GN foreach should mark the list identifier as used.

tools/gn/functions_target_unittest.cc

+// Copyright (c) 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "testing/gtest/include/gtest/gtest.h"
+#include "tools/gn/scheduler.h"
+#include "tools/gn/scope.h"
+#include "tools/gn/test_with_scope.h"
+
+
+// Checks that we find unused identifiers in targets.
+TEST(FunctionsTarget, CheckUnused) {
+  Scheduler scheduler;
+  TestWithScope setup;
+
+  // The target generator needs a place to put the targets or it will fail.
+  Scope::ItemVector item_collector;
+  setup.scope()->set_item_collector(&item_collector);
+
+  // Test a good one first.
+  TestParseInput good_input(
+      "source_set(\"foo\") {\n"
+      "}\n");
+  ASSERT_FALSE(good_input.has_error());
+  Err err;
+  good_input.parsed()->Execute(setup.scope(), &err);
+  ASSERT_FALSE(err.has_error()) << err.message();
+
+  // Test a source set (this covers everything but component) with an unused
+  // variable.
+  TestParseInput source_set_input(
+      "source_set(\"foo\") {\n"
+      "  unused = 5\n"
+      "}\n");
+  ASSERT_FALSE(source_set_input.has_error());
+  err = Err();
+  source_set_input.parsed()->Execute(setup.scope(), &err);
+  ASSERT_TRUE(err.has_error());
+
+  // Test a component, which is a different code path.
+  TestParseInput component_input(
+      "component_mode = \"static_library\"\n"
+      "component(\"bar\") {\n"
+      "  unused = 5\n"
+      "}\n");
+  ASSERT_FALSE(component_input.has_error());
+  err = Err();
+  component_input.parsed()->Execute(setup.scope(), &err);
+  ASSERT_TRUE(err.has_error()) << err.message();
+}