cmd/cgo: reject references to builtin functions other than calls

Here we restrict using cgo builtin references because internally they're go functions
as opposed to C usafe.Pointer values.

Fixes #18889

Change-Id: I1e4332e4884063ccbaf9772c172d4462ec8f3d13
Reviewed-on: https://go-review.googlesource.com/40934
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/errors/issue18889.go

@@ -0,0 +1,7 @@
+package main
+
+import "C"
+
+func main() {
+	_ = C.malloc // ERROR HERE
+}

misc/cgo/errors/test.bash

@@ -47,6 +47,7 @@ expect issue13635.go C.uchar C.schar C.ushort C.uint C.ulong C.longlong C.ulongl
 check issue13830.go
 check issue16116.go
 check issue16591.go
+check issue18889.go
 
 if ! go build issue14669.go; then
 	exit 1

src/cmd/cgo/gcc.go

@@ -1086,6 +1086,10 @@ func (p *Package) rewriteRef(f *File) {
 			}
 		case "expr":
 			if r.Name.Kind == "func" {
+				if builtinDefs[r.Name.C] != "" {
+					error_(r.Pos(), "use of builtin '%s' not in function call", fixGo(r.Name.C))
+				}
+
 				// Function is being used in an expression, to e.g. pass around a C function pointer.
 				// Create a new Name for this Ref which causes the variable to be declared in Go land.
 				fpName := "fp_" + r.Name.Go