go/printer, gofmt: trim trailing whitespace in comments

Also: updated go fix testcases to pass tests.

Fixes #4310.

R=r
CC=golang-dev
https://golang.org/cl/6810055

src/pkg/go/printer/printer.go

@@ -14,6 +14,7 @@ import (
 	"strconv"
 	"strings"
 	"text/tabwriter"
+	"unicode"
 )
 
 const (
@@ -405,6 +406,7 @@ func (p *printer) writeCommentPrefix(pos, next token.Position, prev, comment *as
 // Split comment text into lines
 // (using strings.Split(text, "\n") is significantly slower for
 // this specific purpose, as measured with: go test -bench=Print)
+//
 func split(text string) []string {
 	// count lines (comment text never ends in a newline)
 	n := 1
@@ -432,6 +434,7 @@ func split(text string) []string {
 
 // 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] > ' ' {
@@ -441,6 +444,7 @@ func isBlank(s string) bool {
 	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] == '*') {
@@ -449,6 +453,11 @@ func commonPrefix(a, b string) string {
 	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 is likely that the comment
@@ -611,7 +620,7 @@ func (p *printer) writeComment(comment *ast.Comment) {
 
 	// shortcut common case of //-style comments
 	if text[1] == '/' {
-		p.writeString(pos, text, true)
+		p.writeString(pos, trimRight(text), true)
 		return
 	}
 
@@ -641,7 +650,7 @@ func (p *printer) writeComment(comment *ast.Comment) {
 			pos = p.pos
 		}
 		if len(line) > 0 {
-			p.writeString(pos, line, true)
+			p.writeString(pos, trimRight(line), true)
 		}
 	}
 }

src/pkg/go/printer/testdata/comments.golden

@@ -626,4 +626,13 @@ func _() {
 	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 */

src/pkg/go/printer/testdata/comments.input

@@ -630,5 +630,13 @@ 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 */