go/format, cmd/gofmt: fix issues with partial Go code with indent

Fixes issue 5551.
Fixes issue 4449.

Adds tests for both issues.

Note that the two issues occur only when formatting partial Go code
with indent.

The best way to understand the change is as follows: I took the code
of cmd/gofmt and go/format, combined it into one unified code that
does not suffer from either 4449 nor 5551, and then applied that code
to both cmd/gofmt and go/format.

As a result, there is now much more identical code between the two
packages, making future code deduplication easier (it was not possible
to do that now without adding public APIs, which I was advised not to
do at this time).

More specifically, I took the parse() of cmd/gofmt which correctly
preserves comments (issue 5551) and modified it to fix issue where
it would sometimes modify literal values (issue 4449).

I ended up removing the matchSpace() function because it no longer
needed to do some of its work (insert indent), and a part of its work
had to be done in advance (determining the indentation of first code
line), because that calculation is required for cfg.Fprint() to run.

adjustIndent is used to adjust the indent of cfg.Fprint() to compensate
for the body of wrapper func being indented by one level. This allows
to get rid of the bytes.Replace text manipulation of inner content,
which was problematic and sometimes altered raw string literals (issue
4449). This means that sometimes the value of cfg.Indent is negative,
but that works as expected.

So now the algorithm for formatting partial Go code is:

1. Determine and prepend leading space of original source.
2. Determine and prepend indentation of first code line.
3. Format and write partial Go code (with all of its leading &
   trailing space trimmed).
4. Determine and append trailing space of original source.

LGTM=gri
R=golang-codereviews, bradfitz, gri
CC=golang-codereviews
https://codereview.appspot.com/142360043

Committer: Robert Griesemer <gri@golang.org>

2 files changed, 131 insertions, 82 deletions

diff --git a/src/go/format/format.go b/src/go/format/format.go
index 3d00a645d..08a9047b9 100644
--- a/src/go/format/format.go
+++ b/src/go/format/format.go
@@ -18,6 +18,8 @@ import (
 
 var config = printer.Config{Mode: printer.UseSpaces | printer.TabIndent, Tabwidth: 8}
 
+const parserMode = parser.ParseComments
+
 // Node formats node in canonical gofmt style and writes the result to dst.
 //
 // The node type must be *ast.File, *printer.CommentedNode, []ast.Decl,
@@ -52,7 +54,7 @@ func Node(dst io.Writer, fset *token.FileSet, node interface{}) error {
 		if err != nil {
 			return err
 		}
-		file, err = parser.ParseFile(fset, "", buf.Bytes(), parser.ParseComments)
+		file, err = parser.ParseFile(fset, "", buf.Bytes(), parserMode)
 		if err != nil {
 			// We should never get here. If we do, provide good diagnostic.
 			return fmt.Errorf("format.Node internal error (%s)", err)
@@ -80,66 +82,12 @@ func Node(dst io.Writer, fset *token.FileSet, node interface{}) error {
 //
 func Source(src []byte) ([]byte, error) {
 	fset := token.NewFileSet()
-	node, err := parse(fset, src)
+	file, sourceAdj, indentAdj, err := parse(fset, "", src, true)
 	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
+	return format(fset, file, sourceAdj, indentAdj, src)
 }
 
 func hasUnsortedImports(file *ast.File) bool {
@@ -160,40 +108,137 @@ func hasUnsortedImports(file *ast.File) bool {
 	return false
 }
 
-func isSpace(b byte) bool {
-	return b == ' ' || b == '\t' || b == '\n' || b == '\r'
-}
+// parse parses src, which was read from filename,
+// as a Go source file or statement list.
+func parse(fset *token.FileSet, filename string, src []byte, fragmentOk bool) (
+	file *ast.File,
+	sourceAdj func(src []byte, indent int) []byte,
+	indentAdj int,
+	err error,
+) {
+	// Try as whole source file.
+	file, err = parser.ParseFile(fset, filename, src, parserMode)
+	// If there's no error, return.  If the error is that the source file didn't begin with a
+	// package line and source fragments are ok, fall through to
+	// try as a source fragment.  Stop and return on any other error.
+	if err == nil || !fragmentOk || !strings.Contains(err.Error(), "expected 'package'") {
+		return
+	}
 
-func parse(fset *token.FileSet, src []byte) (interface{}, error) {
-	// Try as a complete source file.
-	file, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	// If this is a declaration list, make it a source file
+	// by inserting a package clause.
+	// Insert using a ;, not a newline, so that the line numbers
+	// in psrc match the ones in src.
+	psrc := append([]byte("package p;"), src...)
+	file, err = parser.ParseFile(fset, filename, psrc, parserMode)
 	if err == nil {
-		return file, nil
+		sourceAdj = func(src []byte, indent int) []byte {
+			// Remove the package clause.
+			// Gofmt has turned the ; into a \n.
+			src = src[indent+len("package p\n"):]
+			return bytes.TrimSpace(src)
+		}
+		return
 	}
-	// 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
+	// If the error is that the source file didn't begin with a
+	// declaration, fall through to try as a statement list.
+	// Stop and return on any other error.
+	if !strings.Contains(err.Error(), "expected declaration") {
+		return
 	}
 
-	// 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 this is a statement list, make it a source file
+	// by inserting a package clause and turning the list
+	// into a function body.  This handles expressions too.
+	// Insert using a ;, not a newline, so that the line numbers
+	// in fsrc match the ones in src.
+	fsrc := append(append([]byte("package p; func _() {"), src...), '\n', '}')
+	file, err = parser.ParseFile(fset, filename, fsrc, parserMode)
 	if err == nil {
-		return file.Decls, nil
+		sourceAdj = func(src []byte, indent int) []byte {
+			// Cap adjusted indent to zero.
+			if indent < 0 {
+				indent = 0
+			}
+			// Remove the wrapping.
+			// Gofmt has turned the ; into a \n\n.
+			// There will be two non-blank lines with indent, hence 2*indent.
+			src = src[2*indent+len("package p\n\nfunc _() {"):]
+			src = src[:len(src)-(indent+len("\n}\n"))]
+			return bytes.TrimSpace(src)
+		}
+		// Gofmt has also indented the function body one level.
+		// Adjust that with indentAdj.
+		indentAdj = -1
 	}
-	// If the source is missing a declaration, try as a statement list; otherwise fail.
-	if !strings.Contains(err.Error(), "expected declaration") {
+
+	// Succeeded, or out of options.
+	return
+}
+
+func format(fset *token.FileSet, file *ast.File, sourceAdj func(src []byte, indent int) []byte, indentAdj int, src []byte) ([]byte, error) {
+	if sourceAdj == nil {
+		// Complete source file.
+		ast.SortImports(fset, file)
+		var buf bytes.Buffer
+		err := config.Fprint(&buf, fset, file)
+		if err != nil {
+			return nil, err
+		}
+		return buf.Bytes(), nil
+	}
+
+	// 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 // byte offset of last line in leading space
+		}
+		j++
+	}
+	var res []byte
+	res = append(res, src[:i]...)
+
+	// Determine and prepend 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
+	}
+	for i := 0; i < indent; i++ {
+		res = append(res, '\t')
+	}
+
+	// Format the source.
+	// Write it without any leading and trailing space.
+	cfg := config
+	cfg.Indent = indent + indentAdj
+	var buf bytes.Buffer
+	err := cfg.Fprint(&buf, fset, file)
+	if err != nil {
 		return nil, err
 	}
+	res = append(res, sourceAdj(buf.Bytes(), cfg.Indent)...)
 
-	// 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
+	// Determine and append trailing space.
+	i = len(src)
+	for i > 0 && isSpace(src[i-1]) {
+		i--
 	}
+	return append(res, src[i:]...), nil
+}
 
-	// Failed, and out of options.
-	return nil, err
+func isSpace(b byte) bool {
+	return b == ' ' || b == '\t' || b == '\n' || b == '\r'
 }
diff --git a/src/go/format/format_test.go b/src/go/format/format_test.go
index 93f099247..d7846bec6 100644
--- a/src/go/format/format_test.go
+++ b/src/go/format/format_test.go
@@ -87,7 +87,11 @@ var tests = []string{
 	"\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
+	"\n\t\t\n\n\t\t\tx := 0\n\t\t\tconst s = `\nfoo\n`\n\n\n",     // no indentation added inside raw strings
+	"\n\t\t\n\n\t\t\tx := 0\n\t\t\tconst s = `\n\t\tfoo\n`\n\n\n", // no indentation removed inside raw strings
+
+	// comments
+	"i := 5 /* Comment */", // Issue 5551.
 
 	// erroneous programs
 	"ERROR1 + 2 +",