cmd/cgo: don't replace newlines with semicolons in expressions

Fixes #29781

Change-Id: Id032d07a54b8c24f0c6d3f6e512932f76920ee04
Reviewed-on: https://go-review.googlesource.com/c/158457
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/cgo/test/issue29781.go

+// Copyright 2019 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.
+
+// Error with newline inserted into constant expression.
+// Compilation test only, nothing to run.
+
+package cgotest
+
+// static void issue29781F(char **p, int n) {}
+// #define ISSUE29781C 0
+import "C"
+
+func issue29781G() {
+	var p *C.char
+	C.issue29781F(&p, C.ISSUE29781C+1)
+}

src/cmd/cgo/godefs.go

@@ -131,12 +131,27 @@ func gofmt(n interface{}) string {
 // AST expression onto a single line. The lexer normally inserts a
 // semicolon at each newline, so we can replace newline with semicolon.
 // However, we can't do that in cases where the lexer would not insert
-// a semicolon. Fortunately we only have to worry about cases that
-// can occur in an expression passed through gofmt, which just means
-// composite literals.
+// a semicolon. We only have to worry about cases that can occur in an
+// expression passed through gofmt, which means composite literals and
+// (due to the printer possibly inserting newlines because of position
+// information) operators.
 var gofmtLineReplacer = strings.NewReplacer(
 	"{\n", "{",
 	",\n", ",",
+	"++\n", "++;",
+	"--\n", "--;",
+	"+\n", "+",
+	"-\n", "-",
+	"*\n", "*",
+	"/\n", "/",
+	"%\n", "%",
+	"&\n", "&",
+	"|\n", "|",
+	"^\n", "^",
+	"<\n", "<",
+	">\n", ">",
+	"=\n", "=",
+	",\n", ",",
 	"\n", ";",
 )