diff -r a41565bef69f -r e8c11a2c96c0 mercurial/interfaces/repository.py
--- a/mercurial/interfaces/repository.py	Thu Feb 18 17:36:52 2021 +0100
+++ b/mercurial/interfaces/repository.py	Thu Feb 18 18:18:35 2021 +0100
@@ -453,6 +453,10 @@
         """
     )
 
+    sidedata = interfaceutil.Attribute(
+        """Raw sidedata bytes for the given revision."""
+    )
+
 
 class ifilerevisionssequence(interfaceutil.Interface):
     """Contains index data for all revisions of a file.