diff -r b3991b72d9f1 -r fb16ad368606 mercurial/revlog.py
--- a/mercurial/revlog.py	Fri Mar 06 17:02:50 2020 +0100
+++ b/mercurial/revlog.py	Tue Mar 10 14:24:08 2020 +0100
@@ -945,9 +945,6 @@
         if rustancestor is not None:
             lazyancestors = rustancestor.LazyAncestors
             arg = self.index
-        elif util.safehasattr(parsers, b'rustlazyancestors'):
-            lazyancestors = ancestor.rustlazyancestors
-            arg = self.index
         else:
             lazyancestors = ancestor.lazyancestors
             arg = self._uncheckedparentrevs