Improve GN value origins for error messages.

Sets the origin for a value when the value is dereferenced. Previously if you did
  foo = 6
  deps = [ foo ]
You would get the message:
  ERROR at //BUILD.gn:15:9: This is not a string.
    foo = 6
          ^
Which in this case is pretty confusing since that's obviously the case, and you really want to know where it was dereferenced.

With this patch the message is:
  ERROR at //BUILD.gn:21:5: This is not a string.
      foo,
      ^--
  Instead I see a integer = 6
which is more actionable.

R=ajwong@chromium.org

Review URL: https://codereview.chromium.org/439513003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288131 0039d316-1c4b-4281-b951-d872f2087c98

tools/gn/parse_tree.cc

@@ -373,12 +373,16 @@ const IdentifierNode* IdentifierNode::AsIdentifier() const {
 }
 
 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 {

tools/gn/parse_tree_unittest.cc

@@ -76,4 +76,28 @@ TEST(ParseTree, BlockUnusedVars) {
 
   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());
 }

tools/gn/value.cc

@@ -177,7 +177,10 @@ bool Value::VerifyTypeIs(Type t, Err* err) const {
   if (type_ == t)
     return true;
 
-  *err = Err(origin(), std::string("This is not a ") + DescribeType(t) + ".");
+  *err = Err(origin(),
+             std::string("This is not a ") + DescribeType(t) + ".",
+             std::string("Instead I see a ") + DescribeType(type_) + " = " +
+             ToString(true));
   return false;
 }