diff --git a/hgext/highlight/__init__.py b/hgext/highlight/__init__.py
--- a/hgext/highlight/__init__.py
+++ b/hgext/highlight/__init__.py
@@ -87,7 +87,7 @@ def generate_css(web):
     ]))
     return web.res.sendresponse()
 
-def extsetup():
+def extsetup(ui):
     # monkeypatch in the new version
     extensions.wrapfunction(webcommands, '_filerevision',
                             filerevision_highlight)
diff --git a/mercurial/extensions.py b/mercurial/extensions.py
--- a/mercurial/extensions.py
+++ b/mercurial/extensions.py
@@ -226,6 +226,8 @@ def _runextsetup(name, ui):
             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)
diff --git a/tests/notcapable b/tests/notcapable
--- a/tests/notcapable
+++ b/tests/notcapable
@@ -7,7 +7,7 @@ fi
 
 cat > notcapable-$CAP.py << EOF
 from mercurial import extensions, localrepo, repository
-def extsetup():
+def extsetup(ui):
     extensions.wrapfunction(repository.peer, 'capable', wrapcapable)
     extensions.wrapfunction(localrepo.localrepository, 'peer', wrappeer)
 def wrapcapable(orig, self, name, *args, **kwargs):
diff --git a/tests/test-dirstate-race.t b/tests/test-dirstate-race.t
--- a/tests/test-dirstate-race.t
+++ b/tests/test-dirstate-race.t
@@ -53,7 +53,7 @@ confused with a file with the exec bit s
   >     context,
   >     extensions,
   > )
-  > def extsetup():
+  > def extsetup(ui):
   >     extensions.wrapfunction(context.workingctx, '_checklookup', overridechecklookup)
   > def overridechecklookup(orig, self, files):
   >     # make an update that changes the dirstate from underneath
diff --git a/tests/test-extension.t b/tests/test-extension.t
--- a/tests/test-extension.t
+++ b/tests/test-extension.t
@@ -279,7 +279,7 @@ limit mark, regardless of importing modu
   > NO_CHECK_EOF
   $ cat > loadabs.py <<NO_CHECK_EOF
   > import mod.ambigabs as ambigabs
-  > def extsetup():
+  > def extsetup(ui):
   >     print('ambigabs.s=%s' % ambigabs.s, flush=True)
   > NO_CHECK_EOF
   $ "$PYTHON" $TESTTMP/unflush.py loadabs.py
@@ -295,7 +295,7 @@ limit mark, regardless of importing modu
   > NO_CHECK_EOF
   $ cat > loadrel.py <<NO_CHECK_EOF
   > import mod.ambigrel as ambigrel
-  > def extsetup():
+  > def extsetup(ui):
   >     print('ambigrel.s=%s' % ambigrel.s, flush=True)
   > NO_CHECK_EOF
   $ "$PYTHON" $TESTTMP/unflush.py loadrel.py
diff --git a/tests/test-http-bundle1.t b/tests/test-http-bundle1.t
--- a/tests/test-http-bundle1.t
+++ b/tests/test-http-bundle1.t
@@ -186,7 +186,7 @@ test http authentication
   >     if base64.b64decode(auth.split()[1]).split(b':', 1) != [b'user',
   >                                                             b'pass']:
   >         raise common.ErrorResponse(common.HTTP_FORBIDDEN, b'no')
-  > def extsetup():
+  > def extsetup(ui):
   >     common.permhooks.insert(0, perform_authentication)
   > EOT
   $ hg serve --config extensions.x=userpass.py -p $HGPORT2 -d --pid-file=pid \
diff --git a/tests/test-http.t b/tests/test-http.t
--- a/tests/test-http.t
+++ b/tests/test-http.t
@@ -181,7 +181,7 @@ test http authentication
   >                 [(b'WWW-Authenticate', b'Basic Realm="mercurial"')])
   >     if base64.b64decode(auth.split()[1]).split(b':', 1) != [b'user', b'pass']:
   >         raise common.ErrorResponse(common.HTTP_FORBIDDEN, b'no')
-  > def extsetup():
+  > def extsetup(ui):
   >     common.permhooks.insert(0, perform_authentication)
   > EOT
   $ hg serve --config extensions.x=userpass.py -p $HGPORT2 -d --pid-file=pid \
@@ -526,7 +526,7 @@ We raise HTTP 500 because its message is
   >     if not cookie:
   >         raise common.ErrorResponse(common.HTTP_SERVER_ERROR, b'no-cookie')
   >     raise common.ErrorResponse(common.HTTP_SERVER_ERROR, b'Cookie: %s' % cookie)
-  > def extsetup():
+  > def extsetup(ui):
   >     common.permhooks.insert(0, perform_authentication)
   > EOF
 
diff --git a/tests/test-largefiles-wireproto.t b/tests/test-largefiles-wireproto.t
--- a/tests/test-largefiles-wireproto.t
+++ b/tests/test-largefiles-wireproto.t
@@ -430,7 +430,7 @@ a large file from the server rather than
   >                 [(b'WWW-Authenticate', b'Basic Realm="mercurial"')])
   >     if base64.b64decode(auth.split()[1]).split(b':', 1) != [b'user', b'pass']:
   >         raise common.ErrorResponse(common.HTTP_FORBIDDEN, b'no')
-  > def extsetup():
+  > def extsetup(ui):
   >     common.permhooks.insert(0, perform_authentication)
   > EOT
   $ hg serve --config extensions.x=userpass.py -R credentialmain \
diff --git a/tests/test-lfs-serve-access.t b/tests/test-lfs-serve-access.t
--- a/tests/test-lfs-serve-access.t
+++ b/tests/test-lfs-serve-access.t
@@ -404,7 +404,7 @@ the GET/PUT request.
   >     if base64.b64decode(auth.split()[1]).split(b':', 1) != [b'user',
   >                                                             b'pass']:
   >         raise common.ErrorResponse(common.HTTP_FORBIDDEN, b'no')
-  > def extsetup():
+  > def extsetup(ui):
   >     common.permhooks.insert(0, perform_authentication)
   > EOF