go/printer: refine handling of one-line functions

Functions that "fit" on one line and were on one
line in the original source are not broken up into
two lines anymore simply because they contain a comment.

- Fine-tuned use of separating blanks after /*-style comments, so:

( /* extra blank after this comment */ )
(a int /* no extra blank after this comment*/)

- Factored out comment state (from printer state) into commentInfo.
- No impact on $GOROOT/src, misc formatting.

Fixes #5543.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/68630043

src/pkg/go/printer/nodes.go

@@ -826,10 +826,16 @@ func (p *printer) expr1(expr ast.Expr, prec1, depth int) {
 		}
 		p.print(x.Lbrace, token.LBRACE)
 		p.exprList(x.Lbrace, x.Elts, 1, commaTerm, x.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)
+		// do not insert extra line break following a /*-style comment
+		// before the closing '}' as it might break the code if there
+		// is no trailing ','
+		mode := noExtraLinebreak
+		// do not insert extra blank following a /*-style comment
+		// before the closing '}' unless the literal is empty
+		if len(x.Elts) > 0 {
+			mode |= noExtraBlank
+		}
+		p.print(mode, x.Rbrace, token.RBRACE, mode)
 
 	case *ast.Ellipsis:
 		p.print(token.ELLIPSIS)
@@ -1461,13 +1467,16 @@ func (p *printer) bodySize(b *ast.BlockStmt, maxSize int) int {
 		// opening and closing brace are on different lines - don't make it a one-liner
 		return maxSize + 1
 	}
-	if len(b.List) > 5 || p.commentBefore(p.posFor(pos2)) {
-		// too many statements or there is a comment inside - don't make it a one-liner
+	if len(b.List) > 5 {
+		// too many statements - don't make it a one-liner
 		return maxSize + 1
 	}
 	// otherwise, estimate body size
-	bodySize := 0
+	bodySize := p.commentSizeBefore(p.posFor(pos2))
 	for i, s := range b.List {
+		if bodySize > maxSize {
+			break // no need to continue
+		}
 		if i > 0 {
 			bodySize += 2 // space for a semicolon and blank
 		}
@@ -1501,7 +1510,7 @@ func (p *printer) adjBlock(headerSize int, sep whiteSpace, b *ast.BlockStmt) {
 			}
 			p.print(blank)
 		}
-		p.print(b.Rbrace, token.RBRACE)
+		p.print(noExtraLinebreak, b.Rbrace, token.RBRACE, noExtraLinebreak)
 		return
 	}
 

src/pkg/go/printer/printer.go

@@ -39,9 +39,17 @@ const (
 type pmode int
 
 const (
-	noExtraLinebreak pmode = 1 << iota
+	noExtraBlank     pmode = 1 << iota // disables extra blank after /*-style comment
+	noExtraLinebreak                   // disables extra line break after /*-style comment
 )
 
+type commentInfo struct {
+	cindex         int               // current comment index
+	comment        *ast.CommentGroup // = printer.comments[cindex]; or nil
+	commentOffset  int               // = printer.posFor(printer.comments[cindex].List[0].Pos()).Offset; or infinity
+	commentNewline bool              // true if the comment group contains newlines
+}
+
 type printer struct {
 	// Configuration (does not change after initialization)
 	Config
@@ -52,7 +60,8 @@ type printer struct {
 	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)
+	lastTok     token.Token  // last token printed (token.ILLEGAL if it's whitespace)
+	prevOpen    token.Token  // previous non-brace "open" token (, [, or token.ILLEGAL
 	wsbuf       []whiteSpace // delayed white space
 
 	// Positions
@@ -67,13 +76,10 @@ type printer struct {
 
 	// The list of all source comments, in order of appearance.
 	comments        []*ast.CommentGroup // may be nil
-	cindex          int                 // current comment index
 	useNodeComments bool                // if not set, ignore lead and line comments of nodes
 
 	// Information about p.comments[p.cindex]; set up by nextComment.
-	comment        *ast.CommentGroup // = p.comments[p.cindex]; or nil
-	commentOffset  int               // = p.posFor(p.comments[p.cindex].List[0].Pos()).Offset; or infinity
-	commentNewline bool              // true if the comment group contains newlines
+	commentInfo
 
 	// Cache of already computed node sizes.
 	nodeSizes map[ast.Node]int
@@ -129,6 +135,33 @@ func (p *printer) nextComment() {
 	p.commentOffset = infinity
 }
 
+// commentBefore returns true iff the current comment group occurs
+// before the next position in the source code and printing it does
+// not introduce implicit semicolons.
+//
+func (p *printer) commentBefore(next token.Position) bool {
+	return p.commentOffset < next.Offset && (!p.impliedSemi || !p.commentNewline)
+}
+
+// commentSizeBefore returns the estimated size of the
+// comments on the same line before the next position.
+//
+func (p *printer) commentSizeBefore(next token.Position) int {
+	// save/restore current p.commentInfo (p.nextComment() modifies it)
+	defer func(info commentInfo) {
+		p.commentInfo = info
+	}(p.commentInfo)
+
+	size := 0
+	for p.commentBefore(next) {
+		for _, c := range p.comment.List {
+			size += len(c.Text)
+		}
+		p.nextComment()
+	}
+	return size
+}
+
 func (p *printer) internalError(msg ...interface{}) {
 	if debug {
 		fmt.Print(p.pos.String() + ": ")
@@ -675,10 +708,14 @@ func (p *printer) intersperseComments(next token.Position, tok token.Token) (wro
 
 	if last != nil {
 		// if the last comment is a /*-style comment and the next item
-		// follows on the same line but is not a comma or a "closing"
-		// token, add an extra blank for separation
-		if last.Text[1] == '*' && p.lineFor(last.Pos()) == next.Line && tok != token.COMMA &&
-			tok != token.RPAREN && tok != token.RBRACK && tok != token.RBRACE {
+		// follows on the same line but is not a comma, and not a "closing"
+		// token immediately following its corresponding "opening" token,
+		// add an extra blank for separation unless explicitly disabled
+		if p.mode&noExtraBlank == 0 &&
+			last.Text[1] == '*' && p.lineFor(last.Pos()) == next.Line &&
+			tok != token.COMMA &&
+			(tok != token.RPAREN || p.prevOpen == token.LPAREN) &&
+			(tok != token.RBRACK || p.prevOpen == token.LBRACK) {
 			p.writeByte(' ', 1)
 		}
 		// ensure that there is a line break after a //-style comment,
@@ -735,12 +772,8 @@ func (p *printer) writeWhitespace(n int) {
 	}
 
 	// shift remaining entries down
-	i := 0
-	for ; n < len(p.wsbuf); n++ {
-		p.wsbuf[i] = p.wsbuf[n]
-		i++
-	}
-	p.wsbuf = p.wsbuf[0:i]
+	l := copy(p.wsbuf, p.wsbuf[n:])
+	p.wsbuf = p.wsbuf[:l]
 }
 
 // ----------------------------------------------------------------------------
@@ -790,6 +823,17 @@ func (p *printer) print(args ...interface{}) {
 		var isLit bool
 		var impliedSemi bool // value for p.impliedSemi after this arg
 
+		// record previous opening token, if any
+		switch p.lastTok {
+		case token.ILLEGAL:
+			// ignore (white space)
+		case token.LPAREN, token.LBRACK:
+			p.prevOpen = p.lastTok
+		default:
+			// other tokens followed any opening token
+			p.prevOpen = token.ILLEGAL
+		}
+
 		switch x := arg.(type) {
 		case pmode:
 			// toggle printer mode
@@ -904,14 +948,6 @@ func (p *printer) print(args ...interface{}) {
 	}
 }
 
-// commentBefore returns true iff the current comment group occurs
-// before the next position in the source code and printing it does
-// not introduce implicit semicolons.
-//
-func (p *printer) commentBefore(next token.Position) (result bool) {
-	return p.commentOffset < next.Offset && (!p.impliedSemi || !p.commentNewline)
-}
-
 // flush prints any pending comments and whitespace occurring textually
 // before the position of the next token tok. The flush result indicates
 // if a newline was written or if a formfeed was dropped from the whitespace

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

@@ -494,16 +494,21 @@ func _() {
 func _( /* this */ x /* is */ /* an */ int) {
 }
 
-func _( /* no params */)	{}
+func _( /* no params - extra blank before and after comment */ )	{}
+func _(a, b int /* params - no extra blank after comment */)		{}
+
+func _()	{ f( /* no args - extra blank before and after comment */ ) }
+func _()	{ f(a, b /* args - no extra blank after comment */) }
 
 func _() {
-	f( /* no args */)
+	f( /* no args - extra blank before and after comment */ )
+	f(a, b /* args - no extra blank after comment */)
 }
 
 func ( /* comment1 */ T /* comment2 */) _()	{}
 
-func _() { /* one-line functions with comments are formatted as multi-line functions */
-}
+func _()	{ /* "short-ish one-line functions with comments are formatted as multi-line functions */ }
+func _()	{ x := 0; /* comment */ y = x /* comment */ }
 
 func _() {
 	_ = 0

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

@@ -500,15 +500,21 @@ func _() {
 func _(/* this */x/* is *//* an */ int) {
 }
 
-func _(/* no params */) {}
+func _(/* no params - extra blank before and after comment */) {}
+func _(a, b int /* params - no extra blank after comment */) {}
+
+func _() { f(/* no args - extra blank before and after comment */) }
+func _() { f(a, b /* args - no extra blank after comment */) }
 
 func _() {
-	f(/* no args */)
+	f(/* no args - extra blank before and after comment */)
+	f(a, b /* args - no extra blank after comment */)
 }
 
 func (/* comment1 */ T /* comment2 */) _() {}
 
-func _() { /* one-line functions with comments are formatted as multi-line functions */ }
+func _() { /* "short-ish one-line functions with comments are formatted as multi-line functions */ }
+func _() { x := 0; /* comment */ y = x /* comment */ }
 
 func _() {
 	_ = 0

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

@@ -723,7 +723,8 @@ func _() int {
 }
 
 // making function declarations safe for new semicolon rules
-func _() { /* multi-line func because of comment */
+func _()	{ /* single-line function because of "short-ish" comment */ }
+func _() { /* multi-line function because of "long-ish" comment - much more comment text is following here */ /* and more */
 }
 
 func _() {

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

@@ -737,7 +737,8 @@ func _() int {
 
 
 // making function declarations safe for new semicolon rules
-func _() { /* multi-line func because of comment */ }
+func _() { /* single-line function because of "short-ish" comment */ }
+func _() { /* multi-line function because of "long-ish" comment - much more comment text is following here */ /* and more */ }
 
 func _() {
 /* multi-line func because block is on multiple lines */ }