cmd/cgo: make C function pointers non-assignable

Fixes #7757.
Fixes #8488.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/118690044

misc/cgo/errors/issue7757.go

+// Copyright 2014 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.
+
+package main
+
+/*
+void foo() {}
+*/
+import "C"
+
+func main() {
+	C.foo = C.foo // ERROR HERE
+}

misc/cgo/errors/test.bash

@@ -27,6 +27,7 @@ check() {
 check err1.go
 check err2.go
 check err3.go
+check issue7757.go
 
 rm -rf errs _obj
 exit 0

src/cmd/cgo/gcc.go

@@ -650,7 +650,13 @@ func (p *Package) rewriteRef(f *File) {
 					f.Name[fpName] = name
 				}
 				r.Name = name
-				expr = ast.NewIdent(name.Mangle)
+				// Rewrite into call to _Cgo_ptr to prevent assignments.  The _Cgo_ptr
+				// function is defined in out.go and simply returns its argument. See
+				// issue 7757.
+				expr = &ast.CallExpr{
+					Fun:  &ast.Ident{NamePos: (*r.Expr).Pos(), Name: "_Cgo_ptr"},
+					Args: []ast.Expr{ast.NewIdent(name.Mangle)},
+				}
 			} else if r.Name.Kind == "type" {
 				// Okay - might be new(T)
 				expr = r.Name.Type.Go

src/cmd/cgo/out.go

@@ -64,7 +64,7 @@ func (p *Package) writeDefs() {
 	if !*gccgo && *importRuntimeCgo {
 		fmt.Fprintf(fgo2, "import _ \"runtime/cgo\"\n\n")
 	}
-	fmt.Fprintf(fgo2, "type _ unsafe.Pointer\n\n")
+	fmt.Fprintf(fgo2, "func _Cgo_ptr(ptr unsafe.Pointer) unsafe.Pointer { return ptr }\n\n")
 	if *importSyscall {
 		fmt.Fprintf(fgo2, "func _Cerrno(dst *error, x int32) { *dst = syscall.Errno(x) }\n")
 	}