Improve GN value origins for error messages.

tools/gn/parse_tree_unittest.cc

 // Copyright 2014 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/input_file.h"
 #include "tools/gn/parse_tree.h"
 #include "tools/gn/scope.h"
 #include "tools/gn/test_with_scope.h"
 
@@ skipping 58 matching lines @@
   TestParseInput input_unused(
       "{\n"
       "  a = 12\n"
       "  b = 13\n"
       "  print(\"$a\")\n"
       "}");
   EXPECT_FALSE(input_unused.has_error());
 
   input_unused.parsed()->Execute(setup.scope(), &err);
   EXPECT_TRUE(err.has_error());
+
+  // Also verify that the unused variable has the correct origin set. The
+  // origin will point to the value assigned to the variable (in this case, the
+  // "13" assigned to "b".
+  EXPECT_EQ(3, err.location().line_number());
+  EXPECT_EQ(7, err.location().char_offset());
+
 }
+
+TEST(ParseTree, OriginForDereference) {
+  TestWithScope setup;
+  TestParseInput input(
+      "a = 6\n"
+      "get_target_outputs(a)");
+  EXPECT_FALSE(input.has_error());
+
+  Err err;
+  input.parsed()->Execute(setup.scope(), &err);
+  EXPECT_TRUE(err.has_error());
+
+  // The origin for the "not a string" error message should be where the value
+  // was dereferenced (the "a" on the second line).
+  EXPECT_EQ(2, err.location().line_number());
+  EXPECT_EQ(20, err.location().char_offset());
+}