cmd/cgo: avoid using common names for sniffing

Current code uses names like "x" and "s" which can conflict with user's
code easily. Use cryptographic names.

Fixes #21668

Change-Id: Ib6d3d6327aa5b92d95c71503d42e3a79d96c8e15
Reviewed-on: https://go-review.googlesource.com/59710Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/test/issue21668.go

+// Copyright 2017 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.
+
+// Fail to guess the kind of the constant "x".
+// No runtime test; just make sure it compiles.
+
+package cgotest
+
+// const int x = 42;
+import "C"
+
+var issue21668_X = C.x

src/cmd/cgo/gcc.go

@@ -296,15 +296,15 @@ func (p *Package) guessKinds(f *File) []*Name {
 	// For each name, we generate these lines, where xxx is the index in toSniff plus one.
 	//
 	//	#line xxx "not-declared"
-	//	void __cgo_f_xxx_1(void) { __typeof__(name) *__cgo_undefined__; }
+	//	void __cgo_f_xxx_1(void) { __typeof__(name) *__cgo_undefined__1; }
 	//	#line xxx "not-type"
-	//	void __cgo_f_xxx_2(void) { name *__cgo_undefined__; }
+	//	void __cgo_f_xxx_2(void) { name *__cgo_undefined__2; }
 	//	#line xxx "not-int-const"
-	//	void __cgo_f_xxx_3(void) { enum { __cgo_undefined__ = (name)*1 }; }
+	//	void __cgo_f_xxx_3(void) { enum { __cgo_undefined__3 = (name)*1 }; }
 	//	#line xxx "not-num-const"
-	//	void __cgo_f_xxx_4(void) { static const double x = (name); }
+	//	void __cgo_f_xxx_4(void) { static const double __cgo_undefined__4 = (name); }
 	//	#line xxx "not-str-lit"
-	//	void __cgo_f_xxx_5(void) { static const char x[] = (name); }
+	//	void __cgo_f_xxx_5(void) { static const char __cgo_undefined__5[] = (name); }
 	//	#line xxx "not-signed-int-const"
 	//	#if 0 < -(name)
 	//	#line xxx "not-signed-int-const"
@@ -327,15 +327,15 @@ func (p *Package) guessKinds(f *File) []*Name {
 
 	for i, n := range names {
 		fmt.Fprintf(&b, "#line %d \"not-declared\"\n"+
-			"void __cgo_f_%d_1(void) { __typeof__(%s) *__cgo_undefined__; }\n"+
+			"void __cgo_f_%d_1(void) { __typeof__(%s) *__cgo_undefined__1; }\n"+
 			"#line %d \"not-type\"\n"+
-			"void __cgo_f_%d_2(void) { %s *__cgo_undefined__; }\n"+
+			"void __cgo_f_%d_2(void) { %s *__cgo_undefined__2; }\n"+
 			"#line %d \"not-int-const\"\n"+
-			"void __cgo_f_%d_3(void) { enum { __cgo_undefined__ = (%s)*1 }; }\n"+
+			"void __cgo_f_%d_3(void) { enum { __cgo_undefined__3 = (%s)*1 }; }\n"+
 			"#line %d \"not-num-const\"\n"+
-			"void __cgo_f_%d_4(void) { static const double x = (%s); }\n"+
+			"void __cgo_f_%d_4(void) { static const double __cgo_undefined__4 = (%s); }\n"+
 			"#line %d \"not-str-lit\"\n"+
-			"void __cgo_f_%d_5(void) { static const char s[] = (%s); }\n"+
+			"void __cgo_f_%d_5(void) { static const char __cgo_undefined__5[] = (%s); }\n"+
 			"#line %d \"not-signed-int-const\"\n"+
 			"#if 0 < (%s)\n"+
 			"#line %d \"not-signed-int-const\"\n"+