go/printer: implement SourcePos mode

If a printer is configured with the SourcePos mode
set, it will emit //-line comments as necessary to
ensure that the result - if reparsed - reflects the
original source position information.

This change required a bit of reworking of the
output section in printer.go. Specifically:

- Introduced new Config mode 'SourcePos'.

- Introduced new position 'out' which tracks the
position of the generated output if it were read
in again. If there is a discrepancy between out
and the current AST/source position, a //line
comment is emitted to correct for it.

- Lazy emission of indentation so that //line
comments can be placed correctly. As a result,
the trimmer will have to do less work.

- Merged writeItem into writeString.

- Merged writeByteN into writeByte.

- Use a []byte instead of a byte.Buffer both in the
printer and in the trimmer (eliminates dependency).

Also: introduced explicit printer.Mode type (in
sync w/ parser.Mode, scanner.Mode, etc.)

Runs all tests. Applied gofmt to src, misc w/o changes.

Fixes issue 1047.
Fixes issue 2697.

R=rsc, rsc
CC=golang-dev
http://codereview.appspot.com/5643066

3 files changed, 11 insertions, 10 deletions

diff --git a/src/cmd/cgo/godefs.go b/src/cmd/cgo/godefs.go
index 683872927..478ed261c 100644
--- a/src/cmd/cgo/godefs.go
+++ b/src/cmd/cgo/godefs.go
@@ -109,7 +109,7 @@ func (p *Package) godefs(f *File, srcfile string) string {
 		}
 	}
 
-	printer.Fprint(&buf, fset, f.AST)
+	conf.Fprint(&buf, fset, f.AST)
 
 	return buf.String()
 }
diff --git a/src/cmd/cgo/out.go b/src/cmd/cgo/out.go
index 2c0107499..bfbcf50dc 100644
--- a/src/cmd/cgo/out.go
+++ b/src/cmd/cgo/out.go
@@ -17,6 +17,8 @@ import (
 	"strings"
 )
 
+var conf = printer.Config{Mode: printer.SourcePos, Tabwidth: 8}
+
 // writeDefs creates output files to be compiled by 6g, 6c, and gcc.
 // (The comments here say 6g and 6c but the code applies to the 8 and 5 tools too.)
 func (p *Package) writeDefs() {
@@ -57,7 +59,7 @@ func (p *Package) writeDefs() {
 
 	for name, def := range typedef {
 		fmt.Fprintf(fgo2, "type %s ", name)
-		printer.Fprint(fgo2, fset, def)
+		conf.Fprint(fgo2, fset, def)
 		fmt.Fprintf(fgo2, "\n\n")
 	}
 	fmt.Fprintf(fgo2, "type _Ctype_void [0]byte\n")
@@ -87,7 +89,7 @@ func (p *Package) writeDefs() {
 		fmt.Fprintf(fc, "\n")
 
 		fmt.Fprintf(fgo2, "var %s ", n.Mangle)
-		printer.Fprint(fgo2, fset, &ast.StarExpr{X: n.Type.Go})
+		conf.Fprint(fgo2, fset, &ast.StarExpr{X: n.Type.Go})
 		fmt.Fprintf(fgo2, "\n")
 	}
 	fmt.Fprintf(fc, "\n")
@@ -255,7 +257,7 @@ func (p *Package) writeDefsFunc(fc, fgo2 *os.File, n *Name) {
 		Name: ast.NewIdent(n.Mangle),
 		Type: gtype,
 	}
-	printer.Fprint(fgo2, fset, d)
+	conf.Fprint(fgo2, fset, d)
 	if *gccgo {
 		fmt.Fprintf(fgo2, " __asm__(\"%s\")\n", n.C)
 	} else {
@@ -327,8 +329,7 @@ func (p *Package) writeOutput(f *File, srcfile string) {
 
 	// Write Go output: Go input with rewrites of C.xxx to _C_xxx.
 	fmt.Fprintf(fgo1, "// Created by cgo - DO NOT EDIT\n\n")
-	fmt.Fprintf(fgo1, "//line %s:1\n", srcfile)
-	printer.Fprint(fgo1, fset, f.AST)
+	conf.Fprint(fgo1, fset, f.AST)
 
 	// While we process the vars and funcs, also write 6c and gcc output.
 	// Gcc output starts with the preamble.
@@ -542,11 +543,11 @@ func (p *Package) writeExports(fgo2, fc, fm *os.File) {
 		// a Go wrapper function.
 		if fn.Recv != nil {
 			fmt.Fprintf(fgo2, "func %s(recv ", goname)
-			printer.Fprint(fgo2, fset, fn.Recv.List[0].Type)
+			conf.Fprint(fgo2, fset, fn.Recv.List[0].Type)
 			forFieldList(fntype.Params,
 				func(i int, atype ast.Expr) {
 					fmt.Fprintf(fgo2, ", p%d ", i)
-					printer.Fprint(fgo2, fset, atype)
+					conf.Fprint(fgo2, fset, atype)
 				})
 			fmt.Fprintf(fgo2, ")")
 			if gccResult != "void" {
@@ -556,7 +557,7 @@ func (p *Package) writeExports(fgo2, fc, fm *os.File) {
 						if i > 0 {
 							fmt.Fprint(fgo2, ", ")
 						}
-						printer.Fprint(fgo2, fset, atype)
+						conf.Fprint(fgo2, fset, atype)
 					})
 				fmt.Fprint(fgo2, ")")
 			}
diff --git a/src/cmd/gofmt/gofmt.go b/src/cmd/gofmt/gofmt.go
index 6d610adc0..55c01beb5 100644
--- a/src/cmd/gofmt/gofmt.go
+++ b/src/cmd/gofmt/gofmt.go
@@ -45,7 +45,7 @@ var (
 	exitCode    = 0
 	rewrite     func(*ast.File) *ast.File
 	parserMode  parser.Mode
-	printerMode uint
+	printerMode printer.Mode
 )
 
 func report(err error) {