GN flag to fail for unused build args.

tools/gn/setup.cc

Index: tools/gn/setup.cc
diff --git a/tools/gn/setup.cc b/tools/gn/setup.cc
index 384a2af17eaaa00368fbc8d0dfccc64f1fd2df86..7f8e28685dc6fe9117e427183e0b9f4f82bc8550 100644
--- a/tools/gn/setup.cc
+++ b/tools/gn/setup.cc
@@ -332,9 +332,13 @@ bool Setup::RunPostMessageLoop() {
     }
 
     if (!build_settings_.build_args().VerifyAllOverridesUsed(&err)) {
-      // TODO(brettw) implement a system of warnings. Until we have a better
-      // system, print the error but don't return failure.
+      // TODO(brettw) implement a system to have a different marker for
+      // warnings. Until we have a better system, print the error but don't
+      // return failure unless requested on the command line.
       err.PrintToStdout();
+      if (base::CommandLine::ForCurrentProcess()->HasSwitch(
+              switches::kFailOnUnusedArgs))
+        return false;
       return true;

[Dirk Pranke, 2016/04/23 01:27:17]

I actually feel (reasonably strongly) that we should fail on unused args by
default, and have the switch *disable* that behavior, at least until we get a
lot more feedback that this is unworkable.

Or, at least, I would say that should be the default and then configurable per
repo via the dotfile.

But, I don't feel so strongly about this that I won't approve this CL.

     }
   }