Add not_needed function

tools/gn/functions_target_unittest.cc

Index: tools/gn/functions_target_unittest.cc
diff --git a/tools/gn/functions_target_unittest.cc b/tools/gn/functions_target_unittest.cc
index 525bc7cbdb809e2eef2872e3935da64c4d219f88..15d2d77e6b4252f2d35c9246117ec0415b5d79a0 100644
--- a/tools/gn/functions_target_unittest.cc
+++ b/tools/gn/functions_target_unittest.cc
@@ -37,6 +37,45 @@ TEST(FunctionsTarget, CheckUnused) {
   ASSERT_TRUE(err.has_error());
 }
 
+// Checks that we find uses of identifiers marked as unused.
+TEST(FunctionsTarget, CheckNotNeeded) {
+  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(
+      "a = {x = 1 y = 2}\n"
+      "not_needed(a, \"*\")\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 varible being used before being declared unused.
+  TestParseInput before_input(
+      "a = {x = 1 y = 2}\n"
+      "b = a.x\n"
+      "not_needed(a, \"*\")\n");
+  ASSERT_FALSE(before_input.has_error());
+  err = Err();
+  before_input.parsed()->Execute(setup.scope(), &err);
+  ASSERT_TRUE(err.has_error());
+
+  // Test a varible being used after being declared unused.
+  TestParseInput after_input(
+      "a = {x = 1 y = 2}\n"
+      "assert_unused(a, \"*\")\n"
+      "assert_unused({b = a.x}, [ \"b\" ])\n");
+  ASSERT_FALSE(after_input.has_error());
+  err = Err();
+  after_input.parsed()->Execute(setup.scope(), &err);
+  ASSERT_TRUE(err.has_error());
+}
+
 // Checks that the defaults applied to a template invoked by target() use
 // the name of the template, rather than the string "target" (which is the
 // name of the actual function being called).