592783,
+				Ctime:		1244592783,
+			},
+		},
+	},
+	&untarTest{
+		file:	"testdata/v7.tar",
+		headers: []*Header{
+			&Header{
+				Name:		"small.txt",
+				Mode:		0444,
+				Uid:		73025,
+				Gid:		5000,
+				Size:		5,
+				Mtime:		1244593104,
+				Typeflag:	'\x00',
+			},
+			&Header{
+				Name:		"small2.txt",
+				Mode:		0444,
+				Uid:		73025,
+				Gid:		5000,
+				Size:		11,
+				Mtime:		1244593104,
+				Typeflag:	'\x00',
+			},
+		},
+	},
+}
+
+var facts = map[int]string{
+	0:	"1",
+	1:	"1",
+	2:	"2",
+	10:	"3628800",
+	20:	"2432902008176640000",
+	100: "933262154439441526816992388562667004907159682643816214685929" +
+		"638952175999932299156089414639761565182862536979208272237582" +
+		"51185210916864000000000000000000000000",
+}
+
+func usage() {
+	fmt.Fprintf(os.Stderr,
+		// TODO(gri): the 2nd string of this string list should not be indented
+		"usage: godoc package [name ...]\n"+
+			"	godoc -http=:6060\n")
+	flag.PrintDefaults()
+	os.Exit(2)
+}
+
+func TestReader(t *testing.T) {
+testLoop:
+	for i, test := range untarTests {
+		f, err := os.Open(test.file, os.O_RDONLY, 0444)
+		if err != nil {
+			t.Errorf("test %d: Unexpected error: %v", i, err)
+			continue
+		}
+		tr := NewReader(f)
+		for j, header := range test.headers {
+			hdr, err := tr.Next()
+			if err != nil || hdr == nil {
+				t.Errorf("test %d, entry %d: Didn't get entry: %v", i, j, err)
+				f.Close()
+				continue testLoop
+			}
+			if !reflect.DeepEqual(hdr, header) {
+				t.Errorf("test %d, entry %d: Incorrect header:\nhave %+v\nwant %+v",
+					i, j, *hdr, *header)
+			}
+		}
+		hdr, err := tr.Next()
+		if hdr != nil || err != nil {
+			t.Errorf("test %d: Unexpected entry or error: hdr=%v err=%v", i, err)
+		}
+		f.Close()
+	}
+}
+
+// Respect line breaks in function calls.
+func _() {
+	f(x)
+	f(x,
+		x)
+	f(x,
+		x,
+	)
+	f(
+		x,
+		x)
+	f(
+		x,
+		x,
+	)
+}
+
+// Respect line breaks in function declarations.
+func _(x T)	{}
+func _(x T,
+	y T) {
+}
+func _(x T,
+	y T,
+) {
+}
+func _(
+	x T,
+	y T) {
+}
+func _(
+	x T,
+	y T,
+) {
+}
+
+// Example from issue 2597.
+func ManageStatus0(
+	in <-chan *Status,
+	req <-chan Request,
+	stat chan<- *TargetInfo,
+	TargetHistorySize int) {
+}
+
+func ManageStatus1(
+	in <-chan *Status,
+	req <-chan Request,
+	stat chan<- *TargetInfo,
+	TargetHistorySize int,
+) {
+}
+
+// There should be exactly one linebreak after this comment.
diff --git a/src/go/printer/testdata/linebreaks.input b/src/go/printer/testdata/linebreaks.input
new file mode 100644
index 000000000..e782bb044
--- /dev/null
+++ b/src/go/printer/testdata/linebreaks.input
@@ -0,0 +1,271 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package linebreaks
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+type writerTestEntry struct {
+	header *Header
+	contents string
+}
+
+type writerTest struct {
+	file string  // filename of expected output
+	entries []*writerTestEntry
+}
+
+var writerTests = []*writerTest{
+	&writerTest{
+		file: "testdata/writer.tar",
+		entries: []*writerTestEntry{
+			&writerTestEntry{
+				header: &Header{
+					Name: "small.txt",
+					Mode: 0640,
+					Uid: 73025,
+					Gid: 5000,
+					Size: 5,
+					Mtime: 1246508266,
+					Typeflag: '0',
+					Uname: "dsymonds",
+					Gname: "eng",
+				},
+				contents: "Kilts",
+			},
+			&writerTestEntry{
+				header: &Header{
+					Name: "small2.txt",
+					Mode: 0640,
+					Uid: 73025,
+					Gid: 5000,
+					Size: 11,
+					Mtime: 1245217492,
+					Typeflag: '0',
+					Uname: "dsymonds",
+					Gname: "eng",
+				},
+				contents: "Google.com\n",
+			},
+		},
+	},
+	// The truncated test file was produced using these commands:
+	//   dd if=/dev/zero bs=1048576 count=16384 > /tmp/16gig.txt
+	//   tar -b 1 -c -f- /tmp/16gig.txt | dd bs=512 count=8 > writer-big.tar
+	&writerTest{
+		file: "testdata/writer-big.tar",
+		entries: []*writerTestEntry{
+			&writerTestEntry{
+				header: &Header{
+					Name: "tmp/16gig.txt",
+					Mode: 0640,
+					Uid: 73025,
+					Gid: 5000,
+					Size: 16 << 30,
+					Mtime: 1254699560,
+					Typeflag: '0',
+					Uname: "dsymonds",
+					Gname: "eng",
+				},
+				// no contents
+			},
+		},
+	},
+}
+
+type untarTest struct {
+	file string
+	headers []*Header
+}
+
+var untarTests = []*untarTest{
+	&untarTest{
+		file: "testdata/gnu.tar",
+		headers: []*Header{
+			&Header{
+				Name: "small.txt",
+				Mode: 0640,
+				Uid: 73025,
+				Gid: 5000,
+				Size: 5,
+				Mtime: 1244428340,
+				Typeflag: '0',
+				Uname: "dsymonds",
+				Gname: "eng",
+			},
+			&Header{
+				Name: "small2.txt",
+				Mode: 0640,
+				Uid: 73025,
+				Gid: 5000,
+				Size: 11,
+				Mtime: 1244436044,
+				Typeflag: '0',
+				Uname: "dsymonds",
+				Gname: "eng",
+			},
+		},
+	},
+	&untarTest{
+		file: "testdata/star.tar",
+		headers: []*Header{
+			&Header{
+				Name: "small.txt",
+				Mode: 0640,
+				Uid: 73025,
+				Gid: 5000,
+				Size: 5,
+				Mtime: 1244592783,
+				Typeflag: '0',
+				Uname: "dsymonds",
+				Gname: "eng",
+				Atime: 1244592783,
+				Ctime: 1244592783,
+			},
+			&Header{
+				Name: "small2.txt",
+				Mode: 0640,
+				Uid: 73025,
+				Gid: 5000,
+				Size: 11,
+				Mtime: 1244592783,
+				Typeflag: '0',
+				Uname: "dsymonds",
+				Gname: "eng",
+				Atime: 1244592783,
+				Ctime: 1244592783,
+			},
+		},
+	},
+	&untarTest{
+		file: "testdata/v7.tar",
+		headers: []*Header{
+			&Header{
+				Name: "small.txt",
+				Mode: 0444,
+				Uid: 73025,
+				Gid: 5000,
+				Size: 5,
+				Mtime: 1244593104,
+				Typeflag: '\x00',
+			},
+			&Header{
+				Name: "small2.txt",
+				Mode: 0444,
+				Uid: 73025,
+				Gid: 5000,
+				Size: 11,
+				Mtime: 1244593104,
+				Typeflag: '\x00',
+			},
+		},
+	},
+}
+
+var facts = map[int] string {
+	0: "1",
+	1: "1",
+	2: "2",
+	10: "3628800",
+	20: "2432902008176640000",
+	100: "933262154439441526816992388562667004907159682643816214685929" +
+		"638952175999932299156089414639761565182862536979208272237582" +
+		"51185210916864000000000000000000000000",
+}
+
+func usage() {
+	fmt.Fprintf(os.Stderr,
+		// TODO(gri): the 2nd string of this string list should not be indented
+		"usage: godoc package [name ...]\n" +
+		"	godoc -http=:6060\n")
+	flag.PrintDefaults()
+	os.Exit(2)
+}
+
+func TestReader(t *testing.T) {
+testLoop:
+	for i, test := range untarTests {
+		f, err := os.Open(test.file, os.O_RDONLY, 0444)
+		if err != nil {
+			t.Errorf("test %d: Unexpected error: %v", i, err)
+			continue
+		}
+		tr := NewReader(f)
+		for j, header := range test.headers {
+			hdr, err := tr.Next()
+			if err != nil || hdr == nil {
+				t.Errorf("test %d, entry %d: Didn't get entry: %v", i, j, err)
+				f.Close()
+				continue testLoop
+			}
+			if !reflect.DeepEqual(hdr, header) {
+				t.Errorf("test %d, entry %d: Incorrect header:\nhave %+v\nwant %+v",
+					 i, j, *hdr, *header)
+			}
+		}
+		hdr, err := tr.Next()
+		if hdr != nil || err != nil {
+			t.Errorf("test %d: Unexpected entry or error: hdr=%v err=%v", i, err)
+		}
+		f.Close()
+	}
+}
+
+// Respect line breaks in function calls.
+func _() {
+	f(x)
+	f(x,
+	  x)
+	f(x,
+	  x,
+	)
+	f(
+	  x,
+	  x)
+	f(
+	  x,
+	  x,
+	)
+}
+
+// Respect line breaks in function declarations.
+func _(x T) {}
+func _(x T,
+       y T) {}
+func _(x T,
+       y T,
+) {}
+func _(
+       x T,
+       y T) {}
+func _(
+       x T,
+       y T,
+) {}
+
+// Example from issue 2597.
+func ManageStatus0(
+	in <-chan *Status,
+	req <-chan Request,
+	stat chan<- *TargetInfo,
+	TargetHistorySize int) {
+}
+    
+func ManageStatus1(
+	in <-chan *Status,
+	req <-chan Request,
+	stat chan<- *TargetInfo,
+	TargetHistorySize int,
+) {
+}
+    
+// There should be exactly one linebreak after this comment.
diff --git a/src/go/printer/testdata/parser.go b/src/go/printer/testdata/parser.go
new file mode 100644
index 000000000..dba8bbd43
--- /dev/null
+++ b/src/go/printer/testdata/parser.go
@@ -0,0 +1,2153 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package parser implements a parser for Go source files. Input may be
+// provided in a variety of forms (see the various Parse* functions); the
+// output is an abstract syntax tree (AST) representing the Go source. The
+// parser is invoked through one of the Parse* functions.
+
+package parser
+
+import (
+	"fmt"
+	"go/ast"
+	"go/scanner"
+	"go/token"
+)
+
+// The mode parameter to the Parse* functions is a set of flags (or 0).
+// They control the amount of source code parsed and other optional
+// parser functionality.
+//
+const (
+	PackageClauseOnly uint = 1 << iota // parsing stops after package clause
+	ImportsOnly                        // parsing stops after import declarations
+	ParseComments                      // parse comments and add them to AST
+	Trace                              // print a trace of parsed productions
+	DeclarationErrors                  // report declaration errors
+)
+
+// The parser structure holds the parser's internal state.
+type parser struct {
+	file *token.File
+	scanner.ErrorVector
+	scanner scanner.Scanner
+
+	// Tracing/debugging
+	mode   uint // parsing mode
+	trace  bool // == (mode & Trace != 0)
+	indent uint // indentation used for tracing output
+
+	// Comments
+	comments    []*ast.CommentGroup
+	leadComment *ast.CommentGroup // last lead comment
+	lineComment *ast.CommentGroup // last line comment
+
+	// Next token
+	pos token.Pos   // token position
+	tok token.Token // one token look-ahead
+	lit string      // token literal
+
+	// Non-syntactic parser control
+	exprLev int // < 0: in control clause, >= 0: in expression
+
+	// Ordinary identifier scopes
+	pkgScope   *ast.Scope        // pkgScope.Outer == nil
+	topScope   *ast.Scope        // top-most scope; may be pkgScope
+	unresolved []*ast.Ident      // unresolved identifiers
+	imports    []*ast.ImportSpec // list of imports
+
+	// Label scope
+	// (maintained by open/close LabelScope)
+	labelScope  *ast.Scope     // label scope for current function
+	targetStack [][]*ast.Ident // stack of unresolved labels
+}
+
+// scannerMode returns the scanner mode bits given the parser's mode bits.
+func scannerMode(mode uint) uint {
+	var m uint = scanner.InsertSemis
+	if mode&ParseComments != 0 {
+		m |= scanner.ScanComments
+	}
+	return m
+}
+
+func (p *parser) init(fset *token.FileSet, filename string, src []byte, mode uint) {
+	p.file = fset.AddFile(filename, fset.Base(), len(src))
+	p.scanner.Init(p.file, src, p, scannerMode(mode))
+
+	p.mode = mode
+	p.trace = mode&Trace != 0 // for convenience (p.trace is used frequently)
+
+	p.next()
+
+	// set up the pkgScope here (as opposed to in parseFile) because
+	// there are other parser entry points (ParseExpr, etc.)
+	p.openScope()
+	p.pkgScope = p.topScope
+
+	// for the same reason, set up a label scope
+	p.openLabelScope()
+}
+
+// ----------------------------------------------------------------------------
+// Scoping support
+
+func (p *parser) openScope() {
+	p.topScope = ast.NewScope(p.topScope)
+}
+
+func (p *parser) closeScope() {
+	p.topScope = p.topScope.Outer
+}
+
+func (p *parser) openLabelScope() {
+	p.labelScope = ast.NewScope(p.labelScope)
+	p.targetStack = append(p.targetStack, nil)
+}
+
+func (p *parser) closeLabelScope() {
+	// resolve labels
+	n := len(p.targetStack) - 1
+	scope := p.labelScope
+	for _, ident := range p.targetStack[n] {
+		ident.Obj = scope.Lookup(ident.Name)
+		if ident.Obj == nil && p.mode&DeclarationErrors != 0 {
+			p.error(ident.Pos(), fmt.Sprintf("label %s undefined", ident.Name))
+		}
+	}
+	// pop label scope
+	p.targetStack = p.targetStack[0:n]
+	p.labelScope = p.labelScope.Outer
+}
+
+func (p *parser) declare(decl interface{}, scope *ast.Scope, kind ast.ObjKind, idents ...*ast.Ident) {
+	for _, ident := range idents {
+		assert(ident.Obj == nil, "identifier already declared or resolved")
+		if ident.Name != "_" {
+			obj := ast.NewObj(kind, ident.Name)
+			// remember the corresponding declaration for redeclaration
+			// errors and global variable resolution/typechecking phase
+			obj.Decl = decl
+			if alt := scope.Insert(obj); alt != nil && p.mode&DeclarationErrors != 0 {
+				prevDecl := ""
+				if pos := alt.Pos(); pos.IsValid() {
+					prevDecl = fmt.Sprintf("\n\tprevious declaration at %s", p.file.Position(pos))
+				}
+				p.error(ident.Pos(), fmt.Sprintf("%s redeclared in this block%s", ident.Name, prevDecl))
+			}
+			ident.Obj = obj
+		}
+	}
+}
+
+func (p *parser) shortVarDecl(idents []*ast.Ident) {
+	// Go spec: A short variable declaration may redeclare variables
+	// provided they were originally declared in the same block with
+	// the same type, and at least one of the non-blank variables is new.
+	n := 0 // number of new variables
+	for _, ident := range idents {
+		assert(ident.Obj == nil, "identifier already declared or resolved")
+		if ident.Name != "_" {
+			obj := ast.NewObj(ast.Var, ident.Name)
+			// short var declarations cannot have redeclaration errors
+			// and are not global => no need to remember the respective
+			// declaration
+			alt := p.topScope.Insert(obj)
+			if alt == nil {
+				n++ // new declaration
+				alt = obj
+			}
+			ident.Obj = alt
+		}
+	}
+	if n == 0 && p.mode&DeclarationErrors != 0 {
+		p.error(idents[0].Pos(), "no new variables on left side of :=")
+	}
+}
+
+// The unresolved object is a sentinel to mark identifiers that have been added
+// to the list of unresolved identifiers. The sentinel is only used for verifying
+// internal consistency.
+var unresolved = new(ast.Object)
+
+func (p *parser) resolve(x ast.Expr) {
+	// nothing to do if x is not an identifier or the blank identifier
+	ident, _ := x.(*ast.Ident)
+	if ident == nil {
+		return
+	}
+	assert(ident.Obj == nil, "identifier already declared or resolved")
+	if ident.Name == "_" {
+		return
+	}
+	// try to resolve the identifier
+	for s := p.topScope; s != nil; s = s.Outer {
+		if obj := s.Lookup(ident.Name); obj != nil {
+			ident.Obj = obj
+			return
+		}
+	}
+	// all local scopes are known, so any unresolved identifier
+	// must be found either in the file scope, package scope
+	// (perhaps in another file), or universe scope --- collect
+	// them so that they can be resolved later
+	ident.Obj = unresolved
+	p.unresolved = append(p.unresolved, ident)
+}
+
+// ----------------------------------------------------------------------------
+// Parsing support
+
+func (p *parser) printTrace(a ...interface{}) {
+	const dots = ". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . " +
+		". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "
+	const n = uint(len(dots))
+	pos := p.file.Position(p.pos)
+	fmt.Printf("%5d:%3d: ", pos.Line, pos.Column)
+	i := 2 * p.indent
+	for ; i > n; i -= n {
+		fmt.Print(dots)
+	}
+	fmt.Print(dots[0:i])
+	fmt.Println(a...)
+}
+
+func trace(p *parser, msg string) *parser {
+	p.printTrace(msg, "(")
+	p.indent++
+	return p
+}
+
+// Usage pattern: defer un(trace(p, "..."));
+func un(p *parser) {
+	p.indent--
+	p.printTrace(")")
+}
+
+// Advance to the next token.
+func (p *parser) next0() {
+	// Because of one-token look-ahead, print the previous token
+	// when tracing as it provides a more readable output. The
+	// very first token (!p.pos.IsValid()) is not initialized
+	// (it is token.ILLEGAL), so don't print it .
+	if p.trace && p.pos.IsValid() {
+		s := p.tok.String()
+		switch {
+		case p.tok.IsLiteral():
+			p.printTrace(s, p.lit)
+		case p.tok.IsOperator(), p.tok.IsKeyword():
+			p.printTrace("\"" + s + "\"")
+		default:
+			p.printTrace(s)
+		}
+	}
+
+	p.pos, p.tok, p.lit = p.scanner.Scan()
+}
+
+// Consume a comment and return it and the line on which it ends.
+func (p *parser) consumeComment() (comment *ast.Comment, endline int) {
+	// /*-style comments may end on a different line than where they start.
+	// Scan the comment for '\n' chars and adjust endline accordingly.
+	endline = p.file.Line(p.pos)
+	if p.lit[1] == '*' {
+		// don't use range here - no need to decode Unicode code points
+		for i := 0; i < len(p.lit); i++ {
+			if p.lit[i] == '\n' {
+				endline++
+			}
+		}
+	}
+
+	comment = &ast.Comment{p.pos, p.lit}
+	p.next0()
+
+	return
+}
+
+// Consume a group of adjacent comments, add it to the parser's
+// comments list, and return it together with the line at which
+// the last comment in the group ends. An empty line or non-comment
+// token terminates a comment group.
+//
+func (p *parser) consumeCommentGroup() (comments *ast.CommentGroup, endline int) {
+	var list []*ast.Comment
+	endline = p.file.Line(p.pos)
+	for p.tok == token.COMMENT && endline+1 >= p.file.Line(p.pos) {
+		var comment *ast.Comment
+		comment, endline = p.consumeComment()
+		list = append(list, comment)
+	}
+
+	// add comment group to the comments list
+	comments = &ast.CommentGroup{list}
+	p.comments = append(p.comments, comments)
+
+	return
+}
+
+// Advance to the next non-comment token. In the process, collect
+// any comment groups encountered, and remember the last lead and
+// and line comments.
+//
+// A lead comment is a comment group that starts and ends in a
+// line without any other tokens and that is followed by a non-comment
+// token on the line immediately after the comment group.
+//
+// A line comment is a comment group that follows a non-comment
+// token on the same line, and that has no tokens after it on the line
+// where it ends.
+//
+// Lead and line comments may be considered documentation that is
+// stored in the AST.
+//
+func (p *parser) next() {
+	p.leadComment = nil
+	p.lineComment = nil
+	line := p.file.Line(p.pos) // current line
+	p.next0()
+
+	if p.tok == token.COMMENT {
+		var comment *ast.CommentGroup
+		var endline int
+
+		if p.file.Line(p.pos) == line {
+			// The comment is on same line as the previous token; it
+			// cannot be a lead comment but may be a line comment.
+			comment, endline = p.consumeCommentGroup()
+			if p.file.Line(p.pos) != endline {
+				// The next token is on a different line, thus
+				// the last comment group is a line comment.
+				p.lineComment = comment
+			}
+		}
+
+		// consume successor comments, if any
+		endline = -1
+		for p.tok == token.COMMENT {
+			comment, endline = p.consumeCommentGroup()
+		}
+
+		if endline+1 == p.file.Line(p.pos) {
+			// The next token is following on the line immediately after the
+			// comment group, thus the last comment group is a lead comment.
+			p.leadComment = comment
+		}
+	}
+}
+
+func (p *parser) error(pos token.Pos, msg string) {
+	p.Error(p.file.Position(pos), msg)
+}
+
+func (p *parser) errorExpected(pos token.Pos, msg string) {
+	msg = "expected " + msg
+	if pos == p.pos {
+		// the error happened at the current position;
+		// make the error message more specific
+		if p.tok == token.SEMICOLON && p.lit[0] == '\n' {
+			msg += ", found newline"
+		} else {
+			msg += ", found '" + p.tok.String() + "'"
+			if p.tok.IsLiteral() {
+				msg += " " + p.lit
+			}
+		}
+	}
+	p.error(pos, msg)
+}
+
+func (p *parser) expect(tok token.Token) token.Pos {
+	pos := p.pos
+	if p.tok != tok {
+		p.errorExpected(pos, "'"+tok.String()+"'")
+	}
+	p.next() // make progress
+	return pos
+}
+
+func (p *parser) expectSemi() {
+	if p.tok != token.RPAREN && p.tok != token.RBRACE {
+		p.expect(token.SEMICOLON)
+	}
+}
+
+func assert(cond bool, msg string) {
+	if !cond {
+		panic("go/parser internal error: " + msg)
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Identifiers
+
+func (p *parser) parseIdent() *ast.Ident {
+	pos := p.pos
+	name := "_"
+	if p.tok == token.IDENT {
+		name = p.lit
+		p.next()
+	} else {
+		p.expect(token.IDENT) // use expect() error handling
+	}
+	return &ast.Ident{pos, name, nil}
+}
+
+func (p *parser) parseIdentList() (list []*ast.Ident) {
+	if p.trace {
+		defer un(trace(p, "IdentList"))
+	}
+
+	list = append(list, p.parseIdent())
+	for p.tok == token.COMMA {
+		p.next()
+		list = append(list, p.parseIdent())
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Common productions
+
+// If lhs is set, result list elements which are identifiers are not resolved.
+func (p *parser) parseExprList(lhs bool) (list []ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "ExpressionList"))
+	}
+
+	list = append(list, p.parseExpr(lhs))
+	for p.tok == token.COMMA {
+		p.next()
+		list = append(list, p.parseExpr(lhs))
+	}
+
+	return
+}
+
+func (p *parser) parseLhsList() []ast.Expr {
+	list := p.parseExprList(true)
+	switch p.tok {
+	case token.DEFINE:
+		// lhs of a short variable declaration
+		p.shortVarDecl(p.makeIdentList(list))
+	case token.COLON:
+		// lhs of a label declaration or a communication clause of a select
+		// statement (parseLhsList is not called when parsing the case clause
+		// of a switch statement):
+		// - labels are declared by the caller of parseLhsList
+		// - for communication clauses, if there is a stand-alone identifier
+		//   followed by a colon, we have a syntax error; there is no need
+		//   to resolve the identifier in that case
+	default:
+		// identifiers must be declared elsewhere
+		for _, x := range list {
+			p.resolve(x)
+		}
+	}
+	return list
+}
+
+func (p *parser) parseRhsList() []ast.Expr {
+	return p.parseExprList(false)
+}
+
+// ----------------------------------------------------------------------------
+// Types
+
+func (p *parser) parseType() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Type"))
+	}
+
+	typ := p.tryType()
+
+	if typ == nil {
+		pos := p.pos
+		p.errorExpected(pos, "type")
+		p.next() // make progress
+		return &ast.BadExpr{pos, p.pos}
+	}
+
+	return typ
+}
+
+// If the result is an identifier, it is not resolved.
+func (p *parser) parseTypeName() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "TypeName"))
+	}
+
+	ident := p.parseIdent()
+	// don't resolve ident yet - it may be a parameter or field name
+
+	if p.tok == token.PERIOD {
+		// ident is a package name
+		p.next()
+		p.resolve(ident)
+		sel := p.parseIdent()
+		return &ast.SelectorExpr{ident, sel}
+	}
+
+	return ident
+}
+
+func (p *parser) parseArrayType(ellipsisOk bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "ArrayType"))
+	}
+
+	lbrack := p.expect(token.LBRACK)
+	var len ast.Expr
+	if ellipsisOk && p.tok == token.ELLIPSIS {
+		len = &ast.Ellipsis{p.pos, nil}
+		p.next()
+	} else if p.tok != token.RBRACK {
+		len = p.parseRhs()
+	}
+	p.expect(token.RBRACK)
+	elt := p.parseType()
+
+	return &ast.ArrayType{lbrack, len, elt}
+}
+
+func (p *parser) makeIdentList(list []ast.Expr) []*ast.Ident {
+	idents := make([]*ast.Ident, len(list))
+	for i, x := range list {
+		ident, isIdent := x.(*ast.Ident)
+		if !isIdent {
+			pos := x.(ast.Expr).Pos()
+			p.errorExpected(pos, "identifier")
+			ident = &ast.Ident{pos, "_", nil}
+		}
+		idents[i] = ident
+	}
+	return idents
+}
+
+func (p *parser) parseFieldDecl(scope *ast.Scope) *ast.Field {
+	if p.trace {
+		defer un(trace(p, "FieldDecl"))
+	}
+
+	doc := p.leadComment
+
+	// fields
+	list, typ := p.parseVarList(false)
+
+	// optional tag
+	var tag *ast.BasicLit
+	if p.tok == token.STRING {
+		tag = &ast.BasicLit{p.pos, p.tok, p.lit}
+		p.next()
+	}
+
+	// analyze case
+	var idents []*ast.Ident
+	if typ != nil {
+		// IdentifierList Type
+		idents = p.makeIdentList(list)
+	} else {
+		// ["*"] TypeName (AnonymousField)
+		typ = list[0] // we always have at least one element
+		p.resolve(typ)
+		if n := len(list); n > 1 || !isTypeName(deref(typ)) {
+			pos := typ.Pos()
+			p.errorExpected(pos, "anonymous field")
+			typ = &ast.BadExpr{pos, list[n-1].End()}
+		}
+	}
+
+	p.expectSemi() // call before accessing p.linecomment
+
+	field := &ast.Field{doc, idents, typ, tag, p.lineComment}
+	p.declare(field, scope, ast.Var, idents...)
+
+	return field
+}
+
+func (p *parser) parseStructType() *ast.StructType {
+	if p.trace {
+		defer un(trace(p, "StructType"))
+	}
+
+	pos := p.expect(token.STRUCT)
+	lbrace := p.expect(token.LBRACE)
+	scope := ast.NewScope(nil) // struct scope
+	var list []*ast.Field
+	for p.tok == token.IDENT || p.tok == token.MUL || p.tok == token.LPAREN {
+		// a field declaration cannot start with a '(' but we accept
+		// it here for more robust parsing and better error messages
+		// (parseFieldDecl will check and complain if necessary)
+		list = append(list, p.parseFieldDecl(scope))
+	}
+	rbrace := p.expect(token.RBRACE)
+
+	// TODO(gri): store struct scope in AST
+	return &ast.StructType{pos, &ast.FieldList{lbrace, list, rbrace}, false}
+}
+
+func (p *parser) parsePointerType() *ast.StarExpr {
+	if p.trace {
+		defer un(trace(p, "PointerType"))
+	}
+
+	star := p.expect(token.MUL)
+	base := p.parseType()
+
+	return &ast.StarExpr{star, base}
+}
+
+func (p *parser) tryVarType(isParam bool) ast.Expr {
+	if isParam && p.tok == token.ELLIPSIS {
+		pos := p.pos
+		p.next()
+		typ := p.tryIdentOrType(isParam) // don't use parseType so we can provide better error message
+		if typ == nil {
+			p.error(pos, "'...' parameter is missing type")
+			typ = &ast.BadExpr{pos, p.pos}
+		}
+		if p.tok != token.RPAREN {
+			p.error(pos, "can use '...' with last parameter type only")
+		}
+		return &ast.Ellipsis{pos, typ}
+	}
+	return p.tryIdentOrType(false)
+}
+
+func (p *parser) parseVarType(isParam bool) ast.Expr {
+	typ := p.tryVarType(isParam)
+	if typ == nil {
+		pos := p.pos
+		p.errorExpected(pos, "type")
+		p.next() // make progress
+		typ = &ast.BadExpr{pos, p.pos}
+	}
+	return typ
+}
+
+func (p *parser) parseVarList(isParam bool) (list []ast.Expr, typ ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "VarList"))
+	}
+
+	// a list of identifiers looks like a list of type names
+	for {
+		// parseVarType accepts any type (including parenthesized ones)
+		// even though the syntax does not permit them here: we
+		// accept them all for more robust parsing and complain
+		// afterwards
+		list = append(list, p.parseVarType(isParam))
+		if p.tok != token.COMMA {
+			break
+		}
+		p.next()
+	}
+
+	// if we had a list of identifiers, it must be followed by a type
+	typ = p.tryVarType(isParam)
+	if typ != nil {
+		p.resolve(typ)
+	}
+
+	return
+}
+
+func (p *parser) parseParameterList(scope *ast.Scope, ellipsisOk bool) (params []*ast.Field) {
+	if p.trace {
+		defer un(trace(p, "ParameterList"))
+	}
+
+	list, typ := p.parseVarList(ellipsisOk)
+	if typ != nil {
+		// IdentifierList Type
+		idents := p.makeIdentList(list)
+		field := &ast.Field{nil, idents, typ, nil, nil}
+		params = append(params, field)
+		// Go spec: The scope of an identifier denoting a function
+		// parameter or result variable is the function body.
+		p.declare(field, scope, ast.Var, idents...)
+		if p.tok == token.COMMA {
+			p.next()
+		}
+
+		for p.tok != token.RPAREN && p.tok != token.EOF {
+			idents := p.parseIdentList()
+			typ := p.parseVarType(ellipsisOk)
+			field := &ast.Field{nil, idents, typ, nil, nil}
+			params = append(params, field)
+			// Go spec: The scope of an identifier denoting a function
+			// parameter or result variable is the function body.
+			p.declare(field, scope, ast.Var, idents...)
+			if p.tok != token.COMMA {
+				break
+			}
+			p.next()
+		}
+
+	} else {
+		// Type { "," Type } (anonymous parameters)
+		params = make([]*ast.Field, len(list))
+		for i, x := range list {
+			p.resolve(x)
+			params[i] = &ast.Field{Type: x}
+		}
+	}
+
+	return
+}
+
+func (p *parser) parseParameters(scope *ast.Scope, ellipsisOk bool) *ast.FieldList {
+	if p.trace {
+		defer un(trace(p, "Parameters"))
+	}
+
+	var params []*ast.Field
+	lparen := p.expect(token.LPAREN)
+	if p.tok != token.RPAREN {
+		params = p.parseParameterList(scope, ellipsisOk)
+	}
+	rparen := p.expect(token.RPAREN)
+
+	return &ast.FieldList{lparen, params, rparen}
+}
+
+func (p *parser) parseResult(scope *ast.Scope) *ast.FieldList {
+	if p.trace {
+		defer un(trace(p, "Result"))
+	}
+
+	if p.tok == token.LPAREN {
+		return p.parseParameters(scope, false)
+	}
+
+	typ := p.tryType()
+	if typ != nil {
+		list := make([]*ast.Field, 1)
+		list[0] = &ast.Field{Type: typ}
+		return &ast.FieldList{List: list}
+	}
+
+	return nil
+}
+
+func (p *parser) parseSignature(scope *ast.Scope) (params, results *ast.FieldList) {
+	if p.trace {
+		defer un(trace(p, "Signature"))
+	}
+
+	params = p.parseParameters(scope, true)
+	results = p.parseResult(scope)
+
+	return
+}
+
+func (p *parser) parseFuncType() (*ast.FuncType, *ast.Scope) {
+	if p.trace {
+		defer un(trace(p, "FuncType"))
+	}
+
+	pos := p.expect(token.FUNC)
+	scope := ast.NewScope(p.topScope) // function scope
+	params, results := p.parseSignature(scope)
+
+	return &ast.FuncType{pos, params, results}, scope
+}
+
+func (p *parser) parseMethodSpec(scope *ast.Scope) *ast.Field {
+	if p.trace {
+		defer un(trace(p, "MethodSpec"))
+	}
+
+	doc := p.leadComment
+	var idents []*ast.Ident
+	var typ ast.Expr
+	x := p.parseTypeName()
+	if ident, isIdent := x.(*ast.Ident); isIdent && p.tok == token.LPAREN {
+		// method
+		idents = []*ast.Ident{ident}
+		scope := ast.NewScope(nil) // method scope
+		params, results := p.parseSignature(scope)
+		typ = &ast.FuncType{token.NoPos, params, results}
+	} else {
+		// embedded interface
+		typ = x
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	spec := &ast.Field{doc, idents, typ, nil, p.lineComment}
+	p.declare(spec, scope, ast.Fun, idents...)
+
+	return spec
+}
+
+func (p *parser) parseInterfaceType() *ast.InterfaceType {
+	if p.trace {
+		defer un(trace(p, "InterfaceType"))
+	}
+
+	pos := p.expect(token.INTERFACE)
+	lbrace := p.expect(token.LBRACE)
+	scope := ast.NewScope(nil) // interface scope
+	var list []*ast.Field
+	for p.tok == token.IDENT {
+		list = append(list, p.parseMethodSpec(scope))
+	}
+	rbrace := p.expect(token.RBRACE)
+
+	// TODO(gri): store interface scope in AST
+	return &ast.InterfaceType{pos, &ast.FieldList{lbrace, list, rbrace}, false}
+}
+
+func (p *parser) parseMapType() *ast.MapType {
+	if p.trace {
+		defer un(trace(p, "MapType"))
+	}
+
+	pos := p.expect(token.MAP)
+	p.expect(token.LBRACK)
+	key := p.parseType()
+	p.expect(token.RBRACK)
+	value := p.parseType()
+
+	return &ast.MapType{pos, key, value}
+}
+
+func (p *parser) parseChanType() *ast.ChanType {
+	if p.trace {
+		defer un(trace(p, "ChanType"))
+	}
+
+	pos := p.pos
+	dir := ast.SEND | ast.RECV
+	if p.tok == token.CHAN {
+		p.next()
+		if p.tok == token.ARROW {
+			p.next()
+			dir = ast.SEND
+		}
+	} else {
+		p.expect(token.ARROW)
+		p.expect(token.CHAN)
+		dir = ast.RECV
+	}
+	value := p.parseType()
+
+	return &ast.ChanType{pos, dir, value}
+}
+
+// If the result is an identifier, it is not resolved.
+func (p *parser) tryIdentOrType(ellipsisOk bool) ast.Expr {
+	switch p.tok {
+	case token.IDENT:
+		return p.parseTypeName()
+	case token.LBRACK:
+		return p.parseArrayType(ellipsisOk)
+	case token.STRUCT:
+		return p.parseStructType()
+	case token.MUL:
+		return p.parsePointerType()
+	case token.FUNC:
+		typ, _ := p.parseFuncType()
+		return typ
+	case token.INTERFACE:
+		return p.parseInterfaceType()
+	case token.MAP:
+		return p.parseMapType()
+	case token.CHAN, token.ARROW:
+		return p.parseChanType()
+	case token.LPAREN:
+		lparen := p.pos
+		p.next()
+		typ := p.parseType()
+		rparen := p.expect(token.RPAREN)
+		return &ast.ParenExpr{lparen, typ, rparen}
+	}
+
+	// no type found
+	return nil
+}
+
+func (p *parser) tryType() ast.Expr {
+	typ := p.tryIdentOrType(false)
+	if typ != nil {
+		p.resolve(typ)
+	}
+	return typ
+}
+
+// ----------------------------------------------------------------------------
+// Blocks
+
+func (p *parser) parseStmtList() (list []ast.Stmt) {
+	if p.trace {
+		defer un(trace(p, "StatementList"))
+	}
+
+	for p.tok != token.CASE && p.tok != token.DEFAULT && p.tok != token.RBRACE && p.tok != token.EOF {
+		list = append(list, p.parseStmt())
+	}
+
+	return
+}
+
+func (p *parser) parseBody(scope *ast.Scope) *ast.BlockStmt {
+	if p.trace {
+		defer un(trace(p, "Body"))
+	}
+
+	lbrace := p.expect(token.LBRACE)
+	p.topScope = scope // open function scope
+	p.openLabelScope()
+	list := p.parseStmtList()
+	p.closeLabelScope()
+	p.closeScope()
+	rbrace := p.expect(token.RBRACE)
+
+	return &ast.BlockStmt{lbrace, list, rbrace}
+}
+
+func (p *parser) parseBlockStmt() *ast.BlockStmt {
+	if p.trace {
+		defer un(trace(p, "BlockStmt"))
+	}
+
+	lbrace := p.expect(token.LBRACE)
+	p.openScope()
+	list := p.parseStmtList()
+	p.closeScope()
+	rbrace := p.expect(token.RBRACE)
+
+	return &ast.BlockStmt{lbrace, list, rbrace}
+}
+
+// ----------------------------------------------------------------------------
+// Expressions
+
+func (p *parser) parseFuncTypeOrLit() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "FuncTypeOrLit"))
+	}
+
+	typ, scope := p.parseFuncType()
+	if p.tok != token.LBRACE {
+		// function type only
+		return typ
+	}
+
+	p.exprLev++
+	body := p.parseBody(scope)
+	p.exprLev--
+
+	return &ast.FuncLit{typ, body}
+}
+
+// parseOperand may return an expression or a raw type (incl. array
+// types of the form [...]T. Callers must verify the result.
+// If lhs is set and the result is an identifier, it is not resolved.
+//
+func (p *parser) parseOperand(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Operand"))
+	}
+
+	switch p.tok {
+	case token.IDENT:
+		x := p.parseIdent()
+		if !lhs {
+			p.resolve(x)
+		}
+		return x
+
+	case token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING:
+		x := &ast.BasicLit{p.pos, p.tok, p.lit}
+		p.next()
+		return x
+
+	case token.LPAREN:
+		lparen := p.pos
+		p.next()
+		p.exprLev++
+		x := p.parseRhs()
+		p.exprLev--
+		rparen := p.expect(token.RPAREN)
+		return &ast.ParenExpr{lparen, x, rparen}
+
+	case token.FUNC:
+		return p.parseFuncTypeOrLit()
+
+	default:
+		if typ := p.tryIdentOrType(true); typ != nil {
+			// could be type for composite literal or conversion
+			_, isIdent := typ.(*ast.Ident)
+			assert(!isIdent, "type cannot be identifier")
+			return typ
+		}
+	}
+
+	pos := p.pos
+	p.errorExpected(pos, "operand")
+	p.next() // make progress
+	return &ast.BadExpr{pos, p.pos}
+}
+
+func (p *parser) parseSelector(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Selector"))
+	}
+
+	sel := p.parseIdent()
+
+	return &ast.SelectorExpr{x, sel}
+}
+
+func (p *parser) parseTypeAssertion(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "TypeAssertion"))
+	}
+
+	p.expect(token.LPAREN)
+	var typ ast.Expr
+	if p.tok == token.TYPE {
+		// type switch: typ == nil
+		p.next()
+	} else {
+		typ = p.parseType()
+	}
+	p.expect(token.RPAREN)
+
+	return &ast.TypeAssertExpr{x, typ}
+}
+
+func (p *parser) parseIndexOrSlice(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "IndexOrSlice"))
+	}
+
+	lbrack := p.expect(token.LBRACK)
+	p.exprLev++
+	var low, high ast.Expr
+	isSlice := false
+	if p.tok != token.COLON {
+		low = p.parseRhs()
+	}
+	if p.tok == token.COLON {
+		isSlice = true
+		p.next()
+		if p.tok != token.RBRACK {
+			high = p.parseRhs()
+		}
+	}
+	p.exprLev--
+	rbrack := p.expect(token.RBRACK)
+
+	if isSlice {
+		return &ast.SliceExpr{x, lbrack, low, high, rbrack}
+	}
+	return &ast.IndexExpr{x, lbrack, low, rbrack}
+}
+
+func (p *parser) parseCallOrConversion(fun ast.Expr) *ast.CallExpr {
+	if p.trace {
+		defer un(trace(p, "CallOrConversion"))
+	}
+
+	lparen := p.expect(token.LPAREN)
+	p.exprLev++
+	var list []ast.Expr
+	var ellipsis token.Pos
+	for p.tok != token.RPAREN && p.tok != token.EOF && !ellipsis.IsValid() {
+		list = append(list, p.parseRhs())
+		if p.tok == token.ELLIPSIS {
+			ellipsis = p.pos
+			p.next()
+		}
+		if p.tok != token.COMMA {
+			break
+		}
+		p.next()
+	}
+	p.exprLev--
+	rparen := p.expect(token.RPAREN)
+
+	return &ast.CallExpr{fun, lparen, list, ellipsis, rparen}
+}
+
+func (p *parser) parseElement(keyOk bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Element"))
+	}
+
+	if p.tok == token.LBRACE {
+		return p.parseLiteralValue(nil)
+	}
+
+	x := p.parseExpr(keyOk) // don't resolve if map key
+	if keyOk {
+		if p.tok == token.COLON {
+			colon := p.pos
+			p.next()
+			return &ast.KeyValueExpr{x, colon, p.parseElement(false)}
+		}
+		p.resolve(x) // not a map key
+	}
+
+	return x
+}
+
+func (p *parser) parseElementList() (list []ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "ElementList"))
+	}
+
+	for p.tok != token.RBRACE && p.tok != token.EOF {
+		list = append(list, p.parseElement(true))
+		if p.tok != token.COMMA {
+			break
+		}
+		p.next()
+	}
+
+	return
+}
+
+func (p *parser) parseLiteralValue(typ ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "LiteralValue"))
+	}
+
+	lbrace := p.expect(token.LBRACE)
+	var elts []ast.Expr
+	p.exprLev++
+	if p.tok != token.RBRACE {
+		elts = p.parseElementList()
+	}
+	p.exprLev--
+	rbrace := p.expect(token.RBRACE)
+	return &ast.CompositeLit{typ, lbrace, elts, rbrace}
+}
+
+// checkExpr checks that x is an expression (and not a type).
+func (p *parser) checkExpr(x ast.Expr) ast.Expr {
+	switch t := unparen(x).(type) {
+	case *ast.BadExpr:
+	case *ast.Ident:
+	case *ast.BasicLit:
+	case *ast.FuncLit:
+	case *ast.CompositeLit:
+	case *ast.ParenExpr:
+		panic("unreachable")
+	case *ast.SelectorExpr:
+	case *ast.IndexExpr:
+	case *ast.SliceExpr:
+	case *ast.TypeAssertExpr:
+		if t.Type == nil {
+			// the form X.(type) is only allowed in type switch expressions
+			p.errorExpected(x.Pos(), "expression")
+			x = &ast.BadExpr{x.Pos(), x.End()}
+		}
+	case *ast.CallExpr:
+	case *ast.StarExpr:
+	case *ast.UnaryExpr:
+		if t.Op == token.RANGE {
+			// the range operator is only allowed at the top of a for statement
+			p.errorExpected(x.Pos(), "expression")
+			x = &ast.BadExpr{x.Pos(), x.End()}
+		}
+	case *ast.BinaryExpr:
+	default:
+		// all other nodes are not proper expressions
+		p.errorExpected(x.Pos(), "expression")
+		x = &ast.BadExpr{x.Pos(), x.End()}
+	}
+	return x
+}
+
+// isTypeName returns true iff x is a (qualified) TypeName.
+func isTypeName(x ast.Expr) bool {
+	switch t := x.(type) {
+	case *ast.BadExpr:
+	case *ast.Ident:
+	case *ast.SelectorExpr:
+		_, isIdent := t.X.(*ast.Ident)
+		return isIdent
+	default:
+		return false // all other nodes are not type names
+	}
+	return true
+}
+
+// isLiteralType returns true iff x is a legal composite literal type.
+func isLiteralType(x ast.Expr) bool {
+	switch t := x.(type) {
+	case *ast.BadExpr:
+	case *ast.Ident:
+	case *ast.SelectorExpr:
+		_, isIdent := t.X.(*ast.Ident)
+		return isIdent
+	case *ast.ArrayType:
+	case *ast.StructType:
+	case *ast.MapType:
+	default:
+		return false // all other nodes are not legal composite literal types
+	}
+	return true
+}
+
+// If x is of the form *T, deref returns T, otherwise it returns x.
+func deref(x ast.Expr) ast.Expr {
+	if p, isPtr := x.(*ast.StarExpr); isPtr {
+		x = p.X
+	}
+	return x
+}
+
+// If x is of the form (T), unparen returns unparen(T), otherwise it returns x.
+func unparen(x ast.Expr) ast.Expr {
+	if p, isParen := x.(*ast.ParenExpr); isParen {
+		x = unparen(p.X)
+	}
+	return x
+}
+
+// checkExprOrType checks that x is an expression or a type
+// (and not a raw type such as [...]T).
+//
+func (p *parser) checkExprOrType(x ast.Expr) ast.Expr {
+	switch t := unparen(x).(type) {
+	case *ast.ParenExpr:
+		panic("unreachable")
+	case *ast.UnaryExpr:
+		if t.Op == token.RANGE {
+			// the range operator is only allowed at the top of a for statement
+			p.errorExpected(x.Pos(), "expression")
+			x = &ast.BadExpr{x.Pos(), x.End()}
+		}
+	case *ast.ArrayType:
+		if len, isEllipsis := t.Len.(*ast.Ellipsis); isEllipsis {
+			p.error(len.Pos(), "expected array length, found '...'")
+			x = &ast.BadExpr{x.Pos(), x.End()}
+		}
+	}
+
+	// all other nodes are expressions or types
+	return x
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+func (p *parser) parsePrimaryExpr(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "PrimaryExpr"))
+	}
+
+	x := p.parseOperand(lhs)
+L:
+	for {
+		switch p.tok {
+		case token.PERIOD:
+			p.next()
+			if lhs {
+				p.resolve(x)
+			}
+			switch p.tok {
+			case token.IDENT:
+				x = p.parseSelector(p.checkExpr(x))
+			case token.LPAREN:
+				x = p.parseTypeAssertion(p.checkExpr(x))
+			default:
+				pos := p.pos
+				p.next() // make progress
+				p.errorExpected(pos, "selector or type assertion")
+				x = &ast.BadExpr{pos, p.pos}
+			}
+		case token.LBRACK:
+			if lhs {
+				p.resolve(x)
+			}
+			x = p.parseIndexOrSlice(p.checkExpr(x))
+		case token.LPAREN:
+			if lhs {
+				p.resolve(x)
+			}
+			x = p.parseCallOrConversion(p.checkExprOrType(x))
+		case token.LBRACE:
+			if isLiteralType(x) && (p.exprLev >= 0 || !isTypeName(x)) {
+				if lhs {
+					p.resolve(x)
+				}
+				x = p.parseLiteralValue(x)
+			} else {
+				break L
+			}
+		default:
+			break L
+		}
+		lhs = false // no need to try to resolve again
+	}
+
+	return x
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+func (p *parser) parseUnaryExpr(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "UnaryExpr"))
+	}
+
+	switch p.tok {
+	case token.ADD, token.SUB, token.NOT, token.XOR, token.AND, token.RANGE:
+		pos, op := p.pos, p.tok
+		p.next()
+		x := p.parseUnaryExpr(false)
+		return &ast.UnaryExpr{pos, op, p.checkExpr(x)}
+
+	case token.ARROW:
+		// channel type or receive expression
+		pos := p.pos
+		p.next()
+		if p.tok == token.CHAN {
+			p.next()
+			value := p.parseType()
+			return &ast.ChanType{pos, ast.RECV, value}
+		}
+
+		x := p.parseUnaryExpr(false)
+		return &ast.UnaryExpr{pos, token.ARROW, p.checkExpr(x)}
+
+	case token.MUL:
+		// pointer type or unary "*" expression
+		pos := p.pos
+		p.next()
+		x := p.parseUnaryExpr(false)
+		return &ast.StarExpr{pos, p.checkExprOrType(x)}
+	}
+
+	return p.parsePrimaryExpr(lhs)
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+func (p *parser) parseBinaryExpr(lhs bool, prec1 int) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "BinaryExpr"))
+	}
+
+	x := p.parseUnaryExpr(lhs)
+	for prec := p.tok.Precedence(); prec >= prec1; prec-- {
+		for p.tok.Precedence() == prec {
+			pos, op := p.pos, p.tok
+			p.next()
+			if lhs {
+				p.resolve(x)
+				lhs = false
+			}
+			y := p.parseBinaryExpr(false, prec+1)
+			x = &ast.BinaryExpr{p.checkExpr(x), pos, op, p.checkExpr(y)}
+		}
+	}
+
+	return x
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+// TODO(gri): parseExpr may return a type or even a raw type ([..]int) -
+//            should reject when a type/raw type is obviously not allowed
+func (p *parser) parseExpr(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Expression"))
+	}
+
+	return p.parseBinaryExpr(lhs, token.LowestPrec+1)
+}
+
+func (p *parser) parseRhs() ast.Expr {
+	return p.parseExpr(false)
+}
+
+// ----------------------------------------------------------------------------
+// Statements
+
+func (p *parser) parseSimpleStmt(labelOk bool) ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "SimpleStmt"))
+	}
+
+	x := p.parseLhsList()
+
+	switch p.tok {
+	case
+		token.DEFINE, token.ASSIGN, token.ADD_ASSIGN,
+		token.SUB_ASSIGN, token.MUL_ASSIGN, token.QUO_ASSIGN,
+		token.REM_ASSIGN, token.AND_ASSIGN, token.OR_ASSIGN,
+		token.XOR_ASSIGN, token.SHL_ASSIGN, token.SHR_ASSIGN, token.AND_NOT_ASSIGN:
+		// assignment statement
+		pos, tok := p.pos, p.tok
+		p.next()
+		y := p.parseRhsList()
+		return &ast.AssignStmt{x, pos, tok, y}
+	}
+
+	if len(x) > 1 {
+		p.errorExpected(x[0].Pos(), "1 expression")
+		// continue with first expression
+	}
+
+	switch p.tok {
+	case token.COLON:
+		// labeled statement
+		colon := p.pos
+		p.next()
+		if label, isIdent := x[0].(*ast.Ident); labelOk && isIdent {
+			// Go spec: The scope of a label is the body of the function
+			// in which it is declared and excludes the body of any nested
+			// function.
+			stmt := &ast.LabeledStmt{label, colon, p.parseStmt()}
+			p.declare(stmt, p.labelScope, ast.Lbl, label)
+			return stmt
+		}
+		p.error(x[0].Pos(), "illegal label declaration")
+		return &ast.BadStmt{x[0].Pos(), colon + 1}
+
+	case token.ARROW:
+		// send statement
+		arrow := p.pos
+		p.next() // consume "<-"
+		y := p.parseRhs()
+		return &ast.SendStmt{x[0], arrow, y}
+
+	case token.INC, token.DEC:
+		// increment or decrement
+		s := &ast.IncDecStmt{x[0], p.pos, p.tok}
+		p.next() // consume "++" or "--"
+		return s
+	}
+
+	// expression
+	return &ast.ExprStmt{x[0]}
+}
+
+func (p *parser) parseCallExpr() *ast.CallExpr {
+	x := p.parseRhs()
+	if call, isCall := x.(*ast.CallExpr); isCall {
+		return call
+	}
+	p.errorExpected(x.Pos(), "function/method call")
+	return nil
+}
+
+func (p *parser) parseGoStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "GoStmt"))
+	}
+
+	pos := p.expect(token.GO)
+	call := p.parseCallExpr()
+	p.expectSemi()
+	if call == nil {
+		return &ast.BadStmt{pos, pos + 2} // len("go")
+	}
+
+	return &ast.GoStmt{pos, call}
+}
+
+func (p *parser) parseDeferStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "DeferStmt"))
+	}
+
+	pos := p.expect(token.DEFER)
+	call := p.parseCallExpr()
+	p.expectSemi()
+	if call == nil {
+		return &ast.BadStmt{pos, pos + 5} // len("defer")
+	}
+
+	return &ast.DeferStmt{pos, call}
+}
+
+func (p *parser) parseReturnStmt() *ast.ReturnStmt {
+	if p.trace {
+		defer un(trace(p, "ReturnStmt"))
+	}
+
+	pos := p.pos
+	p.expect(token.RETURN)
+	var x []ast.Expr
+	if p.tok != token.SEMICOLON && p.tok != token.RBRACE {
+		x = p.parseRhsList()
+	}
+	p.expectSemi()
+
+	return &ast.ReturnStmt{pos, x}
+}
+
+func (p *parser) parseBranchStmt(tok token.Token) *ast.BranchStmt {
+	if p.trace {
+		defer un(trace(p, "BranchStmt"))
+	}
+
+	pos := p.expect(tok)
+	var label *ast.Ident
+	if tok != token.FALLTHROUGH && p.tok == token.IDENT {
+		label = p.parseIdent()
+		// add to list of unresolved targets
+		n := len(p.targetStack) - 1
+		p.targetStack[n] = append(p.targetStack[n], label)
+	}
+	p.expectSemi()
+
+	return &ast.BranchStmt{pos, tok, label}
+}
+
+func (p *parser) makeExpr(s ast.Stmt) ast.Expr {
+	if s == nil {
+		return nil
+	}
+	if es, isExpr := s.(*ast.ExprStmt); isExpr {
+		return p.checkExpr(es.X)
+	}
+	p.error(s.Pos(), "expected condition, found simple statement")
+	return &ast.BadExpr{s.Pos(), s.End()}
+}
+
+func (p *parser) parseIfStmt() *ast.IfStmt {
+	if p.trace {
+		defer un(trace(p, "IfStmt"))
+	}
+
+	pos := p.expect(token.IF)
+	p.openScope()
+	defer p.closeScope()
+
+	var s ast.Stmt
+	var x ast.Expr
+	{
+		prevLev := p.exprLev
+		p.exprLev = -1
+		if p.tok == token.SEMICOLON {
+			p.next()
+			x = p.parseRhs()
+		} else {
+			s = p.parseSimpleStmt(false)
+			if p.tok == token.SEMICOLON {
+				p.next()
+				x = p.parseRhs()
+			} else {
+				x = p.makeExpr(s)
+				s = nil
+			}
+		}
+		p.exprLev = prevLev
+	}
+
+	body := p.parseBlockStmt()
+	var else_ ast.Stmt
+	if p.tok == token.ELSE {
+		p.next()
+		else_ = p.parseStmt()
+	} else {
+		p.expectSemi()
+	}
+
+	return &ast.IfStmt{pos, s, x, body, else_}
+}
+
+func (p *parser) parseTypeList() (list []ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "TypeList"))
+	}
+
+	list = append(list, p.parseType())
+	for p.tok == token.COMMA {
+		p.next()
+		list = append(list, p.parseType())
+	}
+
+	return
+}
+
+func (p *parser) parseCaseClause(exprSwitch bool) *ast.CaseClause {
+	if p.trace {
+		defer un(trace(p, "CaseClause"))
+	}
+
+	pos := p.pos
+	var list []ast.Expr
+	if p.tok == token.CASE {
+		p.next()
+		if exprSwitch {
+			list = p.parseRhsList()
+		} else {
+			list = p.parseTypeList()
+		}
+	} else {
+		p.expect(token.DEFAULT)
+	}
+
+	colon := p.expect(token.COLON)
+	p.openScope()
+	body := p.parseStmtList()
+	p.closeScope()
+
+	return &ast.CaseClause{pos, list, colon, body}
+}
+
+func isExprSwitch(s ast.Stmt) bool {
+	if s == nil {
+		return true
+	}
+	if e, ok := s.(*ast.ExprStmt); ok {
+		if a, ok := e.X.(*ast.TypeAssertExpr); ok {
+			return a.Type != nil // regular type assertion
+		}
+		return true
+	}
+	return false
+}
+
+func (p *parser) parseSwitchStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "SwitchStmt"))
+	}
+
+	pos := p.expect(token.SWITCH)
+	p.openScope()
+	defer p.closeScope()
+
+	var s1, s2 ast.Stmt
+	if p.tok != token.LBRACE {
+		prevLev := p.exprLev
+		p.exprLev = -1
+		if p.tok != token.SEMICOLON {
+			s2 = p.parseSimpleStmt(false)
+		}
+		if p.tok == token.SEMICOLON {
+			p.next()
+			s1 = s2
+			s2 = nil
+			if p.tok != token.LBRACE {
+				s2 = p.parseSimpleStmt(false)
+			}
+		}
+		p.exprLev = prevLev
+	}
+
+	exprSwitch := isExprSwitch(s2)
+	lbrace := p.expect(token.LBRACE)
+	var list []ast.Stmt
+	for p.tok == token.CASE || p.tok == token.DEFAULT {
+		list = append(list, p.parseCaseClause(exprSwitch))
+	}
+	rbrace := p.expect(token.RBRACE)
+	p.expectSemi()
+	body := &ast.BlockStmt{lbrace, list, rbrace}
+
+	if exprSwitch {
+		return &ast.SwitchStmt{pos, s1, p.makeExpr(s2), body}
+	}
+	// type switch
+	// TODO(gri): do all the checks!
+	return &ast.TypeSwitchStmt{pos, s1, s2, body}
+}
+
+func (p *parser) parseCommClause() *ast.CommClause {
+	if p.trace {
+		defer un(trace(p, "CommClause"))
+	}
+
+	p.openScope()
+	pos := p.pos
+	var comm ast.Stmt
+	if p.tok == token.CASE {
+		p.next()
+		lhs := p.parseLhsList()
+		if p.tok == token.ARROW {
+			// SendStmt
+			if len(lhs) > 1 {
+				p.errorExpected(lhs[0].Pos(), "1 expression")
+				// continue with first expression
+			}
+			arrow := p.pos
+			p.next()
+			rhs := p.parseRhs()
+			comm = &ast.SendStmt{lhs[0], arrow, rhs}
+		} else {
+			// RecvStmt
+			pos := p.pos
+			tok := p.tok
+			var rhs ast.Expr
+			if tok == token.ASSIGN || tok == token.DEFINE {
+				// RecvStmt with assignment
+				if len(lhs) > 2 {
+					p.errorExpected(lhs[0].Pos(), "1 or 2 expressions")
+					// continue with first two expressions
+					lhs = lhs[0:2]
+				}
+				p.next()
+				rhs = p.parseRhs()
+			} else {
+				// rhs must be single receive operation
+				if len(lhs) > 1 {
+					p.errorExpected(lhs[0].Pos(), "1 expression")
+					// continue with first expression
+				}
+				rhs = lhs[0]
+				lhs = nil // there is no lhs
+			}
+			if x, isUnary := rhs.(*ast.UnaryExpr); !isUnary || x.Op != token.ARROW {
+				p.errorExpected(rhs.Pos(), "send or receive operation")
+				rhs = &ast.BadExpr{rhs.Pos(), rhs.End()}
+			}
+			if lhs != nil {
+				comm = &ast.AssignStmt{lhs, pos, tok, []ast.Expr{rhs}}
+			} else {
+				comm = &ast.ExprStmt{rhs}
+			}
+		}
+	} else {
+		p.expect(token.DEFAULT)
+	}
+
+	colon := p.expect(token.COLON)
+	body := p.parseStmtList()
+	p.closeScope()
+
+	return &ast.CommClause{pos, comm, colon, body}
+}
+
+func (p *parser) parseSelectStmt() *ast.SelectStmt {
+	if p.trace {
+		defer un(trace(p, "SelectStmt"))
+	}
+
+	pos := p.expect(token.SELECT)
+	lbrace := p.expect(token.LBRACE)
+	var list []ast.Stmt
+	for p.tok == token.CASE || p.tok == token.DEFAULT {
+		list = append(list, p.parseCommClause())
+	}
+	rbrace := p.expect(token.RBRACE)
+	p.expectSemi()
+	body := &ast.BlockStmt{lbrace, list, rbrace}
+
+	return &ast.SelectStmt{pos, body}
+}
+
+func (p *parser) parseForStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "ForStmt"))
+	}
+
+	pos := p.expect(token.FOR)
+	p.openScope()
+	defer p.closeScope()
+
+	var s1, s2, s3 ast.Stmt
+	if p.tok != token.LBRACE {
+		prevLev := p.exprLev
+		p.exprLev = -1
+		if p.tok != token.SEMICOLON {
+			s2 = p.parseSimpleStmt(false)
+		}
+		if p.tok == token.SEMICOLON {
+			p.next()
+			s1 = s2
+			s2 = nil
+			if p.tok != token.SEMICOLON {
+				s2 = p.parseSimpleStmt(false)
+			}
+			p.expectSemi()
+			if p.tok != token.LBRACE {
+				s3 = p.parseSimpleStmt(false)
+			}
+		}
+		p.exprLev = prevLev
+	}
+
+	body := p.parseBlockStmt()
+	p.expectSemi()
+
+	if as, isAssign := s2.(*ast.AssignStmt); isAssign {
+		// possibly a for statement with a range clause; check assignment operator
+		if as.Tok != token.ASSIGN && as.Tok != token.DEFINE {
+			p.errorExpected(as.TokPos, "'=' or ':='")
+			return &ast.BadStmt{pos, body.End()}
+		}
+		// check lhs
+		var key, value ast.Expr
+		switch len(as.Lhs) {
+		case 2:
+			key, value = as.Lhs[0], as.Lhs[1]
+		case 1:
+			key = as.Lhs[0]
+		default:
+			p.errorExpected(as.Lhs[0].Pos(), "1 or 2 expressions")
+			return &ast.BadStmt{pos, body.End()}
+		}
+		// check rhs
+		if len(as.Rhs) != 1 {
+			p.errorExpected(as.Rhs[0].Pos(), "1 expression")
+			return &ast.BadStmt{pos, body.End()}
+		}
+		if rhs, isUnary := as.Rhs[0].(*ast.UnaryExpr); isUnary && rhs.Op == token.RANGE {
+			// rhs is range expression
+			// (any short variable declaration was handled by parseSimpleStat above)
+			return &ast.RangeStmt{pos, key, value, as.TokPos, as.Tok, rhs.X, body}
+		}
+		p.errorExpected(s2.Pos(), "range clause")
+		return &ast.BadStmt{pos, body.End()}
+	}
+
+	// regular for statement
+	return &ast.ForStmt{pos, s1, p.makeExpr(s2), s3, body}
+}
+
+func (p *parser) parseStmt() (s ast.Stmt) {
+	if p.trace {
+		defer un(trace(p, "Statement"))
+	}
+
+	switch p.tok {
+	case token.CONST, token.TYPE, token.VAR:
+		s = &ast.DeclStmt{p.parseDecl()}
+	case
+		// tokens that may start a top-level expression
+		token.IDENT, token.INT, token.FLOAT, token.CHAR, token.STRING, token.FUNC, token.LPAREN, // operand
+		token.LBRACK, token.STRUCT, // composite type
+		token.MUL, token.AND, token.ARROW, token.ADD, token.SUB, token.XOR: // unary operators
+		s = p.parseSimpleStmt(true)
+		// because of the required look-ahead, labeled statements are
+		// parsed by parseSimpleStmt - don't expect a semicolon after
+		// them
+		if _, isLabeledStmt := s.(*ast.LabeledStmt); !isLabeledStmt {
+			p.expectSemi()
+		}
+	case token.GO:
+		s = p.parseGoStmt()
+	case token.DEFER:
+		s = p.parseDeferStmt()
+	case token.RETURN:
+		s = p.parseReturnStmt()
+	case token.BREAK, token.CONTINUE, token.GOTO, token.FALLTHROUGH:
+		s = p.parseBranchStmt(p.tok)
+	case token.LBRACE:
+		s = p.parseBlockStmt()
+		p.expectSemi()
+	case token.IF:
+		s = p.parseIfStmt()
+	case token.SWITCH:
+		s = p.parseSwitchStmt()
+	case token.SELECT:
+		s = p.parseSelectStmt()
+	case token.FOR:
+		s = p.parseForStmt()
+	case token.SEMICOLON:
+		s = &ast.EmptyStmt{p.pos}
+		p.next()
+	case token.RBRACE:
+		// a semicolon may be omitted before a closing "}"
+		s = &ast.EmptyStmt{p.pos}
+	default:
+		// no statement found
+		pos := p.pos
+		p.errorExpected(pos, "statement")
+		p.next() // make progress
+		s = &ast.BadStmt{pos, p.pos}
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Declarations
+
+type parseSpecFunction func(p *parser, doc *ast.CommentGroup, iota int) ast.Spec
+
+func parseImportSpec(p *parser, doc *ast.CommentGroup, _ int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, "ImportSpec"))
+	}
+
+	var ident *ast.Ident
+	switch p.tok {
+	case token.PERIOD:
+		ident = &ast.Ident{p.pos, ".", nil}
+		p.next()
+	case token.IDENT:
+		ident = p.parseIdent()
+	}
+
+	var path *ast.BasicLit
+	if p.tok == token.STRING {
+		path = &ast.BasicLit{p.pos, p.tok, p.lit}
+		p.next()
+	} else {
+		p.expect(token.STRING) // use expect() error handling
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	// collect imports
+	spec := &ast.ImportSpec{doc, ident, path, p.lineComment}
+	p.imports = append(p.imports, spec)
+
+	return spec
+}
+
+func parseConstSpec(p *parser, doc *ast.CommentGroup, iota int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, "ConstSpec"))
+	}
+
+	idents := p.parseIdentList()
+	typ := p.tryType()
+	var values []ast.Expr
+	if typ != nil || p.tok == token.ASSIGN || iota == 0 {
+		p.expect(token.ASSIGN)
+		values = p.parseRhsList()
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	// Go spec: The scope of a constant or variable identifier declared inside
+	// a function begins at the end of the ConstSpec or VarSpec and ends at
+	// the end of the innermost containing block.
+	// (Global identifiers are resolved in a separate phase after parsing.)
+	spec := &ast.ValueSpec{doc, idents, typ, values, p.lineComment}
+	p.declare(spec, p.topScope, ast.Con, idents...)
+
+	return spec
+}
+
+func parseTypeSpec(p *parser, doc *ast.CommentGroup, _ int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, "TypeSpec"))
+	}
+
+	ident := p.parseIdent()
+
+	// Go spec: The scope of a type identifier declared inside a function begins
+	// at the identifier in the TypeSpec and ends at the end of the innermost
+	// containing block.
+	// (Global identifiers are resolved in a separate phase after parsing.)
+	spec := &ast.TypeSpec{doc, ident, nil, nil}
+	p.declare(spec, p.topScope, ast.Typ, ident)
+
+	spec.Type = p.parseType()
+	p.expectSemi() // call before accessing p.linecomment
+	spec.Comment = p.lineComment
+
+	return spec
+}
+
+func parseVarSpec(p *parser, doc *ast.CommentGroup, _ int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, "VarSpec"))
+	}
+
+	idents := p.parseIdentList()
+	typ := p.tryType()
+	var values []ast.Expr
+	if typ == nil || p.tok == token.ASSIGN {
+		p.expect(token.ASSIGN)
+		values = p.parseRhsList()
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	// Go spec: The scope of a constant or variable identifier declared inside
+	// a function begins at the end of the ConstSpec or VarSpec and ends at
+	// the end of the innermost containing block.
+	// (Global identifiers are resolved in a separate phase after parsing.)
+	spec := &ast.ValueSpec{doc, idents, typ, values, p.lineComment}
+	p.declare(spec, p.topScope, ast.Var, idents...)
+
+	return spec
+}
+
+func (p *parser) parseGenDecl(keyword token.Token, f parseSpecFunction) *ast.GenDecl {
+	if p.trace {
+		defer un(trace(p, "GenDecl("+keyword.String()+")"))
+	}
+
+	doc := p.leadComment
+	pos := p.expect(keyword)
+	var lparen, rparen token.Pos
+	var list []ast.Spec
+	if p.tok == token.LPAREN {
+		lparen = p.pos
+		p.next()
+		for iota := 0; p.tok != token.RPAREN && p.tok != token.EOF; iota++ {
+			list = append(list, f(p, p.leadComment, iota))
+		}
+		rparen = p.expect(token.RPAREN)
+		p.expectSemi()
+	} else {
+		list = append(list, f(p, nil, 0))
+	}
+
+	return &ast.GenDecl{doc, pos, keyword, lparen, list, rparen}
+}
+
+func (p *parser) parseReceiver(scope *ast.Scope) *ast.FieldList {
+	if p.trace {
+		defer un(trace(p, "Receiver"))
+	}
+
+	pos := p.pos
+	par := p.parseParameters(scope, false)
+
+	// must have exactly one receiver
+	if par.NumFields() != 1 {
+		p.errorExpected(pos, "exactly one receiver")
+		// TODO determine a better range for BadExpr below
+		par.List = []*ast.Field{{Type: &ast.BadExpr{pos, pos}}}
+		return par
+	}
+
+	// recv type must be of the form ["*"] identifier
+	recv := par.List[0]
+	base := deref(recv.Type)
+	if _, isIdent := base.(*ast.Ident); !isIdent {
+		p.errorExpected(base.Pos(), "(unqualified) identifier")
+		par.List = []*ast.Field{{Type: &ast.BadExpr{recv.Pos(), recv.End()}}}
+	}
+
+	return par
+}
+
+func (p *parser) parseFuncDecl() *ast.FuncDecl {
+	if p.trace {
+		defer un(trace(p, "FunctionDecl"))
+	}
+
+	doc := p.leadComment
+	pos := p.expect(token.FUNC)
+	scope := ast.NewScope(p.topScope) // function scope
+
+	var recv *ast.FieldList
+	if p.tok == token.LPAREN {
+		recv = p.parseReceiver(scope)
+	}
+
+	ident := p.parseIdent()
+
+	params, results := p.parseSignature(scope)
+
+	var body *ast.BlockStmt
+	if p.tok == token.LBRACE {
+		body = p.parseBody(scope)
+	}
+	p.expectSemi()
+
+	decl := &ast.FuncDecl{doc, recv, ident, &ast.FuncType{pos, params, results}, body}
+	if recv == nil {
+		// Go spec: The scope of an identifier denoting a constant, type,
+		// variable, or function (but not method) declared at top level
+		// (outside any function) is the package block.
+		//
+		// init() functions cannot be referred to and there may
+		// be more than one - don't put them in the pkgScope
+		if ident.Name != "init" {
+			p.declare(decl, p.pkgScope, ast.Fun, ident)
+		}
+	}
+
+	return decl
+}
+
+func (p *parser) parseDecl() ast.Decl {
+	if p.trace {
+		defer un(trace(p, "Declaration"))
+	}
+
+	var f parseSpecFunction
+	switch p.tok {
+	case token.CONST:
+		f = parseConstSpec
+
+	case token.TYPE:
+		f = parseTypeSpec
+
+	case token.VAR:
+		f = parseVarSpec
+
+	case token.FUNC:
+		return p.parseFuncDecl()
+
+	default:
+		pos := p.pos
+		p.errorExpected(pos, "declaration")
+		p.next() // make progress
+		decl := &ast.BadDecl{pos, p.pos}
+		return decl
+	}
+
+	return p.parseGenDecl(p.tok, f)
+}
+
+func (p *parser) parseDeclList() (list []ast.Decl) {
+	if p.trace {
+		defer un(trace(p, "DeclList"))
+	}
+
+	for p.tok != token.EOF {
+		list = append(list, p.parseDecl())
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Source files
+
+func (p *parser) parseFile() *ast.File {
+	if p.trace {
+		defer un(trace(p, "File"))
+	}
+
+	// package clause
+	doc := p.leadComment
+	pos := p.expect(token.PACKAGE)
+	// Go spec: The package clause is not a declaration;
+	// the package name does not appear in any scope.
+	ident := p.parseIdent()
+	if ident.Name == "_" {
+		p.error(p.pos, "invalid package name _")
+	}
+	p.expectSemi()
+
+	var decls []ast.Decl
+
+	// Don't bother parsing the rest if we had errors already.
+	// Likely not a Go source file at all.
+
+	if p.ErrorCount() == 0 && p.mode&PackageClauseOnly == 0 {
+		// import decls
+		for p.tok == token.IMPORT {
+			decls = append(decls, p.parseGenDecl(token.IMPORT, parseImportSpec))
+		}
+
+		if p.mode&ImportsOnly == 0 {
+			// rest of package body
+			for p.tok != token.EOF {
+				decls = append(decls, p.parseDecl())
+			}
+		}
+	}
+
+	assert(p.topScope == p.pkgScope, "imbalanced scopes")
+
+	// resolve global identifiers within the same file
+	i := 0
+	for _, ident := range p.unresolved {
+		// i <= index for current ident
+		assert(ident.Obj == unresolved, "object already resolved")
+		ident.Obj = p.pkgScope.Lookup(ident.Name) // also removes unresolved sentinel
+		if ident.Obj == nil {
+			p.unresolved[i] = ident
+			i++
+		}
+	}
+
+	// TODO(gri): store p.imports in AST
+	return &ast.File{doc, pos, ident, decls, p.pkgScope, p.imports, p.unresolved[0:i], p.comments}
+}
diff --git a/src/go/printer/testdata/slow.golden b/src/go/printer/testdata/slow.golden
new file mode 100644
index 000000000..43a15cb1d
--- /dev/null
+++ b/src/go/printer/testdata/slow.golden
@@ -0,0 +1,85 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package deepequal_test
+
+import (
+	"testing"
+	"google3/spam/archer/frontend/deepequal"
+)
+
+func TestTwoNilValues(t *testing.T) {
+	if err := deepequal.Check(nil, nil); err != nil {
+		t.Errorf("expected nil, saw %v", err)
+	}
+}
+
+type Foo struct {
+	bar	*Bar
+	bang	*Bar
+}
+
+type Bar struct {
+	baz	*Baz
+	foo	[]*Foo
+}
+
+type Baz struct {
+	entries		map[int]interface{}
+	whatever	string
+}
+
+func newFoo() *Foo {
+	return &Foo{bar: &Bar{baz: &Baz{
+		entries: map[int]interface{}{
+			42:	&Foo{},
+			21:	&Bar{},
+			11:	&Baz{whatever: "it's just a test"}}}},
+		bang: &Bar{foo: []*Foo{
+			&Foo{bar: &Bar{baz: &Baz{
+				entries: map[int]interface{}{
+					43:	&Foo{},
+					22:	&Bar{},
+					13:	&Baz{whatever: "this is nuts"}}}},
+				bang: &Bar{foo: []*Foo{
+					&Foo{bar: &Bar{baz: &Baz{
+						entries: map[int]interface{}{
+							61:	&Foo{},
+							71:	&Bar{},
+							11:	&Baz{whatever: "no, it's Go"}}}},
+						bang: &Bar{foo: []*Foo{
+							&Foo{bar: &Bar{baz: &Baz{
+								entries: map[int]interface{}{
+									0:	&Foo{},
+									-2:	&Bar{},
+									-11:	&Baz{whatever: "we need to go deeper"}}}},
+								bang: &Bar{foo: []*Foo{
+									&Foo{bar: &Bar{baz: &Baz{
+										entries: map[int]interface{}{
+											-2:	&Foo{},
+											-5:	&Bar{},
+											-7:	&Baz{whatever: "are you serious?"}}}},
+										bang:	&Bar{foo: []*Foo{}}},
+									&Foo{bar: &Bar{baz: &Baz{
+										entries: map[int]interface{}{
+											-100:	&Foo{},
+											50:	&Bar{},
+											20:	&Baz{whatever: "na, not really ..."}}}},
+										bang:	&Bar{foo: []*Foo{}}}}}}}}},
+					&Foo{bar: &Bar{baz: &Baz{
+						entries: map[int]interface{}{
+							2:	&Foo{},
+							1:	&Bar{},
+							-1:	&Baz{whatever: "... it's just a test."}}}},
+						bang:	&Bar{foo: []*Foo{}}}}}}}}}
+}
+
+func TestElaborate(t *testing.T) {
+	a := newFoo()
+	b := newFoo()
+
+	if err := deepequal.Check(a, b); err != nil {
+		t.Errorf("expected nil, saw %v", err)
+	}
+}
diff --git a/src/go/printer/testdata/slow.input b/src/go/printer/testdata/slow.input
new file mode 100644
index 000000000..0e5a23d88
--- /dev/null
+++ b/src/go/printer/testdata/slow.input
@@ -0,0 +1,85 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package deepequal_test
+
+import (
+        "testing"
+        "google3/spam/archer/frontend/deepequal"
+)
+
+func TestTwoNilValues(t *testing.T) {
+        if err := deepequal.Check(nil, nil); err != nil {
+                t.Errorf("expected nil, saw %v", err)
+        }
+}
+
+type Foo struct {
+        bar *Bar
+        bang *Bar
+}
+
+type Bar struct {
+        baz *Baz
+        foo []*Foo
+}
+
+type Baz struct {
+        entries  map[int]interface{}
+        whatever string
+}
+
+func newFoo() (*Foo) {
+return &Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+42: &Foo{},
+21: &Bar{},
+11: &Baz{ whatever: "it's just a test" }}}},
+        bang: &Bar{foo: []*Foo{
+&Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+43: &Foo{},
+22: &Bar{},
+13: &Baz{ whatever: "this is nuts" }}}},
+        bang: &Bar{foo: []*Foo{
+&Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+61: &Foo{},
+71: &Bar{},
+11: &Baz{ whatever: "no, it's Go" }}}},
+        bang: &Bar{foo: []*Foo{
+&Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+0: &Foo{},
+-2: &Bar{},
+-11: &Baz{ whatever: "we need to go deeper" }}}},
+        bang: &Bar{foo: []*Foo{
+&Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+-2: &Foo{},
+-5: &Bar{},
+-7: &Baz{ whatever: "are you serious?" }}}},
+        bang: &Bar{foo: []*Foo{}}},
+&Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+-100: &Foo{},
+50: &Bar{},
+20: &Baz{ whatever: "na, not really ..." }}}},
+        bang: &Bar{foo: []*Foo{}}}}}}}}},
+&Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+2: &Foo{},
+1: &Bar{},
+-1: &Baz{ whatever: "... it's just a test." }}}},
+        bang: &Bar{foo: []*Foo{}}}}}}}}}
+}
+
+func TestElaborate(t *testing.T) {
+        a := newFoo()
+        b := newFoo()
+
+        if err := deepequal.Check(a, b); err != nil {
+                t.Errorf("expected nil, saw %v", err)
+        }
+}
diff --git a/src/go/printer/testdata/statements.golden b/src/go/printer/testdata/statements.golden
new file mode 100644
index 000000000..324b6cdd0
--- /dev/null
+++ b/src/go/printer/testdata/statements.golden
@@ -0,0 +1,644 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package statements
+
+var expr bool
+
+func use(x interface{})	{}
+
+// Formatting of multi-line return statements.
+func _f() {
+	return
+	return x, y, z
+	return T{}
+	return T{1, 2, 3},
+		x, y, z
+	return T{1, 2, 3},
+		x, y,
+		z
+	return T{1,
+		2,
+		3}
+	return T{1,
+		2,
+		3,
+	}
+	return T{
+		1,
+		2,
+		3}
+	return T{
+		1,
+		2,
+		3,
+	}
+	return T{
+		1,
+		T{1, 2, 3},
+		3,
+	}
+	return T{
+		1,
+		T{1,
+			2, 3},
+		3,
+	}
+	return T{
+		1,
+		T{1,
+			2,
+			3},
+		3,
+	}
+	return T{
+		1,
+		2,
+	}, nil
+	return T{
+			1,
+			2,
+		},
+		T{
+			x:	3,
+			y:	4,
+		}, nil
+	return T{
+			1,
+			2,
+		},
+		nil
+	return T{
+			1,
+			2,
+		},
+		T{
+			x:	3,
+			y:	4,
+		},
+		nil
+	return x + y +
+		z
+	return func() {}
+	return func() {
+			_ = 0
+		}, T{
+			1, 2,
+		}
+	return func() {
+		_ = 0
+	}
+	return func() T {
+		return T{
+			1, 2,
+		}
+	}
+}
+
+// Formatting of multi-line returns: test cases from issue 1207.
+func F() (*T, os.Error) {
+	return &T{
+			X:	1,
+			Y:	2,
+		},
+		nil
+}
+
+func G() (*T, *T, os.Error) {
+	return &T{
+			X:	1,
+			Y:	2,
+		},
+		&T{
+			X:	3,
+			Y:	4,
+		},
+		nil
+}
+
+func _() interface{} {
+	return &fileStat{
+		name:		basename(file.name),
+		size:		mkSize(d.FileSizeHigh, d.FileSizeLow),
+		modTime:	mkModTime(d.LastWriteTime),
+		mode:		mkMode(d.FileAttributes),
+		sys:		mkSysFromFI(&d),
+	}, nil
+}
+
+// Formatting of if-statement headers.
+func _() {
+	if true {
+	}
+	if true {
+	}	// no semicolon printed
+	if expr {
+	}
+	if expr {
+	}	// no semicolon printed
+	if expr {
+	}	// no parens printed
+	if expr {
+	}	// no semicolon and parens printed
+	if x := expr; true {
+		use(x)
+	}
+	if x := expr; expr {
+		use(x)
+	}
+}
+
+// Formatting of switch-statement headers.
+func _() {
+	switch {
+	}
+	switch {
+	}	// no semicolon printed
+	switch expr {
+	}
+	switch expr {
+	}	// no semicolon printed
+	switch expr {
+	}	// no parens printed
+	switch expr {
+	}	// no semicolon and parens printed
+	switch x := expr; {
+	default:
+		use(
+			x)
+	}
+	switch x := expr; expr {
+	default:
+		use(x)
+	}
+}
+
+// Formatting of switch statement bodies.
+func _() {
+	switch {
+	}
+
+	switch x := 0; x {
+	case 1:
+		use(x)
+		use(x)	// followed by an empty line
+
+	case 2:	// followed by an empty line
+
+		use(x)	// followed by an empty line
+
+	case 3:	// no empty lines
+		use(x)
+		use(x)
+	}
+
+	switch x {
+	case 0:
+		use(x)
+	case 1:	// this comment should have no effect on the previous or next line
+		use(x)
+	}
+
+	switch x := 0; x {
+	case 1:
+		x = 0
+		// this comment should be indented
+	case 2:
+		x = 0
+	// this comment should not be indented, it is aligned with the next case
+	case 3:
+		x = 0
+		/* indented comment
+		   aligned
+		   aligned
+		*/
+		// bla
+		/* and more */
+	case 4:
+		x = 0
+	/* not indented comment
+	   aligned
+	   aligned
+	*/
+	// bla
+	/* and more */
+	case 5:
+	}
+}
+
+// Formatting of selected select statements.
+func _() {
+	select {}
+	select { /* this comment should not be tab-aligned because the closing } is on the same line */
+	}
+	select {	/* this comment should be tab-aligned */
+	}
+	select {	// this comment should be tab-aligned
+	}
+	select {
+	case <-c:
+	}
+}
+
+// Formatting of for-statement headers for single-line for-loops.
+func _() {
+	for {
+	}
+	for expr {
+	}
+	for expr {
+	}	// no parens printed
+	for {
+	}	// no semicolons printed
+	for x := expr; ; {
+		use(x)
+	}
+	for expr {
+	}	// no semicolons printed
+	for expr {
+	}	// no semicolons and parens printed
+	for ; ; expr = false {
+	}
+	for x := expr; expr; {
+		use(x)
+	}
+	for x := expr; ; expr = false {
+		use(x)
+	}
+	for ; expr; expr = false {
+	}
+	for x := expr; expr; expr = false {
+		use(x)
+	}
+	for x := range []int{} {
+		use(x)
+	}
+	for x := range []int{} {
+		use(x)
+	}	// no parens printed
+}
+
+// Formatting of for-statement headers for multi-line for-loops.
+func _() {
+	for {
+	}
+	for expr {
+	}
+	for expr {
+	}	// no parens printed
+	for {
+	}	// no semicolons printed
+	for x := expr; ; {
+		use(x)
+	}
+	for expr {
+	}	// no semicolons printed
+	for expr {
+	}	// no semicolons and parens printed
+	for ; ; expr = false {
+	}
+	for x := expr; expr; {
+		use(x)
+	}
+	for x := expr; ; expr = false {
+		use(x)
+	}
+	for ; expr; expr = false {
+	}
+	for x := expr; expr; expr = false {
+		use(x)
+	}
+	for range []int{} {
+		println("foo")
+	}
+	for x := range []int{} {
+		use(x)
+	}
+	for x := range []int{} {
+		use(x)
+	}	// no parens printed
+}
+
+// Formatting of selected short single- and multi-line statements.
+func _() {
+	if cond {
+	}
+	if cond {
+	}	// multiple lines
+	if cond {
+	} else {
+	}	// else clause always requires multiple lines
+
+	for {
+	}
+	for i := 0; i < len(a); 1++ {
+	}
+	for i := 0; i < len(a); 1++ {
+		a[i] = i
+	}
+	for i := 0; i < len(a); 1++ {
+		a[i] = i
+	}	// multiple lines
+
+	for range a {
+	}
+	for _ = range a {
+	}
+	for _, _ = range a {
+	}
+	for i := range a {
+	}
+	for i := range a {
+		a[i] = i
+	}
+	for i := range a {
+		a[i] = i
+	}	// multiple lines
+
+	go func() {
+		for {
+			a <- <-b
+		}
+	}()
+	defer func() {
+		if x := recover(); x != nil {
+			err = fmt.Sprintf("error: %s", x.msg)
+		}
+	}()
+}
+
+// Don't remove mandatory parentheses around composite literals in control clauses.
+func _() {
+	// strip parentheses - no composite literals or composite literals don't start with a type name
+	if x {
+	}
+	if x {
+	}
+	if []T{} {
+	}
+	if []T{} {
+	}
+	if []T{} {
+	}
+
+	for x {
+	}
+	for x {
+	}
+	for []T{} {
+	}
+	for []T{} {
+	}
+	for []T{} {
+	}
+
+	switch x {
+	}
+	switch x {
+	}
+	switch []T{} {
+	}
+	switch []T{} {
+	}
+
+	for _ = range []T{T{42}} {
+	}
+
+	// leave parentheses - composite literals start with a type name
+	if (T{}) {
+	}
+	if (T{}) {
+	}
+	if (T{}) {
+	}
+
+	for (T{}) {
+	}
+	for (T{}) {
+	}
+	for (T{}) {
+	}
+
+	switch (T{}) {
+	}
+	switch (T{}) {
+	}
+
+	for _ = range (T1{T{42}}) {
+	}
+
+	if x == (T{42}[0]) {
+	}
+	if (x == T{42}[0]) {
+	}
+	if x == (T{42}[0]) {
+	}
+	if x == (T{42}[0]) {
+	}
+	if x == (T{42}[0]) {
+	}
+	if x == a+b*(T{42}[0]) {
+	}
+	if (x == a+b*T{42}[0]) {
+	}
+	if x == a+b*(T{42}[0]) {
+	}
+	if x == a+(b * (T{42}[0])) {
+	}
+	if x == a+b*(T{42}[0]) {
+	}
+	if (a + b*(T{42}[0])) == x {
+	}
+	if (a + b*(T{42}[0])) == x {
+	}
+
+	if struct{ x bool }{false}.x {
+	}
+	if (struct{ x bool }{false}.x) == false {
+	}
+	if struct{ x bool }{false}.x == false {
+	}
+}
+
+// Extra empty lines inside functions. Do respect source code line
+// breaks between statement boundaries but print at most one empty
+// line at a time.
+func _() {
+
+	const _ = 0
+
+	const _ = 1
+	type _ int
+	type _ float
+
+	var _ = 0
+	var x = 1
+
+	// Each use(x) call below should have at most one empty line before and after.
+	// Known bug: The first use call may have more than one empty line before
+	//            (see go/printer/nodes.go, func linebreak).
+
+	use(x)
+
+	if x < x {
+
+		use(x)
+
+	} else {
+
+		use(x)
+
+	}
+}
+
+// Formatting around labels.
+func _() {
+L:
+}
+
+func _() {
+	// this comment should be indented
+L:	// no semicolon needed
+}
+
+func _() {
+	switch 0 {
+	case 0:
+	L0:
+		;	// semicolon required
+	case 1:
+	L1:
+		;	// semicolon required
+	default:
+	L2:	// no semicolon needed
+	}
+}
+
+func _() {
+	f()
+L1:
+	f()
+L2:
+	;
+L3:
+}
+
+func _() {
+	// this comment should be indented
+L:
+}
+
+func _() {
+L:
+	_ = 0
+}
+
+func _() {
+	// this comment should be indented
+L:
+	_ = 0
+}
+
+func _() {
+	for {
+	L1:
+		_ = 0
+	L2:
+		_ = 0
+	}
+}
+
+func _() {
+	// this comment should be indented
+	for {
+	L1:
+		_ = 0
+	L2:
+		_ = 0
+	}
+}
+
+func _() {
+	if true {
+		_ = 0
+	}
+	_ = 0	// the indentation here should not be affected by the long label name
+AnOverlongLabel:
+	_ = 0
+
+	if true {
+		_ = 0
+	}
+	_ = 0
+
+L:
+	_ = 0
+}
+
+func _() {
+	for {
+		goto L
+	}
+L:
+
+	MoreCode()
+}
+
+func _() {
+	for {
+		goto L
+	}
+L:	// A comment on the same line as the label, followed by a single empty line.
+	// Known bug: There may be more than one empty line before MoreCode()
+	//            (see go/printer/nodes.go, func linebreak).
+
+	MoreCode()
+}
+
+func _() {
+	for {
+		goto L
+	}
+L:
+
+	// There should be a single empty line before this comment.
+	MoreCode()
+}
+
+func _() {
+	for {
+		goto AVeryLongLabelThatShouldNotAffectFormatting
+	}
+AVeryLongLabelThatShouldNotAffectFormatting:
+	// There should be a single empty line after this comment.
+
+	// There should be a single empty line before this comment.
+	MoreCode()
+}
+
+// Formatting of empty statements.
+func _() {
+
+}
+
+func _() {
+}
+
+func _() {
+}
+
+func _() {
+	f()
+}
+
+func _() {
+L:
+	;
+}
+
+func _() {
+L:
+	;
+	f()
+}
diff --git a/src/go/printer/testdata/statements.input b/src/go/printer/testdata/statements.input
new file mode 100644
index 000000000..cade1576b
--- /dev/null
+++ b/src/go/printer/testdata/statements.input
@@ -0,0 +1,555 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package statements
+
+var expr bool
+
+func use(x interface{}) {}
+
+// Formatting of multi-line return statements.
+func _f() {
+	return
+	return x, y, z
+	return T{}
+	return T{1, 2, 3},
+		x, y, z
+	return T{1, 2, 3},
+		x, y,
+		z
+	return T{1,
+		2,
+		3}
+	return T{1,
+		2,
+		3,
+	}
+	return T{
+		1,
+		2,
+		3}
+	return T{
+		1,
+		2,
+		3,
+	}
+	return T{
+		1,
+		T{1, 2, 3},
+		3,
+	}
+	return T{
+		1,
+		T{1,
+			2, 3},
+		3,
+	}
+	return T{
+		1,
+		T{1,
+			2,
+			3},
+		3,
+	}
+	return T{
+			1,
+			2,
+		}, nil
+	return T{
+			1,
+			2,
+		},
+		T{
+			x: 3,
+			y: 4,
+		}, nil
+	return T{
+			1,
+			2,
+		},
+		nil
+	return T{
+			1,
+			2,
+		},
+		T{
+			x: 3,
+			y: 4,
+		},
+		nil
+	return x + y +
+		z
+	return func() {}
+	return func() {
+		_ = 0
+	}, T{
+		1, 2,
+	}
+	return func() {
+		_ = 0
+	}
+	return func() T {
+		return T {
+			1, 2,
+		}
+	}
+}
+
+// Formatting of multi-line returns: test cases from issue 1207.
+func F() (*T, os.Error) {
+       return &T{
+               X: 1,
+               Y: 2,
+       },
+               nil
+}
+
+func G() (*T, *T, os.Error) {
+       return &T{
+               X: 1,
+               Y: 2,
+       },
+               &T{
+                       X: 3,
+                       Y: 4,
+               },
+               nil
+}
+
+func _() interface{} {
+	return &fileStat{
+			name:    basename(file.name),
+			size:    mkSize(d.FileSizeHigh, d.FileSizeLow),
+			modTime: mkModTime(d.LastWriteTime),
+			mode:    mkMode(d.FileAttributes),
+			sys:     mkSysFromFI(&d),
+		}, nil
+}
+
+// Formatting of if-statement headers.
+func _() {
+	if true {}
+	if; true {}  // no semicolon printed
+	if expr{}
+	if;expr{}  // no semicolon printed
+	if (expr){}  // no parens printed
+	if;((expr)){}  // no semicolon and parens printed
+	if x:=expr;true{
+	use(x)}
+	if x:=expr; expr {use(x)}
+}
+
+
+// Formatting of switch-statement headers.
+func _() {
+	switch {}
+	switch;{}  // no semicolon printed
+	switch expr {}
+	switch;expr{}  // no semicolon printed
+	switch (expr) {}  // no parens printed
+	switch;((expr)){}  // no semicolon and parens printed
+	switch x := expr; { default:use(
+x)
+	}
+	switch x := expr; expr {default:use(x)}
+}
+
+
+// Formatting of switch statement bodies.
+func _() {
+	switch {
+	}
+
+	switch x := 0; x {
+	case 1:
+		use(x)
+		use(x)  // followed by an empty line
+
+	case 2:  // followed by an empty line
+
+		use(x)  // followed by an empty line
+
+	case 3:  // no empty lines
+		use(x)
+		use(x)
+	}
+
+	switch x {
+	case 0:
+		use(x)
+	case 1:  // this comment should have no effect on the previous or next line
+		use(x)
+	}
+
+	switch x := 0; x {
+	case 1:
+		x = 0
+		// this comment should be indented
+	case 2:
+		x = 0
+	// this comment should not be indented, it is aligned with the next case
+	case 3:
+		x = 0
+		/* indented comment
+		   aligned
+		   aligned
+		*/
+		// bla
+		/* and more */
+	case 4:
+		x = 0
+	/* not indented comment
+	   aligned
+	   aligned
+	*/
+	// bla
+	/* and more */
+	case 5:
+	}
+}
+
+
+// Formatting of selected select statements.
+func _() {
+	select {
+	}
+	select { /* this comment should not be tab-aligned because the closing } is on the same line */ }
+	select { /* this comment should be tab-aligned */
+	}
+	select { // this comment should be tab-aligned
+	}
+	select { case <-c: }
+}
+
+
+// Formatting of for-statement headers for single-line for-loops.
+func _() {
+	for{}
+	for expr {}
+	for (expr) {}  // no parens printed
+	for;;{}  // no semicolons printed
+	for x :=expr;; {use( x)}
+	for; expr;{}  // no semicolons printed
+	for; ((expr));{}  // no semicolons and parens printed
+	for; ; expr = false {}
+	for x :=expr; expr; {use(x)}
+	for x := expr;; expr=false {use(x)}
+	for;expr;expr =false {}
+	for x := expr;expr;expr = false { use(x) }
+	for x := range []int{} { use(x) }
+	for x := range (([]int{})) { use(x) }  // no parens printed
+}
+
+
+// Formatting of for-statement headers for multi-line for-loops.
+func _() {
+	for{
+	}
+	for expr {
+	}
+	for (expr) {
+	}  // no parens printed
+	for;;{
+	}  // no semicolons printed
+	for x :=expr;; {use( x)
+	}
+	for; expr;{
+	}  // no semicolons printed
+	for; ((expr));{
+	}  // no semicolons and parens printed
+	for; ; expr = false {
+	}
+	for x :=expr; expr; {use(x)
+	}
+	for x := expr;; expr=false {use(x)
+	}
+	for;expr;expr =false {
+	}
+	for x := expr;expr;expr = false {
+	use(x)
+	}
+	for range []int{} {
+	println("foo")}
+	for x := range []int{} {
+	use(x) }
+	for x := range (([]int{})) {
+	use(x) }  // no parens printed
+}
+
+
+// Formatting of selected short single- and multi-line statements.
+func _() {
+	if cond {}
+	if cond {
+	} // multiple lines
+	if cond {} else {} // else clause always requires multiple lines
+
+	for {}
+	for i := 0; i < len(a); 1++ {}
+	for i := 0; i < len(a); 1++ { a[i] = i }
+	for i := 0; i < len(a); 1++ { a[i] = i
+	} // multiple lines
+
+	for range a{}
+	for _ = range a{}
+	for _, _ = range a{}
+	for i := range a {}
+	for i := range a { a[i] = i }
+	for i := range a { a[i] = i
+	} // multiple lines
+
+	go func() { for { a <- <-b } }()
+	defer func() { if x := recover(); x != nil { err = fmt.Sprintf("error: %s", x.msg) } }()
+}
+
+
+// Don't remove mandatory parentheses around composite literals in control clauses.
+func _() {
+	// strip parentheses - no composite literals or composite literals don't start with a type name
+	if (x) {}
+	if (((x))) {}
+	if ([]T{}) {}
+	if (([]T{})) {}
+	if ; (((([]T{})))) {}
+
+	for (x) {}
+	for (((x))) {}
+	for ([]T{}) {}
+	for (([]T{})) {}
+	for ; (((([]T{})))) ; {}
+
+	switch (x) {}
+	switch (((x))) {}
+	switch ([]T{}) {}
+	switch ; (((([]T{})))) {}
+
+	for _ = range ((([]T{T{42}}))) {}
+
+	// leave parentheses - composite literals start with a type name
+	if (T{}) {}
+	if ((T{})) {}
+	if ; ((((T{})))) {}
+
+	for (T{}) {}
+	for ((T{})) {}
+	for ; ((((T{})))) ; {}
+
+	switch (T{}) {}
+	switch ; ((((T{})))) {}
+
+	for _ = range (((T1{T{42}}))) {}
+
+	if x == (T{42}[0]) {}
+	if (x == T{42}[0]) {}
+	if (x == (T{42}[0])) {}
+	if (x == (((T{42}[0])))) {}
+	if (((x == (T{42}[0])))) {}
+	if x == a + b*(T{42}[0]) {}
+	if (x == a + b*T{42}[0]) {}
+	if (x == a + b*(T{42}[0])) {}
+	if (x == a + ((b * (T{42}[0])))) {}
+	if (((x == a + b * (T{42}[0])))) {}
+	if (((a + b * (T{42}[0])) == x)) {}
+	if (((a + b * (T{42}[0])))) == x {}
+
+	if (struct{x bool}{false}.x) {}
+	if (struct{x bool}{false}.x) == false {}
+	if (struct{x bool}{false}.x == false) {}
+}
+
+
+// Extra empty lines inside functions. Do respect source code line
+// breaks between statement boundaries but print at most one empty
+// line at a time.
+func _() {
+
+	const _ = 0
+
+	const _ = 1
+	type _ int
+	type _ float
+
+	var _ = 0
+	var x = 1
+
+	// Each use(x) call below should have at most one empty line before and after.
+	// Known bug: The first use call may have more than one empty line before
+	//            (see go/printer/nodes.go, func linebreak).
+
+
+
+	use(x)
+
+	if x < x {
+
+		use(x)
+
+	} else {
+
+		use(x)
+
+	}
+}
+
+
+// Formatting around labels.
+func _() {
+	L:
+}
+
+
+func _() {
+	// this comment should be indented
+	L: ;  // no semicolon needed
+}
+
+
+func _() {
+	switch 0 {
+	case 0:
+		L0: ;  // semicolon required
+	case 1:
+		L1: ;  // semicolon required
+	default:
+		L2: ;  // no semicolon needed
+	}
+}
+
+
+func _() {
+	f()
+L1:
+	f()
+L2:
+	;
+L3:
+}
+
+
+func _() {
+	// this comment should be indented
+	L:
+}
+
+
+func _() {
+	L: _ = 0
+}
+
+
+func _() {
+	// this comment should be indented
+	L: _ = 0
+}
+
+
+func _() {
+	for {
+	L1: _ = 0
+	L2:
+		_ = 0
+	}
+}
+
+
+func _() {
+		// this comment should be indented
+	for {
+	L1: _ = 0
+	L2:
+		_ = 0
+	}
+}
+
+
+func _() {
+	if true {
+		_ = 0
+	}
+	_ = 0  // the indentation here should not be affected by the long label name
+AnOverlongLabel:
+	_ = 0
+	
+	if true {
+		_ = 0
+	}
+	_ = 0
+
+L:	_ = 0
+}
+
+
+func _() {
+	for {
+		goto L
+	}
+L:
+
+	MoreCode()
+}
+
+
+func _() {
+	for {
+		goto L
+	}
+L:	// A comment on the same line as the label, followed by a single empty line.
+	// Known bug: There may be more than one empty line before MoreCode()
+	//            (see go/printer/nodes.go, func linebreak).
+
+
+
+
+	MoreCode()
+}
+
+
+func _() {
+	for {
+		goto L
+	}
+L:
+
+
+
+
+	// There should be a single empty line before this comment.
+	MoreCode()
+}
+
+
+func _() {
+	for {
+		goto AVeryLongLabelThatShouldNotAffectFormatting
+	}
+AVeryLongLabelThatShouldNotAffectFormatting:
+	// There should be a single empty line after this comment.
+
+	// There should be a single empty line before this comment.
+	MoreCode()
+}
+
+
+// Formatting of empty statements.
+func _() {
+	;;;;;;;;;;;;;;;;;;;;;;;;;
+}
+
+func _() {;;;;;;;;;;;;;;;;;;;;;;;;;
+}
+
+func _() {;;;;;;;;;;;;;;;;;;;;;;;;;}
+
+func _() {
+f();;;;;;;;;;;;;;;;;;;;;;;;;
+}
+
+func _() {
+L:;;;;;;;;;;;;
+}
+
+func _() {
+L:;;;;;;;;;;;;
+	f()
+}
diff --git a/src/go/scanner/errors.go b/src/go/scanner/errors.go
new file mode 100644
index 000000000..22de69c3c
--- /dev/null
+++ b/src/go/scanner/errors.go
@@ -0,0 +1,126 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package scanner
+
+import (
+	"fmt"
+	"go/token"
+	"io"
+	"sort"
+)
+
+// In an ErrorList, an error is represented by an *Error.
+// The position Pos, if valid, points to the beginning of
+// the offending token, and the error condition is described
+// by Msg.
+//
+type Error struct {
+	Pos token.Position
+	Msg string
+}
+
+// Error implements the error interface.
+func (e Error) Error() string {
+	if e.Pos.Filename != "" || e.Pos.IsValid() {
+		// don't print "<unknown position>"
+		// TODO(gri) reconsider the semantics of Position.IsValid
+		return e.Pos.String() + ": " + e.Msg
+	}
+	return e.Msg
+}
+
+// ErrorList is a list of *Errors.
+// The zero value for an ErrorList is an empty ErrorList ready to use.
+//
+type ErrorList []*Error
+
+// Add adds an Error with given position and error message to an ErrorList.
+func (p *ErrorList) Add(pos token.Position, msg string) {
+	*p = append(*p, &Error{pos, msg})
+}
+
+// Reset resets an ErrorList to no errors.
+func (p *ErrorList) Reset() { *p = (*p)[0:0] }
+
+// ErrorList implements the sort Interface.
+func (p ErrorList) Len() int      { return len(p) }
+func (p ErrorList) Swap(i, j int) { p[i], p[j] = p[j], p[i] }
+
+func (p ErrorList) Less(i, j int) bool {
+	e := &p[i].Pos
+	f := &p[j].Pos
+	// Note that it is not sufficient to simply compare file offsets because
+	// the offsets do not reflect modified line information (through //line
+	// comments).
+	if e.Filename < f.Filename {
+		return true
+	}
+	if e.Filename == f.Filename {
+		if e.Line < f.Line {
+			return true
+		}
+		if e.Line == f.Line {
+			return e.Column < f.Column
+		}
+	}
+	return false
+}
+
+// Sort sorts an ErrorList. *Error entries are sorted by position,
+// other errors are sorted by error message, and before any *Error
+// entry.
+//
+func (p ErrorList) Sort() {
+	sort.Sort(p)
+}
+
+// RemoveMultiples sorts an ErrorList and removes all but the first error per line.
+func (p *ErrorList) RemoveMultiples() {
+	sort.Sort(p)
+	var last token.Position // initial last.Line is != any legal error line
+	i := 0
+	for _, e := range *p {
+		if e.Pos.Filename != last.Filename || e.Pos.Line != last.Line {
+			last = e.Pos
+			(*p)[i] = e
+			i++
+		}
+	}
+	(*p) = (*p)[0:i]
+}
+
+// An ErrorList implements the error interface.
+func (p ErrorList) Error() string {
+	switch len(p) {
+	case 0:
+		return "no errors"
+	case 1:
+		return p[0].Error()
+	}
+	return fmt.Sprintf("%s (and %d more errors)", p[0], len(p)-1)
+}
+
+// Err returns an error equivalent to this error list.
+// If the list is empty, Err returns nil.
+func (p ErrorList) Err() error {
+	if len(p) == 0 {
+		return nil
+	}
+	return p
+}
+
+// PrintError is a utility function that prints a list of errors to w,
+// one error per line, if the err parameter is an ErrorList. Otherwise
+// it prints the err string.
+//
+func PrintError(w io.Writer, err error) {
+	if list, ok := err.(ErrorList); ok {
+		for _, e := range list {
+			fmt.Fprintf(w, "%s\n", e)
+		}
+	} else if err != nil {
+		fmt.Fprintf(w, "%s\n", err)
+	}
+}
diff --git a/src/go/scanner/example_test.go b/src/go/scanner/example_test.go
new file mode 100644
index 000000000..9004a4ad3
--- /dev/null
+++ b/src/go/scanner/example_test.go
@@ -0,0 +1,46 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package scanner_test
+
+import (
+	"fmt"
+	"go/scanner"
+	"go/token"
+)
+
+func ExampleScanner_Scan() {
+	// src is the input that we want to tokenize.
+	src := []byte("cos(x) + 1i*sin(x) // Euler")
+
+	// Initialize the scanner.
+	var s scanner.Scanner
+	fset := token.NewFileSet()                      // positions are relative to fset
+	file := fset.AddFile("", fset.Base(), len(src)) // register input "file"
+	s.Init(file, src, nil /* no error handler */, scanner.ScanComments)
+
+	// Repeated calls to Scan yield the token sequence found in the input.
+	for {
+		pos, tok, lit := s.Scan()
+		if tok == token.EOF {
+			break
+		}
+		fmt.Printf("%s\t%s\t%q\n", fset.Position(pos), tok, lit)
+	}
+
+	// output:
+	// 1:1	IDENT	"cos"
+	// 1:4	(	""
+	// 1:5	IDENT	"x"
+	// 1:6	)	""
+	// 1:8	+	""
+	// 1:10	IMAG	"1i"
+	// 1:12	*	""
+	// 1:13	IDENT	"sin"
+	// 1:16	(	""
+	// 1:17	IDENT	"x"
+	// 1:18	)	""
+	// 1:20	;	"\n"
+	// 1:20	COMMENT	"// Euler"
+}
diff --git a/src/go/scanner/scanner.go b/src/go/scanner/scanner.go
new file mode 100644
index 000000000..cec82ea10
--- /dev/null
+++ b/src/go/scanner/scanner.go
@@ -0,0 +1,760 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package scanner implements a scanner for Go source text.
+// It takes a []byte as source which can then be tokenized
+// through repeated calls to the Scan method.
+//
+package scanner
+
+import (
+	"bytes"
+	"fmt"
+	"go/token"
+	"path/filepath"
+	"strconv"
+	"unicode"
+	"unicode/utf8"
+)
+
+// An ErrorHandler may be provided to Scanner.Init. If a syntax error is
+// encountered and a handler was installed, the handler is called with a
+// position and an error message. The position points to the beginning of
+// the offending token.
+//
+type ErrorHandler func(pos token.Position, msg string)
+
+// A Scanner holds the scanner's internal state while processing
+// a given text.  It can be allocated as part of another data
+// structure but must be initialized via Init before use.
+//
+type Scanner struct {
+	// immutable state
+	file *token.File  // source file handle
+	dir  string       // directory portion of file.Name()
+	src  []byte       // source
+	err  ErrorHandler // error reporting; or nil
+	mode Mode         // scanning mode
+
+	// scanning state
+	ch         rune // current character
+	offset     int  // character offset
+	rdOffset   int  // reading offset (position after current character)
+	lineOffset int  // current line offset
+	insertSemi bool // insert a semicolon before next newline
+
+	// public state - ok to modify
+	ErrorCount int // number of errors encountered
+}
+
+const bom = 0xFEFF // byte order mark, only permitted as very first character
+
+// Read the next Unicode char into s.ch.
+// s.ch < 0 means end-of-file.
+//
+func (s *Scanner) next() {
+	if s.rdOffset < len(s.src) {
+		s.offset = s.rdOffset
+		if s.ch == '\n' {
+			s.lineOffset = s.offset
+			s.file.AddLine(s.offset)
+		}
+		r, w := rune(s.src[s.rdOffset]), 1
+		switch {
+		case r == 0:
+			s.error(s.offset, "illegal character NUL")
+		case r >= 0x80:
+			// not ASCII
+			r, w = utf8.DecodeRune(s.src[s.rdOffset:])
+			if r == utf8.RuneError && w == 1 {
+				s.error(s.offset, "illegal UTF-8 encoding")
+			} else if r == bom && s.offset > 0 {
+				s.error(s.offset, "illegal byte order mark")
+			}
+		}
+		s.rdOffset += w
+		s.ch = r
+	} else {
+		s.offset = len(s.src)
+		if s.ch == '\n' {
+			s.lineOffset = s.offset
+			s.file.AddLine(s.offset)
+		}
+		s.ch = -1 // eof
+	}
+}
+
+// A mode value is a set of flags (or 0).
+// They control scanner behavior.
+//
+type Mode uint
+
+const (
+	ScanComments    Mode = 1 << iota // return comments as COMMENT tokens
+	dontInsertSemis                  // do not automatically insert semicolons - for testing only
+)
+
+// Init prepares the scanner s to tokenize the text src by setting the
+// scanner at the beginning of src. The scanner uses the file set file
+// for position information and it adds line information for each line.
+// It is ok to re-use the same file when re-scanning the same file as
+// line information which is already present is ignored. Init causes a
+// panic if the file size does not match the src size.
+//
+// Calls to Scan will invoke the error handler err if they encounter a
+// syntax error and err is not nil. Also, for each error encountered,
+// the Scanner field ErrorCount is incremented by one. The mode parameter
+// determines how comments are handled.
+//
+// Note that Init may call err if there is an error in the first character
+// of the file.
+//
+func (s *Scanner) Init(file *token.File, src []byte, err ErrorHandler, mode Mode) {
+	// Explicitly initialize all fields since a scanner may be reused.
+	if file.Size() != len(src) {
+		panic(fmt.Sprintf("file size (%d) does not match src len (%d)", file.Size(), len(src)))
+	}
+	s.file = file
+	s.dir, _ = filepath.Split(file.Name())
+	s.src = src
+	s.err = err
+	s.mode = mode
+
+	s.ch = ' '
+	s.offset = 0
+	s.rdOffset = 0
+	s.lineOffset = 0
+	s.insertSemi = false
+	s.ErrorCount = 0
+
+	s.next()
+	if s.ch == bom {
+		s.next() // ignore BOM at file beginning
+	}
+}
+
+func (s *Scanner) error(offs int, msg string) {
+	if s.err != nil {
+		s.err(s.file.Position(s.file.Pos(offs)), msg)
+	}
+	s.ErrorCount++
+}
+
+var prefix = []byte("//line ")
+
+func (s *Scanner) interpretLineComment(text []byte) {
+	if bytes.HasPrefix(text, prefix) {
+		// get filename and line number, if any
+		if i := bytes.LastIndex(text, []byte{':'}); i > 0 {
+			if line, err := strconv.Atoi(string(text[i+1:])); err == nil && line > 0 {
+				// valid //line filename:line comment
+				filename := string(bytes.TrimSpace(text[len(prefix):i]))
+				if filename != "" {
+					filename = filepath.Clean(filename)
+					if !filepath.IsAbs(filename) {
+						// make filename relative to current directory
+						filename = filepath.Join(s.dir, filename)
+					}
+				}
+				// update scanner position
+				s.file.AddLineInfo(s.lineOffset+len(text)+1, filename, line) // +len(text)+1 since comment applies to next line
+			}
+		}
+	}
+}
+
+func (s *Scanner) scanComment() string {
+	// initial '/' already consumed; s.ch == '/' || s.ch == '*'
+	offs := s.offset - 1 // position of initial '/'
+	hasCR := false
+
+	if s.ch == '/' {
+		//-style comment
+		s.next()
+		for s.ch != '\n' && s.ch >= 0 {
+			if s.ch == '\r' {
+				hasCR = true
+			}
+			s.next()
+		}
+		if offs == s.lineOffset {
+			// comment starts at the beginning of the current line
+			s.interpretLineComment(s.src[offs:s.offset])
+		}
+		goto exit
+	}
+
+	/*-style comment */
+	s.next()
+	for s.ch >= 0 {
+		ch := s.ch
+		if ch == '\r' {
+			hasCR = true
+		}
+		s.next()
+		if ch == '*' && s.ch == '/' {
+			s.next()
+			goto exit
+		}
+	}
+
+	s.error(offs, "comment not terminated")
+
+exit:
+	lit := s.src[offs:s.offset]
+	if hasCR {
+		lit = stripCR(lit)
+	}
+
+	return string(lit)
+}
+
+func (s *Scanner) findLineEnd() bool {
+	// initial '/' already consumed
+
+	defer func(offs int) {
+		// reset scanner state to where it was upon calling findLineEnd
+		s.ch = '/'
+		s.offset = offs
+		s.rdOffset = offs + 1
+		s.next() // consume initial '/' again
+	}(s.offset - 1)
+
+	// read ahead until a newline, EOF, or non-comment token is found
+	for s.ch == '/' || s.ch == '*' {
+		if s.ch == '/' {
+			//-style comment always contains a newline
+			return true
+		}
+		/*-style comment: look for newline */
+		s.next()
+		for s.ch >= 0 {
+			ch := s.ch
+			if ch == '\n' {
+				return true
+			}
+			s.next()
+			if ch == '*' && s.ch == '/' {
+				s.next()
+				break
+			}
+		}
+		s.skipWhitespace() // s.insertSemi is set
+		if s.ch < 0 || s.ch == '\n' {
+			return true
+		}
+		if s.ch != '/' {
+			// non-comment token
+			return false
+		}
+		s.next() // consume '/'
+	}
+
+	return false
+}
+
+func isLetter(ch rune) bool {
+	return 'a' <= ch && ch <= 'z' || 'A' <= ch && ch <= 'Z' || ch == '_' || ch >= 0x80 && unicode.IsLetter(ch)
+}
+
+func isDigit(ch rune) bool {
+	return '0' <= ch && ch <= '9' || ch >= 0x80 && unicode.IsDigit(ch)
+}
+
+func (s *Scanner) scanIdentifier() string {
+	offs := s.offset
+	for isLetter(s.ch) || isDigit(s.ch) {
+		s.next()
+	}
+	return string(s.src[offs:s.offset])
+}
+
+func digitVal(ch rune) int {
+	switch {
+	case '0' <= ch && ch <= '9':
+		return int(ch - '0')
+	case 'a' <= ch && ch <= 'f':
+		return int(ch - 'a' + 10)
+	case 'A' <= ch && ch <= 'F':
+		return int(ch - 'A' + 10)
+	}
+	return 16 // larger than any legal digit val
+}
+
+func (s *Scanner) scanMantissa(base int) {
+	for digitVal(s.ch) < base {
+		s.next()
+	}
+}
+
+func (s *Scanner) scanNumber(seenDecimalPoint bool) (token.Token, string) {
+	// digitVal(s.ch) < 10
+	offs := s.offset
+	tok := token.INT
+
+	if seenDecimalPoint {
+		offs--
+		tok = token.FLOAT
+		s.scanMantissa(10)
+		goto exponent
+	}
+
+	if s.ch == '0' {
+		// int or float
+		offs := s.offset
+		s.next()
+		if s.ch == 'x' || s.ch == 'X' {
+			// hexadecimal int
+			s.next()
+			s.scanMantissa(16)
+			if s.offset-offs <= 2 {
+				// only scanned "0x" or "0X"
+				s.error(offs, "illegal hexadecimal number")
+			}
+		} else {
+			// octal int or float
+			seenDecimalDigit := false
+			s.scanMantissa(8)
+			if s.ch == '8' || s.ch == '9' {
+				// illegal octal int or float
+				seenDecimalDigit = true
+				s.scanMantissa(10)
+			}
+			if s.ch == '.' || s.ch == 'e' || s.ch == 'E' || s.ch == 'i' {
+				goto fraction
+			}
+			// octal int
+			if seenDecimalDigit {
+				s.error(offs, "illegal octal number")
+			}
+		}
+		goto exit
+	}
+
+	// decimal int or float
+	s.scanMantissa(10)
+
+fraction:
+	if s.ch == '.' {
+		tok = token.FLOAT
+		s.next()
+		s.scanMantissa(10)
+	}
+
+exponent:
+	if s.ch == 'e' || s.ch == 'E' {
+		tok = token.FLOAT
+		s.next()
+		if s.ch == '-' || s.ch == '+' {
+			s.next()
+		}
+		s.scanMantissa(10)
+	}
+
+	if s.ch == 'i' {
+		tok = token.IMAG
+		s.next()
+	}
+
+exit:
+	return tok, string(s.src[offs:s.offset])
+}
+
+// scanEscape parses an escape sequence where rune is the accepted
+// escaped quote. In case of a syntax error, it stops at the offending
+// character (without consuming it) and returns false. Otherwise
+// it returns true.
+func (s *Scanner) scanEscape(quote rune) bool {
+	offs := s.offset
+
+	var n int
+	var base, max uint32
+	switch s.ch {
+	case 'a', 'b', 'f', 'n', 'r', 't', 'v', '\\', quote:
+		s.next()
+		return true
+	case '0', '1', '2', '3', '4', '5', '6', '7':
+		n, base, max = 3, 8, 255
+	case 'x':
+		s.next()
+		n, base, max = 2, 16, 255
+	case 'u':
+		s.next()
+		n, base, max = 4, 16, unicode.MaxRune
+	case 'U':
+		s.next()
+		n, base, max = 8, 16, unicode.MaxRune
+	default:
+		msg := "unknown escape sequence"
+		if s.ch < 0 {
+			msg = "escape sequence not terminated"
+		}
+		s.error(offs, msg)
+		return false
+	}
+
+	var x uint32
+	for n > 0 {
+		d := uint32(digitVal(s.ch))
+		if d >= base {
+			msg := fmt.Sprintf("illegal character %#U in escape sequence", s.ch)
+			if s.ch < 0 {
+				msg = "escape sequence not terminated"
+			}
+			s.error(s.offset, msg)
+			return false
+		}
+		x = x*base + d
+		s.next()
+		n--
+	}
+
+	if x > max || 0xD800 <= x && x < 0xE000 {
+		s.error(offs, "escape sequence is invalid Unicode code point")
+		return false
+	}
+
+	return true
+}
+
+func (s *Scanner) scanRune() string {
+	// '\'' opening already consumed
+	offs := s.offset - 1
+
+	valid := true
+	n := 0
+	for {
+		ch := s.ch
+		if ch == '\n' || ch < 0 {
+			// only report error if we don't have one already
+			if valid {
+				s.error(offs, "rune literal not terminated")
+				valid = false
+			}
+			break
+		}
+		s.next()
+		if ch == '\'' {
+			break
+		}
+		n++
+		if ch == '\\' {
+			if !s.scanEscape('\'') {
+				valid = false
+			}
+			// continue to read to closing quote
+		}
+	}
+
+	if valid && n != 1 {
+		s.error(offs, "illegal rune literal")
+	}
+
+	return string(s.src[offs:s.offset])
+}
+
+func (s *Scanner) scanString() string {
+	// '"' opening already consumed
+	offs := s.offset - 1
+
+	for {
+		ch := s.ch
+		if ch == '\n' || ch < 0 {
+			s.error(offs, "string literal not terminated")
+			break
+		}
+		s.next()
+		if ch == '"' {
+			break
+		}
+		if ch == '\\' {
+			s.scanEscape('"')
+		}
+	}
+
+	return string(s.src[offs:s.offset])
+}
+
+func stripCR(b []byte) []byte {
+	c := make([]byte, len(b))
+	i := 0
+	for _, ch := range b {
+		if ch != '\r' {
+			c[i] = ch
+			i++
+		}
+	}
+	return c[:i]
+}
+
+func (s *Scanner) scanRawString() string {
+	// '`' opening already consumed
+	offs := s.offset - 1
+
+	hasCR := false
+	for {
+		ch := s.ch
+		if ch < 0 {
+			s.error(offs, "raw string literal not terminated")
+			break
+		}
+		s.next()
+		if ch == '`' {
+			break
+		}
+		if ch == '\r' {
+			hasCR = true
+		}
+	}
+
+	lit := s.src[offs:s.offset]
+	if hasCR {
+		lit = stripCR(lit)
+	}
+
+	return string(lit)
+}
+
+func (s *Scanner) skipWhitespace() {
+	for s.ch == ' ' || s.ch == '\t' || s.ch == '\n' && !s.insertSemi || s.ch == '\r' {
+		s.next()
+	}
+}
+
+// Helper functions for scanning multi-byte tokens such as >> += >>= .
+// Different routines recognize different length tok_i based on matches
+// of ch_i. If a token ends in '=', the result is tok1 or tok3
+// respectively. Otherwise, the result is tok0 if there was no other
+// matching character, or tok2 if the matching character was ch2.
+
+func (s *Scanner) switch2(tok0, tok1 token.Token) token.Token {
+	if s.ch == '=' {
+		s.next()
+		return tok1
+	}
+	return tok0
+}
+
+func (s *Scanner) switch3(tok0, tok1 token.Token, ch2 rune, tok2 token.Token) token.Token {
+	if s.ch == '=' {
+		s.next()
+		return tok1
+	}
+	if s.ch == ch2 {
+		s.next()
+		return tok2
+	}
+	return tok0
+}
+
+func (s *Scanner) switch4(tok0, tok1 token.Token, ch2 rune, tok2, tok3 token.Token) token.Token {
+	if s.ch == '=' {
+		s.next()
+		return tok1
+	}
+	if s.ch == ch2 {
+		s.next()
+		if s.ch == '=' {
+			s.next()
+			return tok3
+		}
+		return tok2
+	}
+	return tok0
+}
+
+// Scan scans the next token and returns the token position, the token,
+// and its literal string if applicable. The source end is indicated by
+// token.EOF.
+//
+// If the returned token is a literal (token.IDENT, token.INT, token.FLOAT,
+// token.IMAG, token.CHAR, token.STRING) or token.COMMENT, the literal string
+// has the corresponding value.
+//
+// If the returned token is a keyword, the literal string is the keyword.
+//
+// If the returned token is token.SEMICOLON, the corresponding
+// literal string is ";" if the semicolon was present in the source,
+// and "\n" if the semicolon was inserted because of a newline or
+// at EOF.
+//
+// If the returned token is token.ILLEGAL, the literal string is the
+// offending character.
+//
+// In all other cases, Scan returns an empty literal string.
+//
+// For more tolerant parsing, Scan will return a valid token if
+// possible even if a syntax error was encountered. Thus, even
+// if the resulting token sequence contains no illegal tokens,
+// a client may not assume that no error occurred. Instead it
+// must check the scanner's ErrorCount or the number of calls
+// of the error handler, if there was one installed.
+//
+// Scan adds line information to the file added to the file
+// set with Init. Token positions are relative to that file
+// and thus relative to the file set.
+//
+func (s *Scanner) Scan() (pos token.Pos, tok token.Token, lit string) {
+scanAgain:
+	s.skipWhitespace()
+
+	// current token start
+	pos = s.file.Pos(s.offset)
+
+	// determine token value
+	insertSemi := false
+	switch ch := s.ch; {
+	case isLetter(ch):
+		lit = s.scanIdentifier()
+		if len(lit) > 1 {
+			// keywords are longer than one letter - avoid lookup otherwise
+			tok = token.Lookup(lit)
+			switch tok {
+			case token.IDENT, token.BREAK, token.CONTINUE, token.FALLTHROUGH, token.RETURN:
+				insertSemi = true
+			}
+		} else {
+			insertSemi = true
+			tok = token.IDENT
+		}
+	case '0' <= ch && ch <= '9':
+		insertSemi = true
+		tok, lit = s.scanNumber(false)
+	default:
+		s.next() // always make progress
+		switch ch {
+		case -1:
+			if s.insertSemi {
+				s.insertSemi = false // EOF consumed
+				return pos, token.SEMICOLON, "\n"
+			}
+			tok = token.EOF
+		case '\n':
+			// we only reach here if s.insertSemi was
+			// set in the first place and exited early
+			// from s.skipWhitespace()
+			s.insertSemi = false // newline consumed
+			return pos, token.SEMICOLON, "\n"
+		case '"':
+			insertSemi = true
+			tok = token.STRING
+			lit = s.scanString()
+		case '\'':
+			insertSemi = true
+			tok = token.CHAR
+			lit = s.scanRune()
+		case '`':
+			insertSemi = true
+			tok = token.STRING
+			lit = s.scanRawString()
+		case ':':
+			tok = s.switch2(token.COLON, token.DEFINE)
+		case '.':
+			if '0' <= s.ch && s.ch <= '9' {
+				insertSemi = true
+				tok, lit = s.scanNumber(true)
+			} else if s.ch == '.' {
+				s.next()
+				if s.ch == '.' {
+					s.next()
+					tok = token.ELLIPSIS
+				}
+			} else {
+				tok = token.PERIOD
+			}
+		case ',':
+			tok = token.COMMA
+		case ';':
+			tok = token.SEMICOLON
+			lit = ";"
+		case '(':
+			tok = token.LPAREN
+		case ')':
+			insertSemi = true
+			tok = token.RPAREN
+		case '[':
+			tok = token.LBRACK
+		case ']':
+			insertSemi = true
+			tok = token.RBRACK
+		case '{':
+			tok = token.LBRACE
+		case '}':
+			insertSemi = true
+			tok = token.RBRACE
+		case '+':
+			tok = s.switch3(token.ADD, token.ADD_ASSIGN, '+', token.INC)
+			if tok == token.INC {
+				insertSemi = true
+			}
+		case '-':
+			tok = s.switch3(token.SUB, token.SUB_ASSIGN, '-', token.DEC)
+			if tok == token.DEC {
+				insertSemi = true
+			}
+		case '*':
+			tok = s.switch2(token.MUL, token.MUL_ASSIGN)
+		case '/':
+			if s.ch == '/' || s.ch == '*' {
+				// comment
+				if s.insertSemi && s.findLineEnd() {
+					// reset position to the beginning of the comment
+					s.ch = '/'
+					s.offset = s.file.Offset(pos)
+					s.rdOffset = s.offset + 1
+					s.insertSemi = false // newline consumed
+					return pos, token.SEMICOLON, "\n"
+				}
+				lit = s.scanComment()
+				if s.mode&ScanComments == 0 {
+					// skip comment
+					s.insertSemi = false // newline consumed
+					goto scanAgain
+				}
+				tok = token.COMMENT
+			} else {
+				tok = s.switch2(token.QUO, token.QUO_ASSIGN)
+			}
+		case '%':
+			tok = s.switch2(token.REM, token.REM_ASSIGN)
+		case '^':
+			tok = s.switch2(token.XOR, token.XOR_ASSIGN)
+		case '<':
+			if s.ch == '-' {
+				s.next()
+				tok = token.ARROW
+			} else {
+				tok = s.switch4(token.LSS, token.LEQ, '<', token.SHL, token.SHL_ASSIGN)
+			}
+		case '>':
+			tok = s.switch4(token.GTR, token.GEQ, '>', token.SHR, token.SHR_ASSIGN)
+		case '=':
+			tok = s.switch2(token.ASSIGN, token.EQL)
+		case '!':
+			tok = s.switch2(token.NOT, token.NEQ)
+		case '&':
+			if s.ch == '^' {
+				s.next()
+				tok = s.switch2(token.AND_NOT, token.AND_NOT_ASSIGN)
+			} else {
+				tok = s.switch3(token.AND, token.AND_ASSIGN, '&', token.LAND)
+			}
+		case '|':
+			tok = s.switch3(token.OR, token.OR_ASSIGN, '|', token.LOR)
+		default:
+			// next reports unexpected BOMs - don't repeat
+			if ch != bom {
+				s.error(s.file.Offset(pos), fmt.Sprintf("illegal character %#U", ch))
+			}
+			insertSemi = s.insertSemi // preserve insertSemi info
+			tok = token.ILLEGAL
+			lit = string(ch)
+		}
+	}
+	if s.mode&dontInsertSemis == 0 {
+		s.insertSemi = insertSemi
+	}
+
+	return
+}
diff --git a/src/go/scanner/scanner_test.go b/src/go/scanner/scanner_test.go
new file mode 100644
index 000000000..fc450d8a6
--- /dev/null
+++ b/src/go/scanner/scanner_test.go
@@ -0,0 +1,775 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package scanner
+
+import (
+	"go/token"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+)
+
+var fset = token.NewFileSet()
+
+const /* class */ (
+	special = iota
+	literal
+	operator
+	keyword
+)
+
+func tokenclass(tok token.Token) int {
+	switch {
+	case tok.IsLiteral():
+		return literal
+	case tok.IsOperator():
+		return operator
+	case tok.IsKeyword():
+		return keyword
+	}
+	return special
+}
+
+type elt struct {
+	tok   token.Token
+	lit   string
+	class int
+}
+
+var tokens = [...]elt{
+	// Special tokens
+	{token.COMMENT, "/* a comment */", special},
+	{token.COMMENT, "// a comment \n", special},
+	{token.COMMENT, "/*\r*/", special},
+	{token.COMMENT, "//\r\n", special},
+
+	// Identifiers and basic type literals
+	{token.IDENT, "foobar", literal},
+	{token.IDENT, "a۰۱۸", literal},
+	{token.IDENT, "foo६४", literal},
+	{token.IDENT, "bar９８７６", literal},
+	{token.IDENT, "ŝ", literal},    // was bug (issue 4000)
+	{token.IDENT, "ŝfoo", literal}, // was bug (issue 4000)
+	{token.INT, "0", literal},
+	{token.INT, "1", literal},
+	{token.INT, "123456789012345678890", literal},
+	{token.INT, "01234567", literal},
+	{token.INT, "0xcafebabe", literal},
+	{token.FLOAT, "0.", literal},
+	{token.FLOAT, ".0", literal},
+	{token.FLOAT, "3.14159265", literal},
+	{token.FLOAT, "1e0", literal},
+	{token.FLOAT, "1e+100", literal},
+	{token.FLOAT, "1e-100", literal},
+	{token.FLOAT, "2.71828e-1000", literal},
+	{token.IMAG, "0i", literal},
+	{token.IMAG, "1i", literal},
+	{token.IMAG, "012345678901234567889i", literal},
+	{token.IMAG, "123456789012345678890i", literal},
+	{token.IMAG, "0.i", literal},
+	{token.IMAG, ".0i", literal},
+	{token.IMAG, "3.14159265i", literal},
+	{token.IMAG, "1e0i", literal},
+	{token.IMAG, "1e+100i", literal},
+	{token.IMAG, "1e-100i", literal},
+	{token.IMAG, "2.71828e-1000i", literal},
+	{token.CHAR, "'a'", literal},
+	{token.CHAR, "'\\000'", literal},
+	{token.CHAR, "'\\xFF'", literal},
+	{token.CHAR, "'\\uff16'", literal},
+	{token.CHAR, "'\\U0000ff16'", literal},
+	{token.STRING, "`foobar`", literal},
+	{token.STRING, "`" + `foo
+	                        bar` +
+		"`",
+		literal,
+	},
+	{token.STRING, "`\r`", literal},
+	{token.STRING, "`foo\r\nbar`", literal},
+
+	// Operators and delimiters
+	{token.ADD, "+", operator},
+	{token.SUB, "-", operator},
+	{token.MUL, "*", operator},
+	{token.QUO, "/", operator},
+	{token.REM, "%", operator},
+
+	{token.AND, "&", operator},
+	{token.OR, "|", operator},
+	{token.XOR, "^", operator},
+	{token.SHL, "<<", operator},
+	{token.SHR, ">>", operator},
+	{token.AND_NOT, "&^", operator},
+
+	{token.ADD_ASSIGN, "+=", operator},
+	{token.SUB_ASSIGN, "-=", operator},
+	{token.MUL_ASSIGN, "*=", operator},
+	{token.QUO_ASSIGN, "/=", operator},
+	{token.REM_ASSIGN, "%=", operator},
+
+	{token.AND_ASSIGN, "&=", operator},
+	{token.OR_ASSIGN, "|=", operator},
+	{token.XOR_ASSIGN, "^=", operator},
+	{token.SHL_ASSIGN, "<<=", operator},
+	{token.SHR_ASSIGN, ">>=", operator},
+	{token.AND_NOT_ASSIGN, "&^=", operator},
+
+	{token.LAND, "&&", operator},
+	{token.LOR, "||", operator},
+	{token.ARROW, "<-", operator},
+	{token.INC, "++", operator},
+	{token.DEC, "--", operator},
+
+	{token.EQL, "==", operator},
+	{token.LSS, "<", operator},
+	{token.GTR, ">", operator},
+	{token.ASSIGN, "=", operator},
+	{token.NOT, "!", operator},
+
+	{token.NEQ, "!=", operator},
+	{token.LEQ, "<=", operator},
+	{token.GEQ, ">=", operator},
+	{token.DEFINE, ":=", operator},
+	{token.ELLIPSIS, "...", operator},
+
+	{token.LPAREN, "(", operator},
+	{token.LBRACK, "[", operator},
+	{token.LBRACE, "{", operator},
+	{token.COMMA, ",", operator},
+	{token.PERIOD, ".", operator},
+
+	{token.RPAREN, ")", operator},
+	{token.RBRACK, "]", operator},
+	{token.RBRACE, "}", operator},
+	{token.SEMICOLON, ";", operator},
+	{token.COLON, ":", operator},
+
+	// Keywords
+	{token.BREAK, "break", keyword},
+	{token.CASE, "case", keyword},
+	{token.CHAN, "chan", keyword},
+	{token.CONST, "const", keyword},
+	{token.CONTINUE, "continue", keyword},
+
+	{token.DEFAULT, "default", keyword},
+	{token.DEFER, "defer", keyword},
+	{token.ELSE, "else", keyword},
+	{token.FALLTHROUGH, "fallthrough", keyword},
+	{token.FOR, "for", keyword},
+
+	{token.FUNC, "func", keyword},
+	{token.GO, "go", keyword},
+	{token.GOTO, "goto", keyword},
+	{token.IF, "if", keyword},
+	{token.IMPORT, "import", keyword},
+
+	{token.INTERFACE, "interface", keyword},
+	{token.MAP, "map", keyword},
+	{token.PACKAGE, "package", keyword},
+	{token.RANGE, "range", keyword},
+	{token.RETURN, "return", keyword},
+
+	{token.SELECT, "select", keyword},
+	{token.STRUCT, "struct", keyword},
+	{token.SWITCH, "switch", keyword},
+	{token.TYPE, "type", keyword},
+	{token.VAR, "var", keyword},
+}
+
+const whitespace = "  \t  \n\n\n" // to separate tokens
+
+var source = func() []byte {
+	var src []byte
+	for _, t := range tokens {
+		src = append(src, t.lit...)
+		src = append(src, whitespace...)
+	}
+	return src
+}()
+
+func newlineCount(s string) int {
+	n := 0
+	for i := 0; i < len(s); i++ {
+		if s[i] == '\n' {
+			n++
+		}
+	}
+	return n
+}
+
+func checkPos(t *testing.T, lit string, p token.Pos, expected token.Position) {
+	pos := fset.Position(p)
+	if pos.Filename != expected.Filename {
+		t.Errorf("bad filename for %q: got %s, expected %s", lit, pos.Filename, expected.Filename)
+	}
+	if pos.Offset != expected.Offset {
+		t.Errorf("bad position for %q: got %d, expected %d", lit, pos.Offset, expected.Offset)
+	}
+	if pos.Line != expected.Line {
+		t.Errorf("bad line for %q: got %d, expected %d", lit, pos.Line, expected.Line)
+	}
+	if pos.Column != expected.Column {
+		t.Errorf("bad column for %q: got %d, expected %d", lit, pos.Column, expected.Column)
+	}
+}
+
+// Verify that calling Scan() provides the correct results.
+func TestScan(t *testing.T) {
+	whitespace_linecount := newlineCount(whitespace)
+
+	// error handler
+	eh := func(_ token.Position, msg string) {
+		t.Errorf("error handler called (msg = %s)", msg)
+	}
+
+	// verify scan
+	var s Scanner
+	s.Init(fset.AddFile("", fset.Base(), len(source)), source, eh, ScanComments|dontInsertSemis)
+
+	// set up expected position
+	epos := token.Position{
+		Filename: "",
+		Offset:   0,
+		Line:     1,
+		Column:   1,
+	}
+
+	index := 0
+	for {
+		pos, tok, lit := s.Scan()
+
+		// check position
+		if tok == token.EOF {
+			// correction for EOF
+			epos.Line = newlineCount(string(source))
+			epos.Column = 2
+		}
+		checkPos(t, lit, pos, epos)
+
+		// check token
+		e := elt{token.EOF, "", special}
+		if index < len(tokens) {
+			e = tokens[index]
+			index++
+		}
+		if tok != e.tok {
+			t.Errorf("bad token for %q: got %s, expected %s", lit, tok, e.tok)
+		}
+
+		// check token class
+		if tokenclass(tok) != e.class {
+			t.Errorf("bad class for %q: got %d, expected %d", lit, tokenclass(tok), e.class)
+		}
+
+		// check literal
+		elit := ""
+		switch e.tok {
+		case token.COMMENT:
+			// no CRs in comments
+			elit = string(stripCR([]byte(e.lit)))
+			//-style comment literal doesn't contain newline
+			if elit[1] == '/' {
+				elit = elit[0 : len(elit)-1]
+			}
+		case token.IDENT:
+			elit = e.lit
+		case token.SEMICOLON:
+			elit = ";"
+		default:
+			if e.tok.IsLiteral() {
+				// no CRs in raw string literals
+				elit = e.lit
+				if elit[0] == '`' {
+					elit = string(stripCR([]byte(elit)))
+				}
+			} else if e.tok.IsKeyword() {
+				elit = e.lit
+			}
+		}
+		if lit != elit {
+			t.Errorf("bad literal for %q: got %q, expected %q", lit, lit, elit)
+		}
+
+		if tok == token.EOF {
+			break
+		}
+
+		// update position
+		epos.Offset += len(e.lit) + len(whitespace)
+		epos.Line += newlineCount(e.lit) + whitespace_linecount
+
+	}
+
+	if s.ErrorCount != 0 {
+		t.Errorf("found %d errors", s.ErrorCount)
+	}
+}
+
+func checkSemi(t *testing.T, line string, mode Mode) {
+	var S Scanner
+	file := fset.AddFile("TestSemis", fset.Base(), len(line))
+	S.Init(file, []byte(line), nil, mode)
+	pos, tok, lit := S.Scan()
+	for tok != token.EOF {
+		if tok == token.ILLEGAL {
+			// the illegal token literal indicates what
+			// kind of semicolon literal to expect
+			semiLit := "\n"
+			if lit[0] == '#' {
+				semiLit = ";"
+			}
+			// next token must be a semicolon
+			semiPos := file.Position(pos)
+			semiPos.Offset++
+			semiPos.Column++
+			pos, tok, lit = S.Scan()
+			if tok == token.SEMICOLON {
+				if lit != semiLit {
+					t.Errorf(`bad literal for %q: got %q, expected %q`, line, lit, semiLit)
+				}
+				checkPos(t, line, pos, semiPos)
+			} else {
+				t.Errorf("bad token for %q: got %s, expected ;", line, tok)
+			}
+		} else if tok == token.SEMICOLON {
+			t.Errorf("bad token for %q: got ;, expected no ;", line)
+		}
+		pos, tok, lit = S.Scan()
+	}
+}
+
+var lines = []string{
+	// # indicates a semicolon present in the source
+	// $ indicates an automatically inserted semicolon
+	"",
+	"\ufeff#;", // first BOM is ignored
+	"#;",
+	"foo$\n",
+	"123$\n",
+	"1.2$\n",
+	"'x'$\n",
+	`"x"` + "$\n",
+	"`x`$\n",
+
+	"+\n",
+	"-\n",
+	"*\n",
+	"/\n",
+	"%\n",
+
+	"&\n",
+	"|\n",
+	"^\n",
+	"<<\n",
+	">>\n",
+	"&^\n",
+
+	"+=\n",
+	"-=\n",
+	"*=\n",
+	"/=\n",
+	"%=\n",
+
+	"&=\n",
+	"|=\n",
+	"^=\n",
+	"<<=\n",
+	">>=\n",
+	"&^=\n",
+
+	"&&\n",
+	"||\n",
+	"<-\n",
+	"++$\n",
+	"--$\n",
+
+	"==\n",
+	"<\n",
+	">\n",
+	"=\n",
+	"!\n",
+
+	"!=\n",
+	"<=\n",
+	">=\n",
+	":=\n",
+	"...\n",
+
+	"(\n",
+	"[\n",
+	"{\n",
+	",\n",
+	".\n",
+
+	")$\n",
+	"]$\n",
+	"}$\n",
+	"#;\n",
+	":\n",
+
+	"break$\n",
+	"case\n",
+	"chan\n",
+	"const\n",
+	"continue$\n",
+
+	"default\n",
+	"defer\n",
+	"else\n",
+	"fallthrough$\n",
+	"for\n",
+
+	"func\n",
+	"go\n",
+	"goto\n",
+	"if\n",
+	"import\n",
+
+	"interface\n",
+	"map\n",
+	"package\n",
+	"range\n",
+	"return$\n",
+
+	"select\n",
+	"struct\n",
+	"switch\n",
+	"type\n",
+	"var\n",
+
+	"foo$//comment\n",
+	"foo$//comment",
+	"foo$/*comment*/\n",
+	"foo$/*\n*/",
+	"foo$/*comment*/    \n",
+	"foo$/*\n*/    ",
+
+	"foo    $// comment\n",
+	"foo    $// comment",
+	"foo    $/*comment*/\n",
+	"foo    $/*\n*/",
+	"foo    $/*  */ /* \n */ bar$/**/\n",
+	"foo    $/*0*/ /*1*/ /*2*/\n",
+
+	"foo    $/*comment*/    \n",
+	"foo    $/*0*/ /*1*/ /*2*/    \n",
+	"foo	$/**/ /*-------------*/       /*----\n*/bar       $/*  \n*/baa$\n",
+	"foo    $/* an EOF terminates a line */",
+	"foo    $/* an EOF terminates a line */ /*",
+	"foo    $/* an EOF terminates a line */ //",
+
+	"package main$\n\nfunc main() {\n\tif {\n\t\treturn /* */ }$\n}$\n",
+	"package main$",
+}
+
+func TestSemis(t *testing.T) {
+	for _, line := range lines {
+		checkSemi(t, line, 0)
+		checkSemi(t, line, ScanComments)
+
+		// if the input ended in newlines, the input must tokenize the
+		// same with or without those newlines
+		for i := len(line) - 1; i >= 0 && line[i] == '\n'; i-- {
+			checkSemi(t, line[0:i], 0)
+			checkSemi(t, line[0:i], ScanComments)
+		}
+	}
+}
+
+type segment struct {
+	srcline  string // a line of source text
+	filename string // filename for current token
+	line     int    // line number for current token
+}
+
+var segments = []segment{
+	// exactly one token per line since the test consumes one token per segment
+	{"  line1", filepath.Join("dir", "TestLineComments"), 1},
+	{"\nline2", filepath.Join("dir", "TestLineComments"), 2},
+	{"\nline3  //line File1.go:100", filepath.Join("dir", "TestLineComments"), 3}, // bad line comment, ignored
+	{"\nline4", filepath.Join("dir", "TestLineComments"), 4},
+	{"\n//line File1.go:100\n  line100", filepath.Join("dir", "File1.go"), 100},
+	{"\n//line  \t :42\n  line1", "", 42},
+	{"\n//line File2.go:200\n  line200", filepath.Join("dir", "File2.go"), 200},
+	{"\n//line foo\t:42\n  line42", filepath.Join("dir", "foo"), 42},
+	{"\n //line foo:42\n  line44", filepath.Join("dir", "foo"), 44},           // bad line comment, ignored
+	{"\n//line foo 42\n  line46", filepath.Join("dir", "foo"), 46},            // bad line comment, ignored
+	{"\n//line foo:42 extra text\n  line48", filepath.Join("dir", "foo"), 48}, // bad line comment, ignored
+	{"\n//line ./foo:42\n  line42", filepath.Join("dir", "foo"), 42},
+	{"\n//line a/b/c/File1.go:100\n  line100", filepath.Join("dir", "a", "b", "c", "File1.go"), 100},
+}
+
+var unixsegments = []segment{
+	{"\n//line /bar:42\n  line42", "/bar", 42},
+}
+
+var winsegments = []segment{
+	{"\n//line c:\\bar:42\n  line42", "c:\\bar", 42},
+	{"\n//line c:\\dir\\File1.go:100\n  line100", "c:\\dir\\File1.go", 100},
+}
+
+// Verify that comments of the form "//line filename:line" are interpreted correctly.
+func TestLineComments(t *testing.T) {
+	segs := segments
+	if runtime.GOOS == "windows" {
+		segs = append(segs, winsegments...)
+	} else {
+		segs = append(segs, unixsegments...)
+	}
+
+	// make source
+	var src string
+	for _, e := range segs {
+		src += e.srcline
+	}
+
+	// verify scan
+	var S Scanner
+	file := fset.AddFile(filepath.Join("dir", "TestLineComments"), fset.Base(), len(src))
+	S.Init(file, []byte(src), nil, dontInsertSemis)
+	for _, s := range segs {
+		p, _, lit := S.Scan()
+		pos := file.Position(p)
+		checkPos(t, lit, p, token.Position{
+			Filename: s.filename,
+			Offset:   pos.Offset,
+			Line:     s.line,
+			Column:   pos.Column,
+		})
+	}
+
+	if S.ErrorCount != 0 {
+		t.Errorf("found %d errors", S.ErrorCount)
+	}
+}
+
+// Verify that initializing the same scanner more than once works correctly.
+func TestInit(t *testing.T) {
+	var s Scanner
+
+	// 1st init
+	src1 := "if true { }"
+	f1 := fset.AddFile("src1", fset.Base(), len(src1))
+	s.Init(f1, []byte(src1), nil, dontInsertSemis)
+	if f1.Size() != len(src1) {
+		t.Errorf("bad file size: got %d, expected %d", f1.Size(), len(src1))
+	}
+	s.Scan()              // if
+	s.Scan()              // true
+	_, tok, _ := s.Scan() // {
+	if tok != token.LBRACE {
+		t.Errorf("bad token: got %s, expected %s", tok, token.LBRACE)
+	}
+
+	// 2nd init
+	src2 := "go true { ]"
+	f2 := fset.AddFile("src2", fset.Base(), len(src2))
+	s.Init(f2, []byte(src2), nil, dontInsertSemis)
+	if f2.Size() != len(src2) {
+		t.Errorf("bad file size: got %d, expected %d", f2.Size(), len(src2))
+	}
+	_, tok, _ = s.Scan() // go
+	if tok != token.GO {
+		t.Errorf("bad token: got %s, expected %s", tok, token.GO)
+	}
+
+	if s.ErrorCount != 0 {
+		t.Errorf("found %d errors", s.ErrorCount)
+	}
+}
+
+func TestStdErrorHander(t *testing.T) {
+	const src = "@\n" + // illegal character, cause an error
+		"@ @\n" + // two errors on the same line
+		"//line File2:20\n" +
+		"@\n" + // different file, but same line
+		"//line File2:1\n" +
+		"@ @\n" + // same file, decreasing line number
+		"//line File1:1\n" +
+		"@ @ @" // original file, line 1 again
+
+	var list ErrorList
+	eh := func(pos token.Position, msg string) { list.Add(pos, msg) }
+
+	var s Scanner
+	s.Init(fset.AddFile("File1", fset.Base(), len(src)), []byte(src), eh, dontInsertSemis)
+	for {
+		if _, tok, _ := s.Scan(); tok == token.EOF {
+			break
+		}
+	}
+
+	if len(list) != s.ErrorCount {
+		t.Errorf("found %d errors, expected %d", len(list), s.ErrorCount)
+	}
+
+	if len(list) != 9 {
+		t.Errorf("found %d raw errors, expected 9", len(list))
+		PrintError(os.Stderr, list)
+	}
+
+	list.Sort()
+	if len(list) != 9 {
+		t.Errorf("found %d sorted errors, expected 9", len(list))
+		PrintError(os.Stderr, list)
+	}
+
+	list.RemoveMultiples()
+	if len(list) != 4 {
+		t.Errorf("found %d one-per-line errors, expected 4", len(list))
+		PrintError(os.Stderr, list)
+	}
+}
+
+type errorCollector struct {
+	cnt int            // number of errors encountered
+	msg string         // last error message encountered
+	pos token.Position // last error position encountered
+}
+
+func checkError(t *testing.T, src string, tok token.Token, pos int, lit, err string) {
+	var s Scanner
+	var h errorCollector
+	eh := func(pos token.Position, msg string) {
+		h.cnt++
+		h.msg = msg
+		h.pos = pos
+	}
+	s.Init(fset.AddFile("", fset.Base(), len(src)), []byte(src), eh, ScanComments|dontInsertSemis)
+	_, tok0, lit0 := s.Scan()
+	if tok0 != tok {
+		t.Errorf("%q: got %s, expected %s", src, tok0, tok)
+	}
+	if tok0 != token.ILLEGAL && lit0 != lit {
+		t.Errorf("%q: got literal %q, expected %q", src, lit0, lit)
+	}
+	cnt := 0
+	if err != "" {
+		cnt = 1
+	}
+	if h.cnt != cnt {
+		t.Errorf("%q: got cnt %d, expected %d", src, h.cnt, cnt)
+	}
+	if h.msg != err {
+		t.Errorf("%q: got msg %q, expected %q", src, h.msg, err)
+	}
+	if h.pos.Offset != pos {
+		t.Errorf("%q: got offset %d, expected %d", src, h.pos.Offset, pos)
+	}
+}
+
+var errors = []struct {
+	src string
+	tok token.Token
+	pos int
+	lit string
+	err string
+}{
+	{"\a", token.ILLEGAL, 0, "", "illegal character U+0007"},
+	{`#`, token.ILLEGAL, 0, "", "illegal character U+0023 '#'"},
+	{`…`, token.ILLEGAL, 0, "", "illegal character U+2026 '…'"},
+	{`' '`, token.CHAR, 0, `' '`, ""},
+	{`''`, token.CHAR, 0, `''`, "illegal rune literal"},
+	{`'12'`, token.CHAR, 0, `'12'`, "illegal rune literal"},
+	{`'123'`, token.CHAR, 0, `'123'`, "illegal rune literal"},
+	{`'\0'`, token.CHAR, 3, `'\0'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\07'`, token.CHAR, 4, `'\07'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\8'`, token.CHAR, 2, `'\8'`, "unknown escape sequence"},
+	{`'\08'`, token.CHAR, 3, `'\08'`, "illegal character U+0038 '8' in escape sequence"},
+	{`'\x'`, token.CHAR, 3, `'\x'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\x0'`, token.CHAR, 4, `'\x0'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\x0g'`, token.CHAR, 4, `'\x0g'`, "illegal character U+0067 'g' in escape sequence"},
+	{`'\u'`, token.CHAR, 3, `'\u'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\u0'`, token.CHAR, 4, `'\u0'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\u00'`, token.CHAR, 5, `'\u00'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\u000'`, token.CHAR, 6, `'\u000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\u000`, token.CHAR, 6, `'\u000`, "escape sequence not terminated"},
+	{`'\u0000'`, token.CHAR, 0, `'\u0000'`, ""},
+	{`'\U'`, token.CHAR, 3, `'\U'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U0'`, token.CHAR, 4, `'\U0'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U00'`, token.CHAR, 5, `'\U00'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U000'`, token.CHAR, 6, `'\U000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U0000'`, token.CHAR, 7, `'\U0000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U00000'`, token.CHAR, 8, `'\U00000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U000000'`, token.CHAR, 9, `'\U000000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U0000000'`, token.CHAR, 10, `'\U0000000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U0000000`, token.CHAR, 10, `'\U0000000`, "escape sequence not terminated"},
+	{`'\U00000000'`, token.CHAR, 0, `'\U00000000'`, ""},
+	{`'\Uffffffff'`, token.CHAR, 2, `'\Uffffffff'`, "escape sequence is invalid Unicode code point"},
+	{`'`, token.CHAR, 0, `'`, "rune literal not terminated"},
+	{`'\`, token.CHAR, 2, `'\`, "escape sequence not terminated"},
+	{"'\n", token.CHAR, 0, "'", "rune literal not terminated"},
+	{"'\n   ", token.CHAR, 0, "'", "rune literal not terminated"},
+	{`""`, token.STRING, 0, `""`, ""},
+	{`"abc`, token.STRING, 0, `"abc`, "string literal not terminated"},
+	{"\"abc\n", token.STRING, 0, `"abc`, "string literal not terminated"},
+	{"\"abc\n   ", token.STRING, 0, `"abc`, "string literal not terminated"},
+	{"``", token.STRING, 0, "``", ""},
+	{"`", token.STRING, 0, "`", "raw string literal not terminated"},
+	{"/**/", token.COMMENT, 0, "/**/", ""},
+	{"/*", token.COMMENT, 0, "/*", "comment not terminated"},
+	{"077", token.INT, 0, "077", ""},
+	{"078.", token.FLOAT, 0, "078.", ""},
+	{"07801234567.", token.FLOAT, 0, "07801234567.", ""},
+	{"078e0", token.FLOAT, 0, "078e0", ""},
+	{"078", token.INT, 0, "078", "illegal octal number"},
+	{"07800000009", token.INT, 0, "07800000009", "illegal octal number"},
+	{"0x", token.INT, 0, "0x", "illegal hexadecimal number"},
+	{"0X", token.INT, 0, "0X", "illegal hexadecimal number"},
+	{"\"abc\x00def\"", token.STRING, 4, "\"abc\x00def\"", "illegal character NUL"},
+	{"\"abc\x80def\"", token.STRING, 4, "\"abc\x80def\"", "illegal UTF-8 encoding"},
+	{"\ufeff\ufeff", token.ILLEGAL, 3, "\ufeff\ufeff", "illegal byte order mark"},                        // only first BOM is ignored
+	{"//\ufeff", token.COMMENT, 2, "//\ufeff", "illegal byte order mark"},                                // only first BOM is ignored
+	{"'\ufeff" + `'`, token.CHAR, 1, "'\ufeff" + `'`, "illegal byte order mark"},                         // only first BOM is ignored
+	{`"` + "abc\ufeffdef" + `"`, token.STRING, 4, `"` + "abc\ufeffdef" + `"`, "illegal byte order mark"}, // only first BOM is ignored
+}
+
+func TestScanErrors(t *testing.T) {
+	for _, e := range errors {
+		checkError(t, e.src, e.tok, e.pos, e.lit, e.err)
+	}
+}
+
+func BenchmarkScan(b *testing.B) {
+	b.StopTimer()
+	fset := token.NewFileSet()
+	file := fset.AddFile("", fset.Base(), len(source))
+	var s Scanner
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		s.Init(file, source, nil, ScanComments)
+		for {
+			_, tok, _ := s.Scan()
+			if tok == token.EOF {
+				break
+			}
+		}
+	}
+}
+
+func BenchmarkScanFile(b *testing.B) {
+	b.StopTimer()
+	const filename = "scanner.go"
+	src, err := ioutil.ReadFile(filename)
+	if err != nil {
+		panic(err)
+	}
+	fset := token.NewFileSet()
+	file := fset.AddFile(filename, fset.Base(), len(src))
+	b.SetBytes(int64(len(src)))
+	var s Scanner
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		s.Init(file, src, nil, ScanComments)
+		for {
+			_, tok, _ := s.Scan()
+			if tok == token.EOF {
+				break
+			}
+		}
+	}
+}
diff --git a/src/go/token/position.go b/src/go/token/position.go
new file mode 100644
index 000000000..82d90eeb7
--- /dev/null
+++ b/src/go/token/position.go
@@ -0,0 +1,485 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package token
+
+import (
+	"fmt"
+	"sort"
+	"sync"
+)
+
+// -----------------------------------------------------------------------------
+// Positions
+
+// Position describes an arbitrary source position
+// including the file, line, and column location.
+// A Position is valid if the line number is > 0.
+//
+type Position struct {
+	Filename string // filename, if any
+	Offset   int    // offset, starting at 0
+	Line     int    // line number, starting at 1
+	Column   int    // column number, starting at 1 (character count)
+}
+
+// IsValid returns true if the position is valid.
+func (pos *Position) IsValid() bool { return pos.Line > 0 }
+
+// String returns a string in one of several forms:
+//
+//	file:line:column    valid position with file name
+//	line:column         valid position without file name
+//	file                invalid position with file name
+//	-                   invalid position without file name
+//
+func (pos Position) String() string {
+	s := pos.Filename
+	if pos.IsValid() {
+		if s != "" {
+			s += ":"
+		}
+		s += fmt.Sprintf("%d:%d", pos.Line, pos.Column)
+	}
+	if s == "" {
+		s = "-"
+	}
+	return s
+}
+
+// Pos is a compact encoding of a source position within a file set.
+// It can be converted into a Position for a more convenient, but much
+// larger, representation.
+//
+// The Pos value for a given file is a number in the range [base, base+size],
+// where base and size are specified when adding the file to the file set via
+// AddFile.
+//
+// To create the Pos value for a specific source offset, first add
+// the respective file to the current file set (via FileSet.AddFile)
+// and then call File.Pos(offset) for that file. Given a Pos value p
+// for a specific file set fset, the corresponding Position value is
+// obtained by calling fset.Position(p).
+//
+// Pos values can be compared directly with the usual comparison operators:
+// If two Pos values p and q are in the same file, comparing p and q is
+// equivalent to comparing the respective source file offsets. If p and q
+// are in different files, p < q is true if the file implied by p was added
+// to the respective file set before the file implied by q.
+//
+type Pos int
+
+// The zero value for Pos is NoPos; there is no file and line information
+// associated with it, and NoPos().IsValid() is false. NoPos is always
+// smaller than any other Pos value. The corresponding Position value
+// for NoPos is the zero value for Position.
+//
+const NoPos Pos = 0
+
+// IsValid returns true if the position is valid.
+func (p Pos) IsValid() bool {
+	return p != NoPos
+}
+
+// -----------------------------------------------------------------------------
+// File
+
+// A File is a handle for a file belonging to a FileSet.
+// A File has a name, size, and line offset table.
+//
+type File struct {
+	set  *FileSet
+	name string // file name as provided to AddFile
+	base int    // Pos value range for this file is [base...base+size]
+	size int    // file size as provided to AddFile
+
+	// lines and infos are protected by set.mutex
+	lines []int // lines contains the offset of the first character for each line (the first entry is always 0)
+	infos []lineInfo
+}
+
+// Name returns the file name of file f as registered with AddFile.
+func (f *File) Name() string {
+	return f.name
+}
+
+// Base returns the base offset of file f as registered with AddFile.
+func (f *File) Base() int {
+	return f.base
+}
+
+// Size returns the size of file f as registered with AddFile.
+func (f *File) Size() int {
+	return f.size
+}
+
+// LineCount returns the number of lines in file f.
+func (f *File) LineCount() int {
+	f.set.mutex.RLock()
+	n := len(f.lines)
+	f.set.mutex.RUnlock()
+	return n
+}
+
+// AddLine adds the line offset for a new line.
+// The line offset must be larger than the offset for the previous line
+// and smaller than the file size; otherwise the line offset is ignored.
+//
+func (f *File) AddLine(offset int) {
+	f.set.mutex.Lock()
+	if i := len(f.lines); (i == 0 || f.lines[i-1] < offset) && offset < f.size {
+		f.lines = append(f.lines, offset)
+	}
+	f.set.mutex.Unlock()
+}
+
+// MergeLine merges a line with the following line. It is akin to replacing
+// the newline character at the end of the line with a space (to not change the
+// remaining offsets). To obtain the line number, consult e.g. Position.Line.
+// MergeLine will panic if given an invalid line number.
+//
+func (f *File) MergeLine(line int) {
+	if line <= 0 {
+		panic("illegal line number (line numbering starts at 1)")
+	}
+	f.set.mutex.Lock()
+	defer f.set.mutex.Unlock()
+	if line >= len(f.lines) {
+		panic("illegal line number")
+	}
+	// To merge the line numbered <line> with the line numbered <line+1>,
+	// we need to remove the entry in lines corresponding to the line
+	// numbered <line+1>. The entry in lines corresponding to the line
+	// numbered <line+1> is located at index <line>, since indices in lines
+	// are 0-based and line numbers are 1-based.
+	copy(f.lines[line:], f.lines[line+1:])
+	f.lines = f.lines[:len(f.lines)-1]
+}
+
+// SetLines sets the line offsets for a file and returns true if successful.
+// The line offsets are the offsets of the first character of each line;
+// for instance for the content "ab\nc\n" the line offsets are {0, 3}.
+// An empty file has an empty line offset table.
+// Each line offset must be larger than the offset for the previous line
+// and smaller than the file size; otherwise SetLines fails and returns
+// false.
+//
+func (f *File) SetLines(lines []int) bool {
+	// verify validity of lines table
+	size := f.size
+	for i, offset := range lines {
+		if i > 0 && offset <= lines[i-1] || size <= offset {
+			return false
+		}
+	}
+
+	// set lines table
+	f.set.mutex.Lock()
+	f.lines = lines
+	f.set.mutex.Unlock()
+	return true
+}
+
+// SetLinesForContent sets the line offsets for the given file content.
+// It ignores position-altering //line comments.
+func (f *File) SetLinesForContent(content []byte) {
+	var lines []int
+	line := 0
+	for offset, b := range content {
+		if line >= 0 {
+			lines = append(lines, line)
+		}
+		line = -1
+		if b == '\n' {
+			line = offset + 1
+		}
+	}
+
+	// set lines table
+	f.set.mutex.Lock()
+	f.lines = lines
+	f.set.mutex.Unlock()
+}
+
+// A lineInfo object describes alternative file and line number
+// information (such as provided via a //line comment in a .go
+// file) for a given file offset.
+type lineInfo struct {
+	// fields are exported to make them accessible to gob
+	Offset   int
+	Filename string
+	Line     int
+}
+
+// AddLineInfo adds alternative file and line number information for
+// a given file offset. The offset must be larger than the offset for
+// the previously added alternative line info and smaller than the
+// file size; otherwise the information is ignored.
+//
+// AddLineInfo is typically used to register alternative position
+// information for //line filename:line comments in source files.
+//
+func (f *File) AddLineInfo(offset int, filename string, line int) {
+	f.set.mutex.Lock()
+	if i := len(f.infos); i == 0 || f.infos[i-1].Offset < offset && offset < f.size {
+		f.infos = append(f.infos, lineInfo{offset, filename, line})
+	}
+	f.set.mutex.Unlock()
+}
+
+// Pos returns the Pos value for the given file offset;
+// the offset must be <= f.Size().
+// f.Pos(f.Offset(p)) == p.
+//
+func (f *File) Pos(offset int) Pos {
+	if offset > f.size {
+		panic("illegal file offset")
+	}
+	return Pos(f.base + offset)
+}
+
+// Offset returns the offset for the given file position p;
+// p must be a valid Pos value in that file.
+// f.Offset(f.Pos(offset)) == offset.
+//
+func (f *File) Offset(p Pos) int {
+	if int(p) < f.base || int(p) > f.base+f.size {
+		panic("illegal Pos value")
+	}
+	return int(p) - f.base
+}
+
+// Line returns the line number for the given file position p;
+// p must be a Pos value in that file or NoPos.
+//
+func (f *File) Line(p Pos) int {
+	return f.Position(p).Line
+}
+
+func searchLineInfos(a []lineInfo, x int) int {
+	return sort.Search(len(a), func(i int) bool { return a[i].Offset > x }) - 1
+}
+
+// unpack returns the filename and line and column number for a file offset.
+// If adjusted is set, unpack will return the filename and line information
+// possibly adjusted by //line comments; otherwise those comments are ignored.
+//
+func (f *File) unpack(offset int, adjusted bool) (filename string, line, column int) {
+	filename = f.name
+	if i := searchInts(f.lines, offset); i >= 0 {
+		line, column = i+1, offset-f.lines[i]+1
+	}
+	if adjusted && len(f.infos) > 0 {
+		// almost no files have extra line infos
+		if i := searchLineInfos(f.infos, offset); i >= 0 {
+			alt := &f.infos[i]
+			filename = alt.Filename
+			if i := searchInts(f.lines, alt.Offset); i >= 0 {
+				line += alt.Line - i - 1
+			}
+		}
+	}
+	return
+}
+
+func (f *File) position(p Pos, adjusted bool) (pos Position) {
+	offset := int(p) - f.base
+	pos.Offset = offset
+	pos.Filename, pos.Line, pos.Column = f.unpack(offset, adjusted)
+	return
+}
+
+// PositionFor returns the Position value for the given file position p.
+// If adjusted is set, the position may be adjusted by position-altering
+// //line comments; otherwise those comments are ignored.
+// p must be a Pos value in f or NoPos.
+//
+func (f *File) PositionFor(p Pos, adjusted bool) (pos Position) {
+	if p != NoPos {
+		if int(p) < f.base || int(p) > f.base+f.size {
+			panic("illegal Pos value")
+		}
+		pos = f.position(p, adjusted)
+	}
+	return
+}
+
+// Position returns the Position value for the given file position p.
+// Calling f.Position(p) is equivalent to calling f.PositionFor(p, true).
+//
+func (f *File) Position(p Pos) (pos Position) {
+	return f.PositionFor(p, true)
+}
+
+// -----------------------------------------------------------------------------
+// FileSet
+
+// A FileSet represents a set of source files.
+// Methods of file sets are synchronized; multiple goroutines
+// may invoke them concurrently.
+//
+type FileSet struct {
+	mutex sync.RWMutex // protects the file set
+	base  int          // base offset for the next file
+	files []*File      // list of files in the order added to the set
+	last  *File        // cache of last file looked up
+}
+
+// NewFileSet creates a new file set.
+func NewFileSet() *FileSet {
+	return &FileSet{
+		base: 1, // 0 == NoPos
+	}
+}
+
+// Base returns the minimum base offset that must be provided to
+// AddFile when adding the next file.
+//
+func (s *FileSet) Base() int {
+	s.mutex.RLock()
+	b := s.base
+	s.mutex.RUnlock()
+	return b
+
+}
+
+// AddFile adds a new file with a given filename, base offset, and file size
+// to the file set s and returns the file. Multiple files may have the same
+// name. The base offset must not be smaller than the FileSet's Base(), and
+// size must not be negative. As a special case, if a negative base is provided,
+// the current value of the FileSet's Base() is used instead.
+//
+// Adding the file will set the file set's Base() value to base + size + 1
+// as the minimum base value for the next file. The following relationship
+// exists between a Pos value p for a given file offset offs:
+//
+//	int(p) = base + offs
+//
+// with offs in the range [0, size] and thus p in the range [base, base+size].
+// For convenience, File.Pos may be used to create file-specific position
+// values from a file offset.
+//
+func (s *FileSet) AddFile(filename string, base, size int) *File {
+	s.mutex.Lock()
+	defer s.mutex.Unlock()
+	if base < 0 {
+		base = s.base
+	}
+	if base < s.base || size < 0 {
+		panic("illegal base or size")
+	}
+	// base >= s.base && size >= 0
+	f := &File{s, filename, base, size, []int{0}, nil}
+	base += size + 1 // +1 because EOF also has a position
+	if base < 0 {
+		panic("token.Pos offset overflow (> 2G of source code in file set)")
+	}
+	// add the file to the file set
+	s.base = base
+	s.files = append(s.files, f)
+	s.last = f
+	return f
+}
+
+// Iterate calls f for the files in the file set in the order they were added
+// until f returns false.
+//
+func (s *FileSet) Iterate(f func(*File) bool) {
+	for i := 0; ; i++ {
+		var file *File
+		s.mutex.RLock()
+		if i < len(s.files) {
+			file = s.files[i]
+		}
+		s.mutex.RUnlock()
+		if file == nil || !f(file) {
+			break
+		}
+	}
+}
+
+func searchFiles(a []*File, x int) int {
+	return sort.Search(len(a), func(i int) bool { return a[i].base > x }) - 1
+}
+
+func (s *FileSet) file(p Pos) *File {
+	s.mutex.RLock()
+	// common case: p is in last file
+	if f := s.last; f != nil && f.base <= int(p) && int(p) <= f.base+f.size {
+		s.mutex.RUnlock()
+		return f
+	}
+	// p is not in last file - search all files
+	if i := searchFiles(s.files, int(p)); i >= 0 {
+		f := s.files[i]
+		// f.base <= int(p) by definition of searchFiles
+		if int(p) <= f.base+f.size {
+			s.mutex.RUnlock()
+			s.mutex.Lock()
+			s.last = f // race is ok - s.last is only a cache
+			s.mutex.Unlock()
+			return f
+		}
+	}
+	s.mutex.RUnlock()
+	return nil
+}
+
+// File returns the file that contains the position p.
+// If no such file is found (for instance for p == NoPos),
+// the result is nil.
+//
+func (s *FileSet) File(p Pos) (f *File) {
+	if p != NoPos {
+		f = s.file(p)
+	}
+	return
+}
+
+// PositionFor converts a Pos p in the fileset into a Position value.
+// If adjusted is set, the position may be adjusted by position-altering
+// //line comments; otherwise those comments are ignored.
+// p must be a Pos value in s or NoPos.
+//
+func (s *FileSet) PositionFor(p Pos, adjusted bool) (pos Position) {
+	if p != NoPos {
+		if f := s.file(p); f != nil {
+			pos = f.position(p, adjusted)
+		}
+	}
+	return
+}
+
+// Position converts a Pos p in the fileset into a Position value.
+// Calling s.Position(p) is equivalent to calling s.PositionFor(p, true).
+//
+func (s *FileSet) Position(p Pos) (pos Position) {
+	return s.PositionFor(p, true)
+}
+
+// -----------------------------------------------------------------------------
+// Helper functions
+
+func searchInts(a []int, x int) int {
+	// This function body is a manually inlined version of:
+	//
+	//   return sort.Search(len(a), func(i int) bool { return a[i] > x }) - 1
+	//
+	// With better compiler optimizations, this may not be needed in the
+	// future, but at the moment this change improves the go/printer
+	// benchmark performance by ~30%. This has a direct impact on the
+	// speed of gofmt and thus seems worthwhile (2011-04-29).
+	// TODO(gri): Remove this when compilers have caught up.
+	i, j := 0, len(a)
+	for i < j {
+		h := i + (j-i)/2 // avoid overflow when computing h
+		// i ≤ h < j
+		if a[h] <= x {
+			i = h + 1
+		} else {
+			j = h
+		}
+	}
+	return i - 1
+}
diff --git a/src/go/token/position_test.go b/src/go/token/position_test.go
new file mode 100644
index 000000000..d26939ce2
--- /dev/null
+++ b/src/go/token/position_test.go
@@ -0,0 +1,297 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package token
+
+import (
+	"fmt"
+	"math/rand"
+	"sync"
+	"testing"
+)
+
+func checkPos(t *testing.T, msg string, got, want Position) {
+	if got.Filename != want.Filename {
+		t.Errorf("%s: got filename = %q; want %q", msg, got.Filename, want.Filename)
+	}
+	if got.Offset != want.Offset {
+		t.Errorf("%s: got offset = %d; want %d", msg, got.Offset, want.Offset)
+	}
+	if got.Line != want.Line {
+		t.Errorf("%s: got line = %d; want %d", msg, got.Line, want.Line)
+	}
+	if got.Column != want.Column {
+		t.Errorf("%s: got column = %d; want %d", msg, got.Column, want.Column)
+	}
+}
+
+func TestNoPos(t *testing.T) {
+	if NoPos.IsValid() {
+		t.Errorf("NoPos should not be valid")
+	}
+	var fset *FileSet
+	checkPos(t, "nil NoPos", fset.Position(NoPos), Position{})
+	fset = NewFileSet()
+	checkPos(t, "fset NoPos", fset.Position(NoPos), Position{})
+}
+
+var tests = []struct {
+	filename string
+	source   []byte // may be nil
+	size     int
+	lines    []int
+}{
+	{"a", []byte{}, 0, []int{}},
+	{"b", []byte("01234"), 5, []int{0}},
+	{"c", []byte("\n\n\n\n\n\n\n\n\n"), 9, []int{0, 1, 2, 3, 4, 5, 6, 7, 8}},
+	{"d", nil, 100, []int{0, 5, 10, 20, 30, 70, 71, 72, 80, 85, 90, 99}},
+	{"e", nil, 777, []int{0, 80, 100, 120, 130, 180, 267, 455, 500, 567, 620}},
+	{"f", []byte("package p\n\nimport \"fmt\""), 23, []int{0, 10, 11}},
+	{"g", []byte("package p\n\nimport \"fmt\"\n"), 24, []int{0, 10, 11}},
+	{"h", []byte("package p\n\nimport \"fmt\"\n "), 25, []int{0, 10, 11, 24}},
+}
+
+func linecol(lines []int, offs int) (int, int) {
+	prevLineOffs := 0
+	for line, lineOffs := range lines {
+		if offs < lineOffs {
+			return line, offs - prevLineOffs + 1
+		}
+		prevLineOffs = lineOffs
+	}
+	return len(lines), offs - prevLineOffs + 1
+}
+
+func verifyPositions(t *testing.T, fset *FileSet, f *File, lines []int) {
+	for offs := 0; offs < f.Size(); offs++ {
+		p := f.Pos(offs)
+		offs2 := f.Offset(p)
+		if offs2 != offs {
+			t.Errorf("%s, Offset: got offset %d; want %d", f.Name(), offs2, offs)
+		}
+		line, col := linecol(lines, offs)
+		msg := fmt.Sprintf("%s (offs = %d, p = %d)", f.Name(), offs, p)
+		checkPos(t, msg, f.Position(f.Pos(offs)), Position{f.Name(), offs, line, col})
+		checkPos(t, msg, fset.Position(p), Position{f.Name(), offs, line, col})
+	}
+}
+
+func makeTestSource(size int, lines []int) []byte {
+	src := make([]byte, size)
+	for _, offs := range lines {
+		if offs > 0 {
+			src[offs-1] = '\n'
+		}
+	}
+	return src
+}
+
+func TestPositions(t *testing.T) {
+	const delta = 7 // a non-zero base offset increment
+	fset := NewFileSet()
+	for _, test := range tests {
+		// verify consistency of test case
+		if test.source != nil && len(test.source) != test.size {
+			t.Errorf("%s: inconsistent test case: got file size %d; want %d", test.filename, len(test.source), test.size)
+		}
+
+		// add file and verify name and size
+		f := fset.AddFile(test.filename, fset.Base()+delta, test.size)
+		if f.Name() != test.filename {
+			t.Errorf("got filename %q; want %q", f.Name(), test.filename)
+		}
+		if f.Size() != test.size {
+			t.Errorf("%s: got file size %d; want %d", f.Name(), f.Size(), test.size)
+		}
+		if fset.File(f.Pos(0)) != f {
+			t.Errorf("%s: f.Pos(0) was not found in f", f.Name())
+		}
+
+		// add lines individually and verify all positions
+		for i, offset := range test.lines {
+			f.AddLine(offset)
+			if f.LineCount() != i+1 {
+				t.Errorf("%s, AddLine: got line count %d; want %d", f.Name(), f.LineCount(), i+1)
+			}
+			// adding the same offset again should be ignored
+			f.AddLine(offset)
+			if f.LineCount() != i+1 {
+				t.Errorf("%s, AddLine: got unchanged line count %d; want %d", f.Name(), f.LineCount(), i+1)
+			}
+			verifyPositions(t, fset, f, test.lines[0:i+1])
+		}
+
+		// add lines with SetLines and verify all positions
+		if ok := f.SetLines(test.lines); !ok {
+			t.Errorf("%s: SetLines failed", f.Name())
+		}
+		if f.LineCount() != len(test.lines) {
+			t.Errorf("%s, SetLines: got line count %d; want %d", f.Name(), f.LineCount(), len(test.lines))
+		}
+		verifyPositions(t, fset, f, test.lines)
+
+		// add lines with SetLinesForContent and verify all positions
+		src := test.source
+		if src == nil {
+			// no test source available - create one from scratch
+			src = makeTestSource(test.size, test.lines)
+		}
+		f.SetLinesForContent(src)
+		if f.LineCount() != len(test.lines) {
+			t.Errorf("%s, SetLinesForContent: got line count %d; want %d", f.Name(), f.LineCount(), len(test.lines))
+		}
+		verifyPositions(t, fset, f, test.lines)
+	}
+}
+
+func TestLineInfo(t *testing.T) {
+	fset := NewFileSet()
+	f := fset.AddFile("foo", fset.Base(), 500)
+	lines := []int{0, 42, 77, 100, 210, 220, 277, 300, 333, 401}
+	// add lines individually and provide alternative line information
+	for _, offs := range lines {
+		f.AddLine(offs)
+		f.AddLineInfo(offs, "bar", 42)
+	}
+	// verify positions for all offsets
+	for offs := 0; offs <= f.Size(); offs++ {
+		p := f.Pos(offs)
+		_, col := linecol(lines, offs)
+		msg := fmt.Sprintf("%s (offs = %d, p = %d)", f.Name(), offs, p)
+		checkPos(t, msg, f.Position(f.Pos(offs)), Position{"bar", offs, 42, col})
+		checkPos(t, msg, fset.Position(p), Position{"bar", offs, 42, col})
+	}
+}
+
+func TestFiles(t *testing.T) {
+	fset := NewFileSet()
+	for i, test := range tests {
+		base := fset.Base()
+		if i%2 == 1 {
+			// Setting a negative base is equivalent to
+			// fset.Base(), so test some of each.
+			base = -1
+		}
+		fset.AddFile(test.filename, base, test.size)
+		j := 0
+		fset.Iterate(func(f *File) bool {
+			if f.Name() != tests[j].filename {
+				t.Errorf("got filename = %s; want %s", f.Name(), tests[j].filename)
+			}
+			j++
+			return true
+		})
+		if j != i+1 {
+			t.Errorf("got %d files; want %d", j, i+1)
+		}
+	}
+}
+
+// FileSet.File should return nil if Pos is past the end of the FileSet.
+func TestFileSetPastEnd(t *testing.T) {
+	fset := NewFileSet()
+	for _, test := range tests {
+		fset.AddFile(test.filename, fset.Base(), test.size)
+	}
+	if f := fset.File(Pos(fset.Base())); f != nil {
+		t.Errorf("got %v, want nil", f)
+	}
+}
+
+func TestFileSetCacheUnlikely(t *testing.T) {
+	fset := NewFileSet()
+	offsets := make(map[string]int)
+	for _, test := range tests {
+		offsets[test.filename] = fset.Base()
+		fset.AddFile(test.filename, fset.Base(), test.size)
+	}
+	for file, pos := range offsets {
+		f := fset.File(Pos(pos))
+		if f.Name() != file {
+			t.Errorf("got %q at position %d, want %q", f.Name(), pos, file)
+		}
+	}
+}
+
+// issue 4345. Test concurrent use of FileSet.Pos does not trigger a
+// race in the FileSet position cache.
+func TestFileSetRace(t *testing.T) {
+	fset := NewFileSet()
+	for i := 0; i < 100; i++ {
+		fset.AddFile(fmt.Sprintf("file-%d", i), fset.Base(), 1031)
+	}
+	max := int32(fset.Base())
+	var stop sync.WaitGroup
+	r := rand.New(rand.NewSource(7))
+	for i := 0; i < 2; i++ {
+		r := rand.New(rand.NewSource(r.Int63()))
+		stop.Add(1)
+		go func() {
+			for i := 0; i < 1000; i++ {
+				fset.Position(Pos(r.Int31n(max)))
+			}
+			stop.Done()
+		}()
+	}
+	stop.Wait()
+}
+
+func TestPositionFor(t *testing.T) {
+	src := []byte(`
+foo
+b
+ar
+//line :100
+foobar
+//line bar:3
+done
+`)
+
+	const filename = "foo"
+	fset := NewFileSet()
+	f := fset.AddFile(filename, fset.Base(), len(src))
+	f.SetLinesForContent(src)
+
+	// verify position info
+	for i, offs := range f.lines {
+		got1 := f.PositionFor(f.Pos(offs), false)
+		got2 := f.PositionFor(f.Pos(offs), true)
+		got3 := f.Position(f.Pos(offs))
+		want := Position{filename, offs, i + 1, 1}
+		checkPos(t, "1. PositionFor unadjusted", got1, want)
+		checkPos(t, "1. PositionFor adjusted", got2, want)
+		checkPos(t, "1. Position", got3, want)
+	}
+
+	// manually add //line info on lines l1, l2
+	const l1, l2 = 5, 7
+	f.AddLineInfo(f.lines[l1-1], "", 100)
+	f.AddLineInfo(f.lines[l2-1], "bar", 3)
+
+	// unadjusted position info must remain unchanged
+	for i, offs := range f.lines {
+		got1 := f.PositionFor(f.Pos(offs), false)
+		want := Position{filename, offs, i + 1, 1}
+		checkPos(t, "2. PositionFor unadjusted", got1, want)
+	}
+
+	// adjusted position info should have changed
+	for i, offs := range f.lines {
+		got2 := f.PositionFor(f.Pos(offs), true)
+		got3 := f.Position(f.Pos(offs))
+		want := Position{filename, offs, i + 1, 1}
+		// manually compute wanted filename and line
+		line := want.Line
+		if i+1 >= l1 {
+			want.Filename = ""
+			want.Line = line - l1 + 100
+		}
+		if i+1 >= l2 {
+			want.Filename = "bar"
+			want.Line = line - l2 + 3
+		}
+		checkPos(t, "3. PositionFor adjusted", got2, want)
+		checkPos(t, "3. Position", got3, want)
+	}
+}
diff --git a/src/go/token/serialize.go b/src/go/token/serialize.go
new file mode 100644
index 000000000..4adc8f9e3
--- /dev/null
+++ b/src/go/token/serialize.go
@@ -0,0 +1,56 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package token
+
+type serializedFile struct {
+	// fields correspond 1:1 to fields with same (lower-case) name in File
+	Name  string
+	Base  int
+	Size  int
+	Lines []int
+	Infos []lineInfo
+}
+
+type serializedFileSet struct {
+	Base  int
+	Files []serializedFile
+}
+
+// Read calls decode to deserialize a file set into s; s must not be nil.
+func (s *FileSet) Read(decode func(interface{}) error) error {
+	var ss serializedFileSet
+	if err := decode(&ss); err != nil {
+		return err
+	}
+
+	s.mutex.Lock()
+	s.base = ss.Base
+	files := make([]*File, len(ss.Files))
+	for i := 0; i < len(ss.Files); i++ {
+		f := &ss.Files[i]
+		files[i] = &File{s, f.Name, f.Base, f.Size, f.Lines, f.Infos}
+	}
+	s.files = files
+	s.last = nil
+	s.mutex.Unlock()
+
+	return nil
+}
+
+// Write calls encode to serialize the file set s.
+func (s *FileSet) Write(encode func(interface{}) error) error {
+	var ss serializedFileSet
+
+	s.mutex.Lock()
+	ss.Base = s.base
+	files := make([]serializedFile, len(s.files))
+	for i, f := range s.files {
+		files[i] = serializedFile{f.name, f.base, f.size, f.lines, f.infos}
+	}
+	ss.Files = files
+	s.mutex.Unlock()
+
+	return encode(ss)
+}
diff --git a/src/go/token/serialize_test.go b/src/go/token/serialize_test.go
new file mode 100644
index 000000000..4e925adb6
--- /dev/null
+++ b/src/go/token/serialize_test.go
@@ -0,0 +1,111 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package token
+
+import (
+	"bytes"
+	"encoding/gob"
+	"fmt"
+	"testing"
+)
+
+// equal returns nil if p and q describe the same file set;
+// otherwise it returns an error describing the discrepancy.
+func equal(p, q *FileSet) error {
+	if p == q {
+		// avoid deadlock if p == q
+		return nil
+	}
+
+	// not strictly needed for the test
+	p.mutex.Lock()
+	q.mutex.Lock()
+	defer q.mutex.Unlock()
+	defer p.mutex.Unlock()
+
+	if p.base != q.base {
+		return fmt.Errorf("different bases: %d != %d", p.base, q.base)
+	}
+
+	if len(p.files) != len(q.files) {
+		return fmt.Errorf("different number of files: %d != %d", len(p.files), len(q.files))
+	}
+
+	for i, f := range p.files {
+		g := q.files[i]
+		if f.set != p {
+			return fmt.Errorf("wrong fileset for %q", f.name)
+		}
+		if g.set != q {
+			return fmt.Errorf("wrong fileset for %q", g.name)
+		}
+		if f.name != g.name {
+			return fmt.Errorf("different filenames: %q != %q", f.name, g.name)
+		}
+		if f.base != g.base {
+			return fmt.Errorf("different base for %q: %d != %d", f.name, f.base, g.base)
+		}
+		if f.size != g.size {
+			return fmt.Errorf("different size for %q: %d != %d", f.name, f.size, g.size)
+		}
+		for j, l := range f.lines {
+			m := g.lines[j]
+			if l != m {
+				return fmt.Errorf("different offsets for %q", f.name)
+			}
+		}
+		for j, l := range f.infos {
+			m := g.infos[j]
+			if l.Offset != m.Offset || l.Filename != m.Filename || l.Line != m.Line {
+				return fmt.Errorf("different infos for %q", f.name)
+			}
+		}
+	}
+
+	// we don't care about .last - it's just a cache
+	return nil
+}
+
+func checkSerialize(t *testing.T, p *FileSet) {
+	var buf bytes.Buffer
+	encode := func(x interface{}) error {
+		return gob.NewEncoder(&buf).Encode(x)
+	}
+	if err := p.Write(encode); err != nil {
+		t.Errorf("writing fileset failed: %s", err)
+		return
+	}
+	q := NewFileSet()
+	decode := func(x interface{}) error {
+		return gob.NewDecoder(&buf).Decode(x)
+	}
+	if err := q.Read(decode); err != nil {
+		t.Errorf("reading fileset failed: %s", err)
+		return
+	}
+	if err := equal(p, q); err != nil {
+		t.Errorf("filesets not identical: %s", err)
+	}
+}
+
+func TestSerialization(t *testing.T) {
+	p := NewFileSet()
+	checkSerialize(t, p)
+	// add some files
+	for i := 0; i < 10; i++ {
+		f := p.AddFile(fmt.Sprintf("file%d", i), p.Base()+i, i*100)
+		checkSerialize(t, p)
+		// add some lines and alternative file infos
+		line := 1000
+		for offs := 0; offs < f.Size(); offs += 40 + i {
+			f.AddLine(offs)
+			if offs%7 == 0 {
+				f.AddLineInfo(offs, fmt.Sprintf("file%d", offs), line)
+				line += 33
+			}
+		}
+		checkSerialize(t, p)
+	}
+}
diff --git a/src/go/token/token.go b/src/go/token/token.go
new file mode 100644
index 000000000..865f63f4a
--- /dev/null
+++ b/src/go/token/token.go
@@ -0,0 +1,308 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package token defines constants representing the lexical tokens of the Go
+// programming language and basic operations on tokens (printing, predicates).
+//
+package token
+
+import "strconv"
+
+// Token is the set of lexical tokens of the Go programming language.
+type Token int
+
+// The list of tokens.
+const (
+	// Special tokens
+	ILLEGAL Token = iota
+	EOF
+	COMMENT
+
+	literal_beg
+	// Identifiers and basic type literals
+	// (these tokens stand for classes of literals)
+	IDENT  // main
+	INT    // 12345
+	FLOAT  // 123.45
+	IMAG   // 123.45i
+	CHAR   // 'a'
+	STRING // "abc"
+	literal_end
+
+	operator_beg
+	// Operators and delimiters
+	ADD // +
+	SUB // -
+	MUL // *
+	QUO // /
+	REM // %
+
+	AND     // &
+	OR      // |
+	XOR     // ^
+	SHL     // <<
+	SHR     // >>
+	AND_NOT // &^
+
+	ADD_ASSIGN // +=
+	SUB_ASSIGN // -=
+	MUL_ASSIGN // *=
+	QUO_ASSIGN // /=
+	REM_ASSIGN // %=
+
+	AND_ASSIGN     // &=
+	OR_ASSIGN      // |=
+	XOR_ASSIGN     // ^=
+	SHL_ASSIGN     // <<=
+	SHR_ASSIGN     // >>=
+	AND_NOT_ASSIGN // &^=
+
+	LAND  // &&
+	LOR   // ||
+	ARROW // <-
+	INC   // ++
+	DEC   // --
+
+	EQL    // ==
+	LSS    // <
+	GTR    // >
+	ASSIGN // =
+	NOT    // !
+
+	NEQ      // !=
+	LEQ      // <=
+	GEQ      // >=
+	DEFINE   // :=
+	ELLIPSIS // ...
+
+	LPAREN // (
+	LBRACK // [
+	LBRACE // {
+	COMMA  // ,
+	PERIOD // .
+
+	RPAREN    // )
+	RBRACK    // ]
+	RBRACE    // }
+	SEMICOLON // ;
+	COLON     // :
+	operator_end
+
+	keyword_beg
+	// Keywords
+	BREAK
+	CASE
+	CHAN
+	CONST
+	CONTINUE
+
+	DEFAULT
+	DEFER
+	ELSE
+	FALLTHROUGH
+	FOR
+
+	FUNC
+	GO
+	GOTO
+	IF
+	IMPORT
+
+	INTERFACE
+	MAP
+	PACKAGE
+	RANGE
+	RETURN
+
+	SELECT
+	STRUCT
+	SWITCH
+	TYPE
+	VAR
+	keyword_end
+)
+
+var tokens = [...]string{
+	ILLEGAL: "ILLEGAL",
+
+	EOF:     "EOF",
+	COMMENT: "COMMENT",
+
+	IDENT:  "IDENT",
+	INT:    "INT",
+	FLOAT:  "FLOAT",
+	IMAG:   "IMAG",
+	CHAR:   "CHAR",
+	STRING: "STRING",
+
+	ADD: "+",
+	SUB: "-",
+	MUL: "*",
+	QUO: "/",
+	REM: "%",
+
+	AND:     "&",
+	OR:      "|",
+	XOR:     "^",
+	SHL:     "<<",
+	SHR:     ">>",
+	AND_NOT: "&^",
+
+	ADD_ASSIGN: "+=",
+	SUB_ASSIGN: "-=",
+	MUL_ASSIGN: "*=",
+	QUO_ASSIGN: "/=",
+	REM_ASSIGN: "%=",
+
+	AND_ASSIGN:     "&=",
+	OR_ASSIGN:      "|=",
+	XOR_ASSIGN:     "^=",
+	SHL_ASSIGN:     "<<=",
+	SHR_ASSIGN:     ">>=",
+	AND_NOT_ASSIGN: "&^=",
+
+	LAND:  "&&",
+	LOR:   "||",
+	ARROW: "<-",
+	INC:   "++",
+	DEC:   "--",
+
+	EQL:    "==",
+	LSS:    "<",
+	GTR:    ">",
+	ASSIGN: "=",
+	NOT:    "!",
+
+	NEQ:      "!=",
+	LEQ:      "<=",
+	GEQ:      ">=",
+	DEFINE:   ":=",
+	ELLIPSIS: "...",
+
+	LPAREN: "(",
+	LBRACK: "[",
+	LBRACE: "{",
+	COMMA:  ",",
+	PERIOD: ".",
+
+	RPAREN:    ")",
+	RBRACK:    "]",
+	RBRACE:    "}",
+	SEMICOLON: ";",
+	COLON:     ":",
+
+	BREAK:    "break",
+	CASE:     "case",
+	CHAN:     "chan",
+	CONST:    "const",
+	CONTINUE: "continue",
+
+	DEFAULT:     "default",
+	DEFER:       "defer",
+	ELSE:        "else",
+	FALLTHROUGH: "fallthrough",
+	FOR:         "for",
+
+	FUNC:   "func",
+	GO:     "go",
+	GOTO:   "goto",
+	IF:     "if",
+	IMPORT: "import",
+
+	INTERFACE: "interface",
+	MAP:       "map",
+	PACKAGE:   "package",
+	RANGE:     "range",
+	RETURN:    "return",
+
+	SELECT: "select",
+	STRUCT: "struct",
+	SWITCH: "switch",
+	TYPE:   "type",
+	VAR:    "var",
+}
+
+// String returns the string corresponding to the token tok.
+// For operators, delimiters, and keywords the string is the actual
+// token character sequence (e.g., for the token ADD, the string is
+// "+"). For all other tokens the string corresponds to the token
+// constant name (e.g. for the token IDENT, the string is "IDENT").
+//
+func (tok Token) String() string {
+	s := ""
+	if 0 <= tok && tok < Token(len(tokens)) {
+		s = tokens[tok]
+	}
+	if s == "" {
+		s = "token(" + strconv.Itoa(int(tok)) + ")"
+	}
+	return s
+}
+
+// A set of constants for precedence-based expression parsing.
+// Non-operators have lowest precedence, followed by operators
+// starting with precedence 1 up to unary operators. The highest
+// precedence serves as "catch-all" precedence for selector,
+// indexing, and other operator and delimiter tokens.
+//
+const (
+	LowestPrec  = 0 // non-operators
+	UnaryPrec   = 6
+	HighestPrec = 7
+)
+
+// Precedence returns the operator precedence of the binary
+// operator op. If op is not a binary operator, the result
+// is LowestPrecedence.
+//
+func (op Token) Precedence() int {
+	switch op {
+	case LOR:
+		return 1
+	case LAND:
+		return 2
+	case EQL, NEQ, LSS, LEQ, GTR, GEQ:
+		return 3
+	case ADD, SUB, OR, XOR:
+		return 4
+	case MUL, QUO, REM, SHL, SHR, AND, AND_NOT:
+		return 5
+	}
+	return LowestPrec
+}
+
+var keywords map[string]Token
+
+func init() {
+	keywords = make(map[string]Token)
+	for i := keyword_beg + 1; i < keyword_end; i++ {
+		keywords[tokens[i]] = i
+	}
+}
+
+// Lookup maps an identifier to its keyword token or IDENT (if not a keyword).
+//
+func Lookup(ident string) Token {
+	if tok, is_keyword := keywords[ident]; is_keyword {
+		return tok
+	}
+	return IDENT
+}
+
+// Predicates
+
+// IsLiteral returns true for tokens corresponding to identifiers
+// and basic type literals; it returns false otherwise.
+//
+func (tok Token) IsLiteral() bool { return literal_beg < tok && tok < literal_end }
+
+// IsOperator returns true for tokens corresponding to operators and
+// delimiters; it returns false otherwise.
+//
+func (tok Token) IsOperator() bool { return operator_beg < tok && tok < operator_end }
+
+// IsKeyword returns true for tokens corresponding to keywords;
+// it returns false otherwise.
+//
+func (tok Token) IsKeyword() bool { return keyword_beg < tok && tok < keyword_end }