Rework approach to allowing extra flags for CrOS builds.

build/config/compiler/BUILD.gn

 # 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.
 
 import("//build/config/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/toolchain.gni")
@@ skipping 1559 matching lines @@
 
 if (is_ios || is_mac) {
   # On Mac and iOS, this enables support for ARC (automatic ref-counting).
   # See http://clang.llvm.org/docs/AutomaticReferenceCounting.html.
   config("enable_arc") {
     common_flags = [ "-fobjc-arc" ]
     cflags_objc = common_flags
     cflags_objcc = common_flags
   }
 }
+
+#
+# Extra flags
+#
+# This is a placeholder config so that users with custom toolchains
+# can more easily set extra flags to be passed to each target, while
+# still allowing for those flags to be turned off on a per-target basis.
+# We generally do not want people to specify extra flags, so for now
+# this hook is only exposed to the CrOS build.
+
+config("extra_flags") {
+  if (default_toolchain == "//build/config/cros:target") {
+    configs = [ "//build/toolchain/cros:extra_flags" ]
+  }

[Dirk Pranke, 2016/08/02 00:28:10]

Perhaps instead of hardcoding the toolchain and config names, this should be a
build arg instead?

[brettw, 2016/08/03 23:02:14]

Can this just be:
  if (target_os == "chromeos") ?

[Dirk Pranke, 2016/08/03 23:09:07]

No, because that would break the "desktop chromeos" config,
where we build using the chromium checked-in clang toolchain.

Ideally some day we'd separate out "want chromeos ui and functionality" from
"want to run on the chromeos operating system", in which case this distinction
would be a lot clearer.

[brettw, 2016/08/08 22:10:56]

Ah, then can you add an explicit comment explaining this? It's very subtle.

+}