cmd/cgo: don't let inserted /*line*/ become a // comment

Fixes #29383

Change-Id: I0fb2929863e153b96d32d851e25e536231e4ae65
Reviewed-on: https://go-review.googlesource.com/c/155638
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

misc/cgo/test/issue29383.go

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// cgo's /*line*/ comments failed when inserted after '/',
+// because the result looked like a "//" comment.
+// No runtime test; just make sure it compiles.
+
+package cgotest
+
+// #include <stddef.h>
+import "C"
+
+func Issue29383(n, size uint) int {
+	if ^C.size_t(0)/C.size_t(n) < C.size_t(size) {
+		return 0
+	}
+	return 0
+}

src/cmd/cgo/gcc.go

@@ -1389,7 +1389,9 @@ func (p *Package) rewriteRef(f *File) {
 
 		// Record source-level edit for cgo output.
 		if !r.Done {
-			repl := gofmtPos(expr, old.Pos())
+			// Prepend a space in case the earlier code ends
+			// with '/', which would give us a "//" comment.
+			repl := " " + gofmtPos(expr, old.Pos())
 			end := fset.Position(old.End())
 			// Subtract 1 from the column if we are going to
 			// append a close parenthesis. That will set the
@@ -1399,7 +1401,7 @@ func (p *Package) rewriteRef(f *File) {
 				sub = 1
 			}
 			if end.Column > sub {
-				repl = fmt.Sprintf("%s/*line :%d:%d*/", repl, end.Line, end.Column-sub)
+				repl = fmt.Sprintf("%s /*line :%d:%d*/", repl, end.Line, end.Column-sub)
 			}
 			if r.Name.Kind != "type" {
 				repl = "(" + repl + ")"