diff -r 4a0730b7127e -r aa4dbc14f735 mercurial/commands.py
--- a/mercurial/commands.py	Thu Dec 10 12:06:55 2020 -0800
+++ b/mercurial/commands.py	Thu Dec 10 14:39:22 2020 -0800
@@ -2455,7 +2455,7 @@
 @command(
     b'diff',
     [
-        (b'r', b'rev', [], _(b'revision'), _(b'REV')),
+        (b'r', b'rev', [], _(b'revision (DEPRECATED)'), _(b'REV')),
         (b'', b'from', b'', _(b'revision to diff from'), _(b'REV1')),
         (b'', b'to', b'', _(b'revision to diff to'), _(b'REV2')),
         (b'c', b'change', b'', _(b'change made by revision'), _(b'REV')),