diff -r 3f9e68864ccc -r b3a677c82a35 mercurial/policy.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mercurial/policy.py	Wed Mar 09 19:55:45 2016 +0000
@@ -0,0 +1,40 @@
+# policy.py - module policy logic for Mercurial.
+#
+# Copyright 2015 Gregory Szorc <gregory.szorc@gmail.com>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+from __future__ import absolute_import
+
+import os
+import sys
+
+# Rules for how modules can be loaded. Values are:
+#
+#    c - require C extensions
+#    allow - allow pure Python implementation when C loading fails
+#    py - only load pure Python modules
+#
+# By default, require the C extensions for performance reasons.
+policy = 'c'
+try:
+    from . import __modulepolicy__
+    policy = __modulepolicy__.modulepolicy
+except ImportError:
+    pass
+
+# PyPy doesn't load C extensions.
+#
+# The canonical way to do this is to test platform.python_implementation().
+# But we don't import platform and don't bloat for it here.
+if '__pypy__' in sys.builtin_module_names:
+    policy = 'py'
+
+# Our C extensions aren't yet compatible with Python 3. So use pure Python
+# on Python 3 for now.
+if sys.version_info[0] >= 3:
+    policy = 'py'
+
+# Environment variable can always force settings.
+policy = os.environ.get('HGMODULEPOLICY', policy)