diff -r 9d1a8829f959 -r 6be2a7ca4b1d mercurial/revlog.py
--- a/mercurial/revlog.py	Tue May 04 14:16:26 2021 +0200
+++ b/mercurial/revlog.py	Tue May 04 14:18:06 2021 +0200
@@ -1063,7 +1063,7 @@
             checkrev(r)
         # and we're sure ancestors aren't filtered as well
 
-        if rustancestor is not None:
+        if rustancestor is not None and self.index.rust_ext_compat:
             lazyancestors = rustancestor.LazyAncestors
             arg = self.index
         else:
@@ -1150,7 +1150,7 @@
         if common is None:
             common = [nullrev]
 
-        if rustancestor is not None:
+        if rustancestor is not None and self.index.rust_ext_compat:
             return rustancestor.MissingAncestors(self.index, common)
         return ancestor.incrementalmissingancestors(self.parentrevs, common)
 
@@ -1370,7 +1370,7 @@
                 return self.index.headrevs()
             except AttributeError:
                 return self._headrevs()
-        if rustdagop is not None:
+        if rustdagop is not None and self.index.rust_ext_compat:
             return rustdagop.headrevs(self.index, revs)
         return dagop.headrevs(revs, self._uncheckedparentrevs)