Add initial prototype for the GN meta-buildsystem.

tools/gn/function_define_rule.cc

Index: tools/gn/function_define_rule.cc
diff --git a/tools/gn/function_define_rule.cc b/tools/gn/function_define_rule.cc
new file mode 100644
index 0000000000000000000000000000000000000000..afbed9e6204e32bb1aba209fc7a108d7063a6ffd
--- /dev/null
+++ b/tools/gn/function_define_rule.cc
@@ -0,0 +1,37 @@
+// 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/value.h"
+
+Value ExecuteDefineRule(Scope* scope,
+                        const FunctionCallNode* function,
+                        const std::vector<Value>& args,
+                        BlockNode* block,
+                        Err* err) {
+  // TODO(brettw) determine if the function is built-in and throw an error if
+  // it is.
+  if (args.size() != 1) {
+    *err = Err(function->function(),
+               "Need exactly one string arg to define_rule.");
+    return Value();
+  }
+  if (!args[0].VerifyTypeIs(Value::STRING, err))
+    return Value();
+  std::string rule_name = args[0].string_value();
+
+  const FunctionCallNode* existing_rule = scope->GetRule(rule_name);
+  if (existing_rule) {
+    *err = Err(function, "Duplicate rule definition.",
+               "A rule with this name was already defined.");
+    err->AppendSubErr(Err(existing_rule->function(), "Previous definition."));
+    return Value();
+  }
+
+  scope->AddRule(rule_name, function);
+  return Value();
+}