go/printer: measure lines/construct in generated output rather than incoming source

No change to $GOROOT/src, misc formatting.

Nice side-effect: almost 3% faster runs because it's much faster to compute
line number differences in the generated output than the incoming source.

Benchmark run, best of 5 runs, before and after:
BenchmarkPrint	     200	  12347587 ns/op
BenchmarkPrint	     200	  11999061 ns/op

Fixes #4504.

LGTM=adonovan
R=golang-codereviews, adonovan
CC=golang-codereviews
https://golang.org/cl/69260045

src/pkg/go/printer/nodes.go

@@ -378,10 +378,6 @@ func (p *printer) setLineComment(text string) {
 	p.setComment(&ast.CommentGroup{List: []*ast.Comment{{Slash: token.NoPos, Text: text}}})
 }
 
-func (p *printer) isMultiLine(n ast.Node) bool {
-	return p.lineFor(n.End())-p.lineFor(n.Pos()) > 0
-}
-
 func (p *printer) fieldList(fields *ast.FieldList, isStruct, isIncomplete bool) {
 	lbrace := fields.Opening
 	list := fields.List
@@ -428,13 +424,14 @@ func (p *printer) fieldList(fields *ast.FieldList, isStruct, isIncomplete bool)
 		if len(list) == 1 {
 			sep = blank
 		}
-		newSection := false
+		var line int
 		for i, f := range list {
 			if i > 0 {
-				p.linebreak(p.lineFor(f.Pos()), 1, ignore, newSection)
+				p.linebreak(p.lineFor(f.Pos()), 1, ignore, p.linesFrom(line) > 0)
 			}
 			extraTabs := 0
 			p.setComment(f.Doc)
+			p.recordLine(&line)
 			if len(f.Names) > 0 {
 				// named fields
 				p.identList(f.Names, false)
@@ -460,7 +457,6 @@ func (p *printer) fieldList(fields *ast.FieldList, isStruct, isIncomplete bool)
 				}
 				p.setComment(f.Comment)
 			}
-			newSection = p.isMultiLine(f)
 		}
 		if isIncomplete {
 			if len(list) > 0 {
@@ -472,12 +468,13 @@ func (p *printer) fieldList(fields *ast.FieldList, isStruct, isIncomplete bool)
 
 	} else { // interface
 
-		newSection := false
+		var line int
 		for i, f := range list {
 			if i > 0 {
-				p.linebreak(p.lineFor(f.Pos()), 1, ignore, newSection)
+				p.linebreak(p.lineFor(f.Pos()), 1, ignore, p.linesFrom(line) > 0)
 			}
 			p.setComment(f.Doc)
+			p.recordLine(&line)
 			if ftyp, isFtyp := f.Type.(*ast.FuncType); isFtyp {
 				// method
 				p.expr(f.Names[0])
@@ -487,7 +484,6 @@ func (p *printer) fieldList(fields *ast.FieldList, isStruct, isIncomplete bool)
 				p.expr(f.Type)
 			}
 			p.setComment(f.Comment)
-			newSection = p.isMultiLine(f)
 		}
 		if isIncomplete {
 			if len(list) > 0 {
@@ -907,7 +903,7 @@ func (p *printer) stmtList(list []ast.Stmt, nindent int, nextIsRBrace bool) {
 	if nindent > 0 {
 		p.print(indent)
 	}
-	multiLine := false
+	var line int
 	i := 0
 	for _, s := range list {
 		// ignore empty statements (was issue 3466)
@@ -917,14 +913,21 @@ func (p *printer) stmtList(list []ast.Stmt, nindent int, nextIsRBrace bool) {
 			if len(p.output) > 0 {
 				// only print line break if we are not at the beginning of the output
 				// (i.e., we are not printing only a partial program)
-				p.linebreak(p.lineFor(s.Pos()), 1, ignore, i == 0 || nindent == 0 || multiLine)
+				p.linebreak(p.lineFor(s.Pos()), 1, ignore, i == 0 || nindent == 0 || p.linesFrom(line) > 0)
 			}
+			p.recordLine(&line)
 			p.stmt(s, nextIsRBrace && i == len(list)-1)
 			// labeled statements put labels on a separate line, but here
-			// we only care about whether the actual statement w/o label
-			// is a multi-line statement - remove the label first
-			// (was issue 5623)
-			multiLine = p.isMultiLine(unlabeledStmt(s))
+			// we only care about the start line of the actual statement
+			// without label - correct line for each label
+			for t := s; ; {
+				lt, _ := t.(*ast.LabeledStmt)
+				if lt == nil {
+					break
+				}
+				line++
+				t = lt.Stmt
+			}
 			i++
 		}
 	}
@@ -933,15 +936,6 @@ func (p *printer) stmtList(list []ast.Stmt, nindent int, nextIsRBrace bool) {
 	}
 }
 
-// unlabeledStmt returns the statement of a labeled statement s;
-// otherwise it return s.
-func unlabeledStmt(s ast.Stmt) ast.Stmt {
-	if s, _ := s.(*ast.LabeledStmt); s != nil {
-		return unlabeledStmt(s.Stmt)
-	}
-	return s
-}
-
 // block prints an *ast.BlockStmt; it always spans at least two lines.
 func (p *printer) block(b *ast.BlockStmt, nindent int) {
 	p.print(b.Lbrace, token.LBRACE)
@@ -1394,22 +1388,22 @@ func (p *printer) genDecl(d *ast.GenDecl) {
 				// two or more grouped const/var declarations:
 				// determine if the type column must be kept
 				keepType := keepTypeColumn(d.Specs)
-				newSection := false
+				var line int
 				for i, s := range d.Specs {
 					if i > 0 {
-						p.linebreak(p.lineFor(s.Pos()), 1, ignore, newSection)
+						p.linebreak(p.lineFor(s.Pos()), 1, ignore, p.linesFrom(line) > 0)
 					}
+					p.recordLine(&line)
 					p.valueSpec(s.(*ast.ValueSpec), keepType[i])
-					newSection = p.isMultiLine(s)
 				}
 			} else {
-				newSection := false
+				var line int
 				for i, s := range d.Specs {
 					if i > 0 {
-						p.linebreak(p.lineFor(s.Pos()), 1, ignore, newSection)
+						p.linebreak(p.lineFor(s.Pos()), 1, ignore, p.linesFrom(line) > 0)
 					}
+					p.recordLine(&line)
 					p.spec(s, n, false)
-					newSection = p.isMultiLine(s)
 				}
 			}
 			p.print(unindent, formfeed)

src/pkg/go/printer/printer.go

@@ -73,6 +73,7 @@ type printer struct {
 	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
+	linePtr *int           // if set, record out.Line for the next token in *linePtr
 
 	// The list of all source comments, in order of appearance.
 	comments        []*ast.CommentGroup // may be nil
@@ -99,6 +100,14 @@ func (p *printer) init(cfg *Config, fset *token.FileSet, nodeSizes map[ast.Node]
 	p.cachedPos = -1
 }
 
+func (p *printer) internalError(msg ...interface{}) {
+	if debug {
+		fmt.Print(p.pos.String() + ": ")
+		fmt.Println(msg...)
+		panic("go/printer")
+	}
+}
+
 // commentsHaveNewline reports whether a list of comments belonging to
 // an *ast.CommentGroup contains newlines. Because the position information
 // may only be partially correct, we also have to read the comment text.
@@ -162,12 +171,22 @@ func (p *printer) commentSizeBefore(next token.Position) int {
 	return size
 }
 
-func (p *printer) internalError(msg ...interface{}) {
-	if debug {
-		fmt.Print(p.pos.String() + ": ")
-		fmt.Println(msg...)
-		panic("go/printer")
-	}
+// recordLine records the output line number for the next non-whitespace
+// token in *linePtr. It is used to compute an accurate line number for a
+// formatted construct, independent of pending (not yet emitted) whitespace
+// or comments.
+//
+func (p *printer) recordLine(linePtr *int) {
+	p.linePtr = linePtr
+}
+
+// linesFrom returns the number of output lines between the current
+// output line and the line argument, ignoring any pending (not yet
+// emitted) whitespace or comments. It is used to compute an accurate
+// size (in number of lines) for a formatted construct.
+//
+func (p *printer) linesFrom(line int) int {
+	return p.out.Line - line
 }
 
 func (p *printer) posFor(pos token.Pos) token.Position {
@@ -943,6 +962,12 @@ func (p *printer) print(args ...interface{}) {
 			}
 		}
 
+		// the next token starts now - record its line number if requested
+		if p.linePtr != nil {
+			*p.linePtr = p.out.Line
+			p.linePtr = nil
+		}
+
 		p.writeString(next, data, isLit)
 		p.impliedSemi = impliedSemi
 	}

src/pkg/go/printer/testdata/comments2.golden

@@ -91,6 +91,7 @@ LLLLLLL:
 	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	// comment
 
 LL:
+LLLLL:
 	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	/* comment */
 	_ = yyyyyyyyyyyyyyyy			/* comment - should be aligned */
 

src/pkg/go/printer/testdata/comments2.input

@@ -91,6 +91,7 @@ LLLLLLL:
    _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx // comment
 
 LL:
+LLLLL:
    _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx /* comment */
    _ = yyyyyyyyyyyyyyyy /* comment - should be aligned */
 

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

@@ -397,6 +397,21 @@ func _() {
 	}
 }
 
+// use the formatted output rather than the input to decide when to align
+// (was issue 4505)
+const (
+	short		= 2 * (1 + 2)
+	aMuchLongerName	= 3
+)
+
+var (
+	short		= X{}
+	aMuchLongerName	= X{}
+
+	x1	= X{}	// foo
+	x2	= X{}	// foo
+)
+
 func _() {
 	type (
 		xxxxxx	int

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

@@ -409,6 +409,24 @@ func _() {
 	}
 }
 
+// use the formatted output rather than the input to decide when to align
+// (was issue 4505)
+const (
+	short = 2 * (
+	1 + 2)
+	aMuchLongerName = 3
+)
+
+var (
+	short = X{
+	}
+	aMuchLongerName = X{}
+
+	x1 = X{} // foo
+	x2 = X{
+	} // foo
+)
+
 func _() {
 	type (
 		xxxxxx int