Add initial prototype for the GN meta-buildsystem.

tools/gn/function_set_default_toolchain.cc

Index: tools/gn/function_set_default_toolchain.cc
diff --git a/tools/gn/function_set_default_toolchain.cc b/tools/gn/function_set_default_toolchain.cc
new file mode 100644
index 0000000000000000000000000000000000000000..4939ac84cee92cc45d27216bed1ae2091398070b
--- /dev/null
+++ b/tools/gn/function_set_default_toolchain.cc
@@ -0,0 +1,70 @@
+// 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/build_settings.h"
+#include "tools/gn/functions.h"
+#include "tools/gn/parse_tree.h"
+#include "tools/gn/scope.h"
+#include "tools/gn/settings.h"
+#include "tools/gn/toolchain_manager.h"
+
+/*
+set_default_toolchain: Sets the default toolchain name.
+
+  set_default_toolchain(toolchain_label)
+
+  The given label should identify a toolchain definition (see "toolchain").
+  This toolchain will be used for all targets unless otherwise specified.
+
+  This function is only valid to call during the processing of the build
+  configuration file. Since the build configuration file is processed
+  separately for each toolchain, this function will be a no-op when called
+  under any non-default toolchains.
+
+  For example, the default toolchain should be appropriate for the current
+  environment. If the current environment is 32-bit and somebody references
+  a target with a 64-bit toolchain, we wouldn't want processing of the build
+  config file for the 64-bit toolchain to reset the default toolchain to
+  64-bit, we want to keep it 32-bits.
+
+Argument:
+
+  toolchain_label:
+      Toolchain name.
+
+Example:
+
+  set_default_toolchain("//build/config/win:vs32")
+*/
+
+Value ExecuteSetDefaultToolchain(Scope* scope,
+                                 const FunctionCallNode* function,
+                                 const std::vector<Value>& args,
+                                 Err* err) {
+  if (!scope->IsProcessingBuildConfig()) {
+    *err = Err(function->function(), "Must be called from build config.",
+        "set_default_toolchain can only be called from the build configuration "
+        "file.");
+    return Value();
+  }
+
+  // Ignore non-default-build-config invocations.
+  if (!scope->IsProcessingDefaultBuildConfig())
+    return Value();
+
+  const SourceDir& current_dir = SourceDirForFunctionCall(function);
+  const Label& default_toolchain = ToolchainLabelForScope(scope);
+
+  if (!EnsureSingleStringArg(function, args, err))
+    return Value();
+  Label toolchain_label(
+      Label::Resolve(current_dir, default_toolchain, args[0], err));
+  if (toolchain_label.is_null())
+    return Value();
+
+  ToolchainManager& mgr =
+      scope->settings()->build_settings()->toolchain_manager();
+  mgr.SetDefaultToolchainUnlocked(toolchain_label, function->GetRange(), err);
+  return Value();
+}