cmd/cgo: also rewrite C.var in selector expressions

While we're here, rename TestIssue7234 to Test7234 for consistency
with other tests.

Fixes #9557.

Change-Id: I22b0a212b31e7b4f199f6a70deb73374beb80f84
Reviewed-on: https://go-review.googlesource.com/2654Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/test/cgo_test.go

@@ -63,5 +63,6 @@ func Test8811(t *testing.T)                  { test8811(t) }
 func TestReturnAfterGrow(t *testing.T)       { testReturnAfterGrow(t) }
 func TestReturnAfterGrowFromGo(t *testing.T) { testReturnAfterGrowFromGo(t) }
 func Test9026(t *testing.T)                  { test9026(t) }
+func Test9557(t *testing.T)                  { test9557(t) }
 
 func BenchmarkCgoCall(b *testing.B) { benchCgoCall(b) }

misc/cgo/test/issue7234_test.go

@@ -14,7 +14,7 @@ import "testing"
 
 var v7234 = [...]string{"runtime/cgo"}
 
-func TestIssue7234(t *testing.T) {
+func Test7234(t *testing.T) {
 	if v7234[0] != "runtime/cgo" {
 		t.Errorf("bad string constant %q", v7234[0])
 	}

misc/cgo/test/issue9557.go

+// Copyright 2015 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 rewrote C.var to *_Cvar_var, but left
+// C.var.field as _Cvar.var.field.  It now rewrites
+// the latter as (*_Cvar_var).field.
+// See https://golang.org/issue/9557.
+
+package cgotest
+
+// struct issue9557_t {
+//   int a;
+// } test9557bar = { 42 };
+//
+// struct issue9557_t *issue9557foo = &test9557bar;
+import "C"
+import "testing"
+
+func test9557(t *testing.T) {
+	// implicitly dereference a Go variable
+	foo := C.issue9557foo
+	if v := foo.a; v != 42 {
+		t.Fatalf("foo.a expected 42, but got %d", v)
+	}
+
+	// explicitly dereference a C variable
+	if v := (*C.issue9557foo).a; v != 42 {
+		t.Fatalf("(*C.issue9557foo).a expected 42, but is %d", v)
+	}
+
+	// implicitly dereference a C variable
+	if v := C.issue9557foo.a; v != 42 {
+		t.Fatalf("C.issue9557foo.a expected 42, but is %d", v)
+	}
+}

src/cmd/cgo/gcc.go

@@ -659,6 +659,13 @@ func (p *Package) rewriteRef(f *File) {
 				expr = &ast.StarExpr{Star: (*r.Expr).Pos(), X: expr}
 			}
 
+		case "selector":
+			if r.Name.Kind == "var" {
+				expr = &ast.StarExpr{Star: (*r.Expr).Pos(), X: expr}
+			} else {
+				error_(r.Pos(), "only C variables allowed in selector expression", fixGo(r.Name.Go))
+			}
+
 		case "type":
 			if r.Name.Kind != "type" {
 				error_(r.Pos(), "expression C.%s used as type", fixGo(r.Name.Go))