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/Android.bp

@@ -50,6 +50,7 @@ bootstrap_go_package {
         "fuzz_test.go",
         "image_test.go",
         "library_test.go",
+        "proc_macro_test.go",
         "project_json_test.go",
         "protobuf_test.go",
         "rust_test.go",

rust/proc_macro.go

@@ -63,6 +63,12 @@ func (procMacro *procMacroDecorator) compilerProps() []interface{} {
 		&procMacro.Properties)
 }
 
+func (procMacro *procMacroDecorator) compilerFlags(ctx ModuleContext, flags Flags) Flags {
+	flags = procMacro.baseCompiler.compilerFlags(ctx, flags)
+	flags.RustFlags = append(flags.RustFlags, "--extern proc_macro")
+	return flags
+}
+
 func (procMacro *procMacroDecorator) compile(ctx ModuleContext, flags Flags, deps PathDeps) android.Path {
 	fileName := procMacro.getStem(ctx) + ctx.toolchain().ProcMacroSuffix()
 	outputFile := android.PathForModuleOut(ctx, fileName)

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"])
+	}
+}