GN foreach should mark the list identifier as used.

tools/gn/function_foreach_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/test_with_scope.h"
 
 TEST(FunctionForeach, CollisionOnLoopVar) {
   TestWithScope setup;
   TestParseInput input(
@@ skipping 33 matching lines @@
   TestParseInput input_bad(
       "foreach(i, [1, 2, 3]) {\n"
       "  print(i)\n"
       "}\n"
       "print(i)");
   ASSERT_FALSE(input_bad.has_error());  // Should parse OK.
 
   input_bad.parsed()->Execute(setup.scope(), &err);
   ASSERT_TRUE(err.has_error());  // Shouldn't actually run.
 }
+
+// Checks that the identifier used as the list is marked as "used".
+TEST(FunctionForeach, MarksIdentAsUsed) {
+  TestWithScope setup;
+  TestParseInput input_good(
+      "a = [1, 2]\n"
+      "foreach(i, a) {\n"
+      "  print(i)\n"
+      "}\n");
+  ASSERT_FALSE(input_good.has_error());
+
+  Err err;
+  input_good.parsed()->Execute(setup.scope(), &err);
+  ASSERT_FALSE(err.has_error()) << err.message();
+
+  EXPECT_EQ("1\n2\n", setup.print_output());
+  setup.print_output().clear();
+
+  // Check for unused vars.
+  EXPECT_TRUE(setup.scope()->CheckForUnusedVars(&err));
+  EXPECT_FALSE(err.has_error());
+}