diff -r f2601cbce209 -r 28a4fb793ba1 mercurial/extensions.py
--- a/mercurial/extensions.py	Thu Dec 27 21:27:43 2018 -0500
+++ b/mercurial/extensions.py	Thu Dec 27 21:46:03 2018 -0500
@@ -226,6 +226,8 @@
             except TypeError:
                 if pycompat.getargspec(extsetup).args:
                     raise
+                ui.deprecwarn("extsetup for '%s' must take a ui argument"
+                              % name, "4.9")
                 extsetup() # old extsetup with no ui argument
         except Exception as inst:
             ui.traceback(force=True)