Add a GN assert that output dir doesn't have a platform name

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index abd6c2f09d97144ac1b22973600d040ba3392238..e0dcb893142a201575db9074622ba72445597d01 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -27,6 +27,32 @@ declare_args() {
   dcheck_always_on = false
 }
 
+# Because of the source assignment filter, many targets end up over-filtering
+# their sources if the output directory contains a platform name. Assert that
+# this doesn't happen. http://crbug.com/548283
+template("assert_valid_out_dir") {
+  assert(target_name != "")  # Mark as used.
+  set_sources_assignment_filter([

[brettw, 2016/02/16 19:19:58]

This is a clever idea!

I *think* the default sources assignment filter will apply without you doing
this. Does this test work if you just delete the set_... call? I would expect
that test will validate the current filter on the current platform without the
possibility of any skew in what the filter is and what you list out here.

[agrieve, 2016/02/16 20:04:20]

I had thought the same, but then figured it wouldn't catch the case of the
current platform being used. Just tried it however, and it worked just fine
(triggered the assert for is_android & "android").

Maybe this is the case due to a dep on a target($host_toolchain), which would
then have a linux filter?

Either way, it works, so i've made the change. :)

+                                  "*\bandroid/*",
+                                  "*\bchromeos/*",
+                                  "*\bcocoa/*",
+                                  "*\bios/*",
+                                  "*\blinux/*",
+                                  "*\bmac/*",
+                                  "*\bposix/*",
+                                  "*\bwin/*",
+                                ])
+
+  assert(
+      invoker.sources == invoker.actual_sources,
+      "Do not use a platform name in your output directory (found \"$root_build_dir\"). http://crbug.com/548283")
+}
+
+assert_valid_out_dir("_check_output_dir_has_no_platform_name") {
+  actual_sources = [ "$root_build_dir/foo" ]
+  sources = actual_sources
+}
+
 # TODO(brettw) Most of these should be removed. Instead of global feature
 # flags, we should have more modular flags that apply only to a target and its
 # dependents. For example, depending on the "x11" meta-target should define