diff -r 80695628adcb -r 856f381ad74b mercurial/utils/interfaceutil.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mercurial/utils/interfaceutil.py	Sun Apr 22 11:54:10 2018 -0700
@@ -0,0 +1,40 @@
+# interfaceutil.py - Utilities for declaring interfaces.
+#
+# Copyright 2018 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.
+
+# zope.interface imposes a run-time cost due to module import overhead and
+# bookkeeping for declaring interfaces. So, we use stubs for various
+# zope.interface primitives unless instructed otherwise.
+
+from __future__ import absolute_import
+
+from .. import (
+    encoding,
+)
+
+if encoding.environ.get('HGREALINTERFACES'):
+    from ..thirdparty.zope import (
+        interface as zi,
+    )
+
+    Attribute = zi.Attribute
+    Interface = zi.Interface
+    implementer = zi.implementer
+else:
+    class Attribute(object):
+        def __init__(self, __name__, __doc__=''):
+            pass
+
+    class Interface(object):
+        def __init__(self, name, bases=(), attrs=None, __doc__=None,
+                 __module__=None):
+            pass
+
+    def implementer(*ifaces):
+        def wrapper(cls):
+            return cls
+
+        return wrapper