Enforce WARN_UNUSED_RESULT attribute on Windows

build/common.gypi

 # Copyright (c) 2012 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 5746 matching lines @@
                 'AdditionalOptions': [
                   # Many files use intrinsics without including this header.
                   # TODO(hans): Fix those files, or move this to sub-GYPs.
                   '/FIIntrin.h',
 
                   # TODO(hans): Make this list shorter eventually, http://crbug.com/504657
                   '-Qunused-arguments',  # http://crbug.com/504658
                   '-Wno-microsoft-enum-value',  # http://crbug.com/505296
                   '-Wno-unknown-pragmas',  # http://crbug.com/505314
                   '-Wno-unused-value',  # http://crbug.com/505318
+                  '-Wunused-result',

[dcheng, 2015/10/07 16:09:59]

/W4 doesn't seem to enable this warning, so I had to manually add it to this
list.

[hans, 2015/10/07 16:25:26]

Hmm, clang-cl /W4 does enable it for me. Can you double check? That we're
disabling -Wunused-value above suggests that these warnings are on by default.

Also, this is a list of disabled warnings (see the comment about making it
shorter above). If we need to enable this explicitly, let's move it to
clang_warning_flags above.

[dcheng, 2015/10/07 16:41:34]

Hm...

Well it's definitely not enabled by default on the Chrome build, but I have no
idea what flag is causing it yet.

(It's easy enough to prove this by patching in just the compiler_specific.h
changes and then introducing an intentional error in base_unittests somewhere)

[hans, 2015/10/07 16:51:29]

D'oh, it's right there: -Wno-unused-value also disables the unused-result
warning. Sorry, I should have noticed that.

Maybe something like:

  # Disable unused-value (crbug.com/505318) except -Wunused-result.
  '-Wno-unused-value',
  '-Wunused-result',

[dcheng, 2015/10/07 17:46:15]

Done.

                 ],
               },
             }],
             ['clang==1 and clang_use_chrome_plugins==1', {
               'VCCLCompilerTool': {
                 'AdditionalOptions': [
                   '<@(clang_chrome_plugins_flags)',
                 ],
               },
             }],
@@ skipping 500 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }