go/printer: fix alignment of comments in labeled statements

Does not change src, misc formatting.

Fixes #5623.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/68400043

src/pkg/go/printer/nodes.go

@@ -906,15 +906,19 @@ func (p *printer) stmtList(list []ast.Stmt, nindent int, nextIsRBrace bool) {
 	for _, s := range list {
 		// ignore empty statements (was issue 3466)
 		if _, isEmpty := s.(*ast.EmptyStmt); !isEmpty {
-			// _indent == 0 only for lists of switch/select case clauses;
+			// nindent == 0 only for lists of switch/select case clauses;
 			// in those cases each clause is a new section
 			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.stmt(s, nextIsRBrace && i == len(list)-1)
-			multiLine = p.isMultiLine(s)
+			// 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))
 			i++
 		}
 	}
@@ -923,6 +927,15 @@ 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)

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

@@ -77,3 +77,28 @@ func main() {
 		println("test")
 	}
 }
+
+func issue5623() {
+L:
+	_ = yyyyyyyyyyyyyyyy			// comment - should be aligned
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	/* comment */
+
+	_ = yyyyyyyyyyyyyyyy			/* comment - should be aligned */
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	// comment
+
+LLLLLLL:
+	_ = yyyyyyyyyyyyyyyy			// comment - should be aligned
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	// comment
+
+LL:
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	/* comment */
+	_ = yyyyyyyyyyyyyyyy			/* comment - should be aligned */
+
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	// comment
+	_ = yyyyyyyyyyyyyyyy			// comment - should be aligned
+
+	// test case from issue
+label:
+	mask := uint64(1)<<c - 1		// Allocation mask
+	used := atomic.LoadUint64(&h.used)	// Current allocations
+}

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

@@ -76,4 +76,29 @@ prints test 5 times
    for i := 0; i < 5; i++ {
       println("test")
    }
-}
+}
+
+func issue5623() {
+L:
+   _ = yyyyyyyyyyyyyyyy // comment - should be aligned
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx /* comment */
+
+   _ = yyyyyyyyyyyyyyyy /* comment - should be aligned */
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx // comment
+
+LLLLLLL:
+   _ = yyyyyyyyyyyyyyyy // comment - should be aligned
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx // comment
+
+LL:
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx /* comment */
+   _ = yyyyyyyyyyyyyyyy /* comment - should be aligned */
+
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx // comment
+   _ = yyyyyyyyyyyyyyyy // comment - should be aligned
+
+// test case from issue
+label:
+   mask := uint64(1)<<c - 1 // Allocation mask
+   used := atomic.LoadUint64(&h.used) // Current allocations
+}