Check for unused variables in GN.

tools/gn/functions.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 "tools/gn/functions.h"
 
 #include <iostream>
 
 #include "base/environment.h"
 #include "base/strings/string_util.h"
@@ skipping 721 matching lines @@
   if (found_function->second.executed_block_runner) {
     if (!block) {
       FillNeedsBlockError(function, err);
       return Value();
     }
 
     Scope block_scope(scope);
     block->ExecuteBlockInScope(&block_scope, err);
     if (err->has_error())
       return Value();
-    return found_function->second.executed_block_runner(
+
+    Value result = found_function->second.executed_block_runner(
         function, args.list_value(), &block_scope, err);
+
+    if (!block_scope.CheckForUnusedVars(err))
+      return Value();
+    return result;
   }
 
   // Otherwise it's a no-block function.
   return found_function->second.no_block_runner(scope, function,
                                                 args.list_value(), err);
 }
 
 }  // namespace functions