Improve GN value origins for error messages.

tools/gn/parse_tree.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/parse_tree.h"
 
 #include <string>
 
 #include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
@@ skipping 355 matching lines @@
 }
 
 IdentifierNode::~IdentifierNode() {
 }
 
 const IdentifierNode* IdentifierNode::AsIdentifier() const {
   return this;
 }
 
 Value IdentifierNode::Execute(Scope* scope, Err* err) const {
-  const Value* result = scope->GetValue(value_.value(), true);
-  if (!result) {
+  const Value* value = scope->GetValue(value_.value(), true);
+  Value result;
+  if (!value) {
     *err = MakeErrorDescribing("Undefined identifier");
-    return Value();
+    return result;
   }
-  return *result;
+
+  result = *value;
+  result.set_origin(this);
+  return result;
 }
 
 LocationRange IdentifierNode::GetRange() const {
   return value_.range();
 }
 
 Err IdentifierNode::MakeErrorDescribing(const std::string& msg,
                                         const std::string& help) const {
   return Err(value_, msg, help);
 }
@@ skipping 128 matching lines @@
 
 Err UnaryOpNode::MakeErrorDescribing(const std::string& msg,
                                      const std::string& help) const {
   return Err(op_, msg, help);
 }
 
 void UnaryOpNode::Print(std::ostream& out, int indent) const {
   out << IndentFor(indent) << "UNARY(" << op_.value() << ")\n";
   operand_->Print(out, indent + 1);
 }