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 #1047.
Fixes #2697.

R=rsc, rsc
CC=golang-dev
https://golang.org/cl/5643066

doc/go1.html

@@ -1049,6 +1049,15 @@ The <code>Duration</code> flag is new and affects no existing code.
 Several packages under <code>go</code> have slightly revised APIs.
 </p>
 
+<p>
+A concrete <code>Mode</code> type was introduced for configuration mode flags
+in the packages
+<a href="/pkg/go/scanner/"><code>go/scanner</code></a>,
+<a href="/pkg/go/parser/"><code>go/parser</code></a>,
+<a href="/pkg/go/printer/"><code>go/printer</code></a>, and
+<a href="/pkg/go/doc/"><code>go/doc</code></a>.
+</p>
+
 <p>
 The modes <code>AllowIllegalChars</code> and <code>InsertSemis</code> have been removed
 from the <a href="/pkg/go/scanner/"><code>go/scanner</code></a> package. They were mostly
@@ -1075,6 +1084,17 @@ convenience functions <a href="/pkg/go/parser/#ParseDir"><code>ParseDir</code></
 and <a href="/pkg/go/parser/#ParseExpr"><code>ParseExpr</code></a>.
 </p>
 
+<p>
+The <a href="/pkg/go/printer/"><code>go/printer</code></a> package supports an additional
+configuration mode <a href="/pkg/go/printer/#Mode"><code>SourcePos</code></a>;
+if set, the printer will emit <code>//line</code> comments such that the generated
+output contains the original source code position information. The new type
+<a href="/pkg/go/printer/#CommentedNode"><code>CommentedNode</code></a> can be
+used to provide comments associated with an arbitrary
+<a href="/pkg/go/ast/#Node"><code>ast.Node</code></a> (until now only
+<a href="/pkg/go/ast/#File"><code>ast.File</code></a> carried comment information).
+</p>
+
 <p>
 The type names of the <a href="/pkg/go/doc/"><code>go/doc</code></a> package have been
 streamlined by removing the <code>Doc</code> suffix: <code>PackageDoc</code>

doc/go1.tmpl

@@ -952,6 +952,15 @@ The <code>Duration</code> flag is new and affects no existing code.
 Several packages under <code>go</code> have slightly revised APIs.
 </p>
 
+<p>
+A concrete <code>Mode</code> type was introduced for configuration mode flags
+in the packages
+<a href="/pkg/go/scanner/"><code>go/scanner</code></a>,
+<a href="/pkg/go/parser/"><code>go/parser</code></a>,
+<a href="/pkg/go/printer/"><code>go/printer</code></a>, and
+<a href="/pkg/go/doc/"><code>go/doc</code></a>.
+</p>
+
 <p>
 The modes <code>AllowIllegalChars</code> and <code>InsertSemis</code> have been removed
 from the <a href="/pkg/go/scanner/"><code>go/scanner</code></a> package. They were mostly
@@ -978,6 +987,17 @@ convenience functions <a href="/pkg/go/parser/#ParseDir"><code>ParseDir</code></
 and <a href="/pkg/go/parser/#ParseExpr"><code>ParseExpr</code></a>.
 </p>
 
+<p>
+The <a href="/pkg/go/printer/"><code>go/printer</code></a> package supports an additional
+configuration mode <a href="/pkg/go/printer/#Mode"><code>SourcePos</code></a>;
+if set, the printer will emit <code>//line</code> comments such that the generated
+output contains the original source code position information. The new type
+<a href="/pkg/go/printer/#CommentedNode"><code>CommentedNode</code></a> can be
+used to provide comments associated with an arbitrary
+<a href="/pkg/go/ast/#Node"><code>ast.Node</code></a> (until now only
+<a href="/pkg/go/ast/#File"><code>ast.File</code></a> carried comment information).
+</p>
+
 <p>
 The type names of the <a href="/pkg/go/doc/"><code>go/doc</code></a> package have been
 streamlined by removing the <code>Doc</code> suffix: <code>PackageDoc</code>

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()
 }

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, ")")
 			}

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) {

src/pkg/go/printer/printer.go

@@ -6,7 +6,6 @@
 package printer
 
 import (
-	"bytes"
 	"fmt"
 	"go/ast"
 	"go/token"
@@ -51,22 +50,22 @@ type printer struct {
 	fset *token.FileSet
 
 	// Current state
-	output      bytes.Buffer // raw printer result
+	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  // the last token printed (token.ILLEGAL if it's whitespace)
 	wsbuf       []whiteSpace // delayed white space
 
-	// The (possibly estimated) position in the generated output;
-	// in AST space (i.e., pos is set whenever a token position is
-	// known accurately, and updated dependending on what has been
-	// written).
-	pos token.Position
-
-	// The value of pos immediately after the last item has been
-	// written using writeItem.
-	last token.Position
+	// 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
 
 	// The list of all source comments, in order of appearance.
 	comments        []*ast.CommentGroup // may be nil
@@ -89,6 +88,8 @@ type printer struct {
 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
@@ -151,41 +152,57 @@ func (p *printer) lineFor(pos token.Pos) int {
 	return p.cachedLine
 }
 
-// writeByte writes ch to p.output and updates p.pos.
-func (p *printer) writeByte(ch byte) {
-	p.output.WriteByte(ch)
-	p.pos.Offset++
-	p.pos.Column++
+// 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
+	for i := 0; i < p.indent; i++ {
+		p.output = append(p.output, '\t')
+	}
+
+	// update positions
+	i := p.indent
+	p.pos.Offset += i
+	p.pos.Column += i
+	p.out.Column += i
+}
+
+// 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' {
-		// write indentation
-		// use "hard" htabs - indentation columns
-		// must not be discarded by the tabwriter
-		const htabs = "\t\t\t\t\t\t\t\t"
-		j := p.indent
-		for j > len(htabs) {
-			p.output.WriteString(htabs)
-			j -= len(htabs)
-		}
-		p.output.WriteString(htabs[0:j])
-
-		// update p.pos
-		p.pos.Line++
-		p.pos.Offset += p.indent
-		p.pos.Column = 1 + p.indent
+		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
 }
 
-// writeByteN writes ch n times to p.output and updates p.pos.
-func (p *printer) writeByteN(ch byte, n int) {
-	for n > 0 {
-		p.writeByte(ch)
-		n--
-	}
-}
-
-// writeString writes the string s to p.output and updates p.pos.
-// If isLit is set, s is escaped w/ tabwriter.Escape characters
+// 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
@@ -195,59 +212,66 @@ func (p *printer) writeByteN(ch byte, n int) {
 // avoids processing extra escape characters and reduces run time of the
 // printer benchmark by up to 10%.
 //
-func (p *printer) writeString(s string, isLit bool) {
+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
+		// reset state if the file changed
+		// (used when printing merged ASTs of different files
+		// e.g., the result of ast.MergePackageFiles)
+		if p.last.IsValid() && p.last.Filename != pos.Filename {
+			p.indent = 0
+			p.mode = 0
+			p.wsbuf = p.wsbuf[0:0]
+		}
+	}
+
 	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.WriteByte(tabwriter.Escape)
+		p.output = append(p.output, tabwriter.Escape)
 	}
 
-	p.output.WriteString(s)
+	if debug {
+		p.output = append(p.output, fmt.Sprintf("/*%s*/", pos)...) // do not update p.pos!
+	}
+	p.output = append(p.output, s...)
 
-	// update p.pos
+	// update positions
 	nlines := 0
-	column := p.pos.Column + len(s)
+	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++
-			column = len(s) - i
+			li = i
 		}
 	}
 	p.pos.Offset += len(s)
-	p.pos.Line += nlines
-	p.pos.Column = column
+	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.WriteByte(tabwriter.Escape)
+		p.output = append(p.output, tabwriter.Escape)
 	}
-}
 
-// writeItem writes data at position pos. data is the text corresponding to
-// a single lexical token, but may also be comment text. pos is the actual
-// (or at least very accurately estimated) position of the data in the original
-// source text. writeItem updates p.last to the position immediately following
-// the data.
-//
-func (p *printer) writeItem(pos token.Position, data string, isLit bool) {
-	if pos.IsValid() {
-		// continue with previous position if we don't have a valid pos
-		if p.last.IsValid() && p.last.Filename != pos.Filename {
-			// the file has changed - reset state
-			// (used when printing merged ASTs of different files
-			// e.g., the result of ast.MergePackageFiles)
-			p.indent = 0
-			p.mode = 0
-			p.wsbuf = p.wsbuf[0:0]
-		}
-		p.pos = pos
-	}
-	if debug {
-		// do not update p.pos - use write0
-		fmt.Fprintf(&p.output, "/*%s*/", pos)
-	}
-	p.writeString(data, isLit)
 	p.last = p.pos
 }
 
@@ -262,14 +286,14 @@ const linePrefix = "//line "
 // next item is a keyword.
 //
 func (p *printer) writeCommentPrefix(pos, next token.Position, prev, comment *ast.Comment, isKeyword bool) {
-	if p.output.Len() == 0 {
+	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.writeByteN('\f', maxNewlines)
+		p.writeByte('\f', maxNewlines)
 		return
 	}
 
@@ -309,7 +333,7 @@ func (p *printer) writeCommentPrefix(pos, next token.Position, prev, comment *as
 				// with a blank instead of a tab
 				sep = ' '
 			}
-			p.writeByte(sep)
+			p.writeByte(sep, 1)
 		}
 
 	} else {
@@ -381,7 +405,7 @@ func (p *printer) writeCommentPrefix(pos, next token.Position, prev, comment *as
 			// use formfeeds to break columns before a comment;
 			// this is analogous to using formfeeds to separate
 			// individual lines of /*-style comments
-			p.writeByteN('\f', nlimit(n))
+			p.writeByte('\f', nlimit(n))
 			p.indent = indent // restore indent
 		}
 	}
@@ -587,7 +611,7 @@ func (p *printer) writeComment(comment *ast.Comment) {
 
 	// shortcut common case of //-style comments
 	if text[1] == '/' {
-		p.writeItem(p.posFor(comment.Pos()), text, true)
+		p.writeString(p.posFor(comment.Pos()), text, true)
 		return
 	}
 
@@ -601,11 +625,11 @@ func (p *printer) writeComment(comment *ast.Comment) {
 	pos := p.posFor(comment.Pos())
 	for i, line := range lines {
 		if i > 0 {
-			p.writeByte('\f')
+			p.writeByte('\f', 1)
 			pos = p.pos
 		}
 		if len(line) > 0 {
-			p.writeItem(pos, line, true)
+			p.writeString(pos, line, true)
 		}
 	}
 }
@@ -643,7 +667,7 @@ func (p *printer) writeCommentSuffix(needsLinebreak bool) (wroteNewline, dropped
 
 	// make sure we have a line break
 	if needsLinebreak {
-		p.writeByte('\n')
+		p.writeByte('\n', 1)
 		wroteNewline = true
 	}
 
@@ -671,7 +695,7 @@ func (p *printer) intersperseComments(next token.Position, tok token.Token) (wro
 		if last.Text[1] == '*' && p.lineFor(last.Pos()) == next.Line {
 			// the last comment is a /*-style comment and the next item
 			// follows on the same line: separate with an extra blank
-			p.writeByte(' ')
+			p.writeByte(' ', 1)
 		}
 		// ensure that there is a line break after a //-style comment,
 		// before a closing '}' unless explicitly disabled, or at eof
@@ -722,7 +746,7 @@ func (p *printer) writeWhitespace(n int) {
 			}
 			fallthrough
 		default:
-			p.writeByte(byte(ch))
+			p.writeByte(byte(ch), 1)
 		}
 	}
 
@@ -886,12 +910,12 @@ func (p *printer) print(args ...interface{}) {
 				if droppedFF {
 					ch = '\f' // use formfeed since we dropped one before
 				}
-				p.writeByteN(ch, n)
+				p.writeByte(ch, n)
 				impliedSemi = false
 			}
 		}
 
-		p.writeItem(next, data, isLit)
+		p.writeString(next, data, isLit)
 		p.impliedSemi = impliedSemi
 	}
 }
@@ -1027,7 +1051,7 @@ unsupported:
 type trimmer struct {
 	output io.Writer
 	state  int
-	space  bytes.Buffer
+	space  []byte
 }
 
 // trimmer is implemented as a state machine.
@@ -1038,6 +1062,11 @@ const (
 	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.
@@ -1062,36 +1091,33 @@ func (p *trimmer) Write(data []byte) (n int, err error) {
 		case inSpace:
 			switch b {
 			case '\t', ' ':
-				p.space.WriteByte(b) // WriteByte returns no errors
+				p.space = append(p.space, b)
 			case '\n', '\f':
-				p.space.Reset() // discard trailing space
+				p.resetSpace() // discard trailing space
 				_, err = p.output.Write(aNewline)
 			case tabwriter.Escape:
-				_, err = p.output.Write(p.space.Bytes())
+				_, err = p.output.Write(p.space)
 				p.state = inEscape
 				m = n + 1 // +1: skip tabwriter.Escape
 			default:
-				_, err = p.output.Write(p.space.Bytes())
+				_, 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.state = inSpace
-				p.space.Reset()
+				p.resetSpace()
 			}
 		case inText:
 			switch b {
 			case '\t', ' ':
 				_, err = p.output.Write(data[m:n])
-				p.state = inSpace
-				p.space.Reset()
-				p.space.WriteByte(b) // WriteByte returns no errors
+				p.resetSpace()
+				p.space = append(p.space, b)
 			case '\n', '\f':
 				_, err = p.output.Write(data[m:n])
-				p.state = inSpace
-				p.space.Reset()
+				p.resetSpace()
 				_, err = p.output.Write(aNewline)
 			case tabwriter.Escape:
 				_, err = p.output.Write(data[m:n])
@@ -1110,8 +1136,7 @@ func (p *trimmer) Write(data []byte) (n int, err error) {
 	switch p.state {
 	case inEscape, inText:
 		_, err = p.output.Write(data[m:n])
-		p.state = inSpace
-		p.space.Reset()
+		p.resetSpace()
 	}
 
 	return
@@ -1120,16 +1145,19 @@ func (p *trimmer) Write(data []byte) (n int, err error) {
 // ----------------------------------------------------------------------------
 // Public interface
 
-// General printing is controlled with these Config.Mode flags.
+// A Mode value is a set of flags (or 0). They coontrol printing. 
+type Mode uint
+
 const (
-	RawFormat uint = 1 << iota // do not use a tabwriter; if set, UseSpaces is ignored
+	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     uint // default: 0
+	Mode     Mode // default: 0
 	Tabwidth int  // default: 8
 }
 
@@ -1170,7 +1198,7 @@ func (cfg *Config) fprint(output io.Writer, fset *token.FileSet, node interface{
 	}
 
 	// write printer result via tabwriter/trimmer to output
-	if _, err = output.Write(p.output.Bytes()); err != nil {
+	if _, err = output.Write(p.output); err != nil {
 		return
 	}
 

src/pkg/go/printer/printer_test.go

@@ -223,7 +223,8 @@ func TestBadNodes(t *testing.T) {
 	}
 }
 
-// Print and parse f with 
+// 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
@@ -280,3 +281,128 @@ func fibo(n int) {
 	testComment(t, f, len(src), &ast.Comment{pos, "/*-style \n comment */"})
 	testComment(t, f, len(src), &ast.Comment{pos, "/*-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())
+	}
+}
+
+// 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 _() {}
+`
+	// parse original
+	f, err := parser.ParseFile(fset, "src", src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// pretty-print original
+	var buf bytes.Buffer
+	if err = (&Config{Mode: UseSpaces, Tabwidth: 8}).Fprint(&buf, fset, f); err != nil {
+		t.Fatal(err)
+	}
+
+	// parse pretty printed original
+	if _, err := parser.ParseFile(fset, "", buf.Bytes(), 0); err != nil {
+		t.Fatalf("%s\n%s", err, buf.Bytes())
+	}
+
+}