gofmt, go/printer: do not insert extra linebreaks where they may break the code

Introduced a printer mode (pmode) type and corresponding
pmode values which permit easy toggling of the current
printer mode for fine-tuning of layout.

Use the printer mode to disable potential introduction of
line breaks before a closing '}' in composite literals.

Added extra test case. Applied gofmt to src and misc.

Fixes #1365.

R=r2
CC=golang-dev
https://golang.org/cl/4008041

src/pkg/go/printer/nodes.go

@@ -872,7 +872,10 @@ func (p *printer) expr1(expr ast.Expr, prec1, depth int, ctxt exprContext, multi
 		}
 		p.print(x.Lbrace, token.LBRACE)
 		p.exprList(x.Lbrace, x.Elts, 1, commaSep|commaTerm, multiLine, x.Rbrace)
-		p.print(x.Rbrace, token.RBRACE)
+		// do not insert extra line breaks because of comments before
+		// the closing '}' as it might break the code if there is no
+		// trailing ','
+		p.print(noExtraLinebreak, x.Rbrace, token.RBRACE, noExtraLinebreak)
 
 	case *ast.Ellipsis:
 		p.print(token.ELLIPSIS)
@@ -1388,7 +1391,7 @@ func (p *printer) funcBody(b *ast.BlockStmt, headerSize int, isLit bool, multiLi
 		if isLit {
 			sep = blank
 		}
-		p.print(sep, b.Pos(), token.LBRACE)
+		p.print(sep, b.Lbrace, token.LBRACE)
 		if len(b.List) > 0 {
 			p.print(blank)
 			for i, s := range b.List {

src/pkg/go/printer/printer.go

@@ -58,6 +58,15 @@ var infinity = 1 << 30
 var ignoreMultiLine = new(bool)
 
 
+// A pmode value represents the current printer mode.
+type pmode int
+
+const (
+	inLiteral pmode = 1 << iota
+	noExtraLinebreak
+)
+
+
 type printer struct {
 	// Configuration (does not change after initialization)
 	output io.Writer
@@ -69,7 +78,7 @@ type printer struct {
 	nesting int         // nesting level (0: top-level (package scope), >0: functions/decls.)
 	written int         // number of bytes written
 	indent  int         // current indentation
-	escape  bool        // true if in escape sequence
+	mode    pmode       // current printer mode
 	lastTok token.Token // the last token printed (token.ILLEGAL if it's whitespace)
 
 	// Buffered whitespace
@@ -162,7 +171,7 @@ func (p *printer) write(data []byte) {
 			p.pos.Line++
 			p.pos.Column = 1
 
-			if !p.escape {
+			if p.mode&inLiteral == 0 {
 				// write indentation
 				// use "hard" htabs - indentation columns
 				// must not be discarded by the tabwriter
@@ -211,7 +220,7 @@ func (p *printer) write(data []byte) {
 			}
 
 		case tabwriter.Escape:
-			p.escape = !p.escape
+			p.mode ^= inLiteral
 
 			// ignore escape chars introduced by printer - they are
 			// invisible and must not affect p.pos (was issue #1089)
@@ -272,7 +281,7 @@ func (p *printer) writeItem(pos token.Position, data []byte, tag HTMLTag) {
 			// (used when printing merged ASTs of different files
 			// e.g., the result of ast.MergePackageFiles)
 			p.indent = 0
-			p.escape = false
+			p.mode = 0
 			p.buffer = p.buffer[0:0]
 			fileChanged = true
 		}
@@ -683,9 +692,13 @@ func (p *printer) intersperseComments(next token.Position, tok token.Token) (dro
 			// follows on the same line: separate with an extra blank
 			p.write([]byte{' '})
 		}
-		// ensure that there is a newline after a //-style comment
+		// ensure that there is a line break after a //-style comment,
-		// or if we are before a closing '}' or at the end of a file
+		// before a closing '}' unless explicitly disabled, or at eof
-		return p.writeCommentSuffix(last.Text[1] == '/' || tok == token.RBRACE || tok == token.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
@@ -787,6 +800,9 @@ func (p *printer) print(args ...interface{}) {
 		var tok token.Token
 
 		switch x := f.(type) {
+		case pmode:
+			// toggle printer mode
+			p.mode ^= x
 		case whiteSpace:
 			if x == ignore {
 				// don't add ignore's to the buffer; they
@@ -818,10 +834,14 @@ func (p *printer) print(args ...interface{}) {
 				data = x.Value
 			}
 			// escape all literals so they pass through unchanged
-			// (note that valid Go programs cannot contain esc ('\xff')
+			// (note that valid Go programs cannot contain
-			// bytes since they do not appear in legal UTF-8 sequences)
+			// tabwriter.Escape bytes since they do not appear in
-			// TODO(gri): do this more efficiently.
+			// legal UTF-8 sequences)
-			data = []byte("\xff" + string(data) + "\xff")
+			escData := make([]byte, 0, len(data)+2)
+			escData = append(escData, tabwriter.Escape)
+			escData = append(escData, data...)
+			escData = append(escData, tabwriter.Escape)
+			data = escData
 			tok = x.Kind
 		case token.Token:
 			s := x.String()

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

@@ -422,7 +422,7 @@ func _() {
 
 func ( /* comment1 */ T /* comment2 */ ) _()	{}
 
-func _() { /* one-liner */
+func _() { /* one-line functions with comments are formatted as multi-line functions */
 }
 
 func _() {
@@ -430,6 +430,10 @@ func _() {
 	/* closing curly brace should be on new line */
 }
 
+func _() {
+	_ = []int{0, 1 /* don't introduce a newline after this comment - was issue 1365 */ }
+}
+
 
 // Comments immediately adjacent to punctuation (for which the go/printer
 // may obly have estimated position information) must remain after the punctuation.

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

@@ -422,12 +422,16 @@ func _() {
 
 func (/* comment1 */ T /* comment2 */) _() {}
 
-func _() { /* one-liner */ }
+func _() { /* one-line functions with comments are formatted as multi-line functions */ }
 
 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 */}
+}
+
 
 // Comments immediately adjacent to punctuation (for which the go/printer
 // may obly have estimated position information) must remain after the punctuation.