Enable Blink GC plugin check for stack allocated classes.

third_party/WebKit/Source/BUILD.gn

 # Copyright 2014 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("config.gni")
 if (is_clang) {
   import("//build/config/clang/clang.gni")
 }
 
 visibility = [ "//third_party/WebKit/*" ]
@@ skipping 11 matching lines @@
 
   # Set to true to have the clang Blink GC plugin additionally check if
   # a class has an empty destructor which would be unnecessarily invoked
   # when finalized.
   blink_gc_plugin_option_warn_unneeded_finalizer = false
 
   # Set to true to have the clang Blink GC plugin use Chromium-style naming
   # rather than legacy Blink name.
   # TODO(https://crbug.com/675879): Remove this option after the Blink rename.
   blink_gc_plugin_option_use_chromium_style_naming = false
+
+  # Set to true to have the clang Blink GC plugin additionally check if
+  # stack only classes (i.e., STACK_ALLOCATED()-annotated) define an unused
+  # and unwanted trace method.
+  # TODO(https://crbug.com/689864): remove once the GC plugin has been rolled
+  # with this check default enabled.
+  blink_gc_plugin_option_warn_stack_allocated_trace_methods = true

[Nico, 2017/03/02 14:08:56]

if the plan is to remove this soon again…

 }
 
 # features ---------------------------------------------------------------------
 
 config("features") {
   defines = feature_defines_list
 }
 
 # inside_blink -----------------------------------------------------------------
 
@@ skipping 102 matching lines @@
       ]
     }
     if (blink_gc_plugin_option_use_chromium_style_naming) {
       cflags += [
         "-Xclang",
         "-plugin-arg-blink-gc-plugin",
         "-Xclang",
         "use-chromium-style-naming",
       ]
     }
+    if (blink_gc_plugin_option_warn_stack_allocated_trace_methods) {

[Nico, 2017/03/02 14:08:56]

…maybe just add it unconditionally here?

[sof, 2017/03/02 14:28:41]

Yes, selectively turning it off shouldn't be needed in practice. Done.

+      cflags += [
+        "-Xclang",
+        "-plugin-arg-blink-gc-plugin",
+        "-Xclang",
+        "warn-stack-allocated-trace-method",
+      ]
+    }
   }
 }
 
 # The follow configs apply to all targets except for unit tests, which rely on
 # static initializers.
 config("non_test_config") {
   cflags = []
 
   if (is_clang) {
     cflags += [ "-Wglobal-constructors" ]
   }
 }
 
 if (is_mac) {
   # This sets up precompiled headers for Mac.
   config("mac_precompiled_headers") {
     precompiled_header = rebase_path("build/mac/Prefix.h", root_build_dir)
     precompiled_source = "//third_party/WebKit/Source/build/mac/Prefix.h"
   }
 }