Pass "--extern proc_macro" to rust_proc_macros.

This is actually required for some code.

Test: Modify the new test so it fails and see it fail.
Test: Build crates that fail without it.
Change-Id: I527752b765e5552aa2de7e201f056955e053e1f3

rust/proc_macro_test.go

@@ -0,0 +1,36 @@
+// Copyright 2021 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package rust
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestRustProcMacro(t *testing.T) {
+	ctx := testRust(t, `
+          rust_proc_macro {
+	    name: "libprocmacro",
+	    srcs: ["foo.rs"],
+	    crate_name: "procmacro",
+	  }
+	`)
+
+	libprocmacro := ctx.ModuleForTests("libprocmacro", "linux_glibc_x86_64").Rule("rustc")
+
+	if !strings.Contains(libprocmacro.Args["rustcFlags"], "--extern proc_macro") {
+		t.Errorf("--extern proc_macro flag not being passed to rustc for proc macro %#v", libprocmacro.Args["rustcFlags"])
+	}
+}