--- a/mercurial/ui.py	Tue Aug 09 16:45:28 2016 +0100
+++ b/mercurial/ui.py	Wed Aug 10 15:05:20 2016 +0100
@@ -1175,6 +1175,7 @@
                            % ((msg,) + calframe[stacklevel][1:4]))
             self.log('develwarn', '%s at: %s:%s (%s)\n',
                      msg, *calframe[stacklevel][1:4])
+            curframe = calframe = None  # avoid cycles
 
     def deprecwarn(self, msg, version):
         """issue a deprecation warning