--- a/CLOBBER
+++ b/CLOBBER
@@ -17,9 +17,9 @@
 #
 # Modifying this file will now automatically clobber the buildbot machines \o/
 #
 
 # Are you updating CLOBBER because you think it's needed for your WebIDL
 # changes to stick? As of bug 928195, this shouldn't be necessary! Please
 # don't change CLOBBER for WebIDL changes any more.
 
-Bug 1454912 - Changed out GENERATED_FILES are handled in the RecursiveMake backend
+Bug 1253064 - Prefer Clang to GCC in local developer builds

--- a/build/moz.configure/toolchain.configure
+++ b/build/moz.configure/toolchain.configure
@@ -1,14 +1,31 @@
 # -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
 # vim: set filetype=python:
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
+imply_option('--enable-release', mozilla_official)
+imply_option('--enable-release', depends_if('MOZ_AUTOMATION')(lambda x: True))
+
+js_option('--enable-release',
+          help='Build with more conservative, release engineering-oriented '
+               'options. This may slow down builds.')
+
+
+@depends('--enable-release')
+def developer_options(value):
+    if not value:
+        return True
+
+
+add_old_configure_assignment('DEVELOPER_OPTIONS', developer_options)
+set_config('DEVELOPER_OPTIONS', developer_options)
+
 # PGO
 # ==============================================================
 js_option(env='MOZ_PGO', help='Build with profile guided optimizations')
 
 set_config('MOZ_PGO', depends('MOZ_PGO')(lambda x: bool(x)))
 add_old_configure_assignment('MOZ_PGO', depends('MOZ_PGO')(lambda x: bool(x)))
 
 # Code optimization
@@ -642,31 +659,35 @@ def toolchain_search_path(vc_compiler_pa
 def default_c_compilers(host_or_target):
     '''Template defining the set of default C compilers for the host and
     target platforms.
     `host_or_target` is either `host` or `target` (the @depends functions
     from init.configure.
     '''
     assert host_or_target in (host, target)
 
-    @depends(host_or_target, target, toolchain_prefix, android_clang_compiler)
-    def default_c_compilers(host_or_target, target, toolchain_prefix, android_clang_compiler):
+    @depends(host_or_target, target, toolchain_prefix, android_clang_compiler,
+             developer_options)
+    def default_c_compilers(host_or_target, target, toolchain_prefix,
+                            android_clang_compiler, developer_options):
         gcc = ('gcc',)
         if toolchain_prefix and host_or_target is target:
             gcc = tuple('%sgcc' % p for p in toolchain_prefix) + gcc
         # Android sets toolchain_prefix and android_clang_compiler, but
         # we want the latter to take precedence, because the latter can
         # point at clang, which is what we want to use.
         if android_clang_compiler and host_or_target is target:
-            gcc = (android_clang_compiler,) + gcc
+            return (android_clang_compiler,) + gcc
 
         if host_or_target.kernel == 'WINNT':
             return ('cl', 'clang-cl') + gcc + ('clang',)
         if host_or_target.kernel == 'Darwin':
             return ('clang',)
+        if developer_options:
+            return ('clang',) + gcc
         return gcc + ('clang',)
 
     return default_c_compilers
 
 
 @template
 def default_cxx_compilers(c_compiler):
     '''Template defining the set of default C++ compilers for the host and
@@ -1358,33 +1379,16 @@ imply_option('--enable-pie', depends_if(
 # ==============================================================
 
 option(env='RUSTFLAGS',
        nargs=1,
        help='Rust compiler flags')
 set_config('RUSTFLAGS', depends('RUSTFLAGS')(lambda flags: flags))
 
 
-imply_option('--enable-release', mozilla_official)
-imply_option('--enable-release', depends_if('MOZ_AUTOMATION')(lambda x: True))
-
-js_option('--enable-release',
-          help='Build with more conservative, release engineering-oriented '
-               'options. This may slow down builds.')
-
-
-@depends('--enable-release')
-def developer_options(value):
-    if not value:
-        return True
-
-
-add_old_configure_assignment('DEVELOPER_OPTIONS', developer_options)
-set_config('DEVELOPER_OPTIONS', developer_options)
-
 # Rust compiler flags
 # ==============================================================
 
 js_option(env='RUSTC_OPT_LEVEL',
           nargs=1,
           help='Rust compiler optimization level (-C opt-level=%s)')
 
 # --enable-release kicks in full optimizations.

--- a/python/mozbuild/mozbuild/test/configure/test_toolchain_configure.py
+++ b/python/mozbuild/mozbuild/test/configure/test_toolchain_configure.py
@@ -351,16 +351,18 @@ class BaseToolchainTest(BaseConfigureTes
           they can be omitted. Likewise for host_cxx_compiler vs.
           cxx_compiler.
         '''
         environ = dict(environ)
         if 'PATH' not in environ:
             environ['PATH'] = os.pathsep.join(
                 mozpath.abspath(p) for p in ('/bin', '/usr/bin'))
 
+        args = args + ['--enable-release']
+
         sandbox = self.get_sandbox(paths, {}, args, environ,
                                    logger=self.logger)
 
         for var in ('c_compiler', 'cxx_compiler', 'host_c_compiler',
                     'host_cxx_compiler'):
             if var in results:
                 result = results[var]
             elif var.startswith('host_'):