GN: Mark all variables used when defining a template.

tools/gn/function_template.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 "tools/gn/parse_tree.h"
 #include "tools/gn/scope.h"
 #include "tools/gn/template.h"
 #include "tools/gn/value.h"
@@ skipping 174 matching lines @@
   const Template* existing_template = scope->GetTemplate(template_name);
   if (existing_template) {
     *err = Err(function, "Duplicate template definition.",
                "A template with this name was already defined.");
     err->AppendSubErr(Err(existing_template->GetDefinitionRange(),
                           "Previous definition."));
     return Value();
   }
 
   scope->AddTemplate(template_name, new Template(scope, function));
+
+  // The template object above created a closure around the variables in the
+  // current scope. The template code will execute in that context when it's
+  // invoked. But this means that any variables defined above that are used
+  // by the template won't get marked used just by defining the template. The
+  // result can be spurious unused variable errors.
+  //
+  // The "right" thing to do would be to walk the syntax tree inside the
+  // template, find all identifier references, and mark those variables used.
+  // This is annoying and error-prone to implement and takes extra time to run
+  // for this narrow use case.
+  //
+  // Templates are most often defined in .gni files which don't get
+  // used-variable checking anyway, and this case is annoying enough that the
+  // incremental value of unused variable checking isn't worth the
+  // alternatives. So all values in scope before this template definition are
+  // exempted from unused variable checking.
+  scope->MarkAllUsed();
+
   return Value();
 }
 
 }  // namespace functions