diff -r d79f3afb079e -r fcb517ff9562 mercurial/configitems.py
--- a/mercurial/configitems.py	Mon Jul 16 11:38:56 2018 -0700
+++ b/mercurial/configitems.py	Mon Jul 16 18:02:30 2018 +0200
@@ -372,6 +372,9 @@
 coreconfigitem('devel', 'warn-config-unknown',
     default=None,
 )
+coreconfigitem('devel', 'debug.extensions',
+    default=False,
+)
 coreconfigitem('devel', 'debug.peer-request',
     default=False,
 )