diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -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)
 
diff --git a/mercurial/setdiscovery.py b/mercurial/setdiscovery.py
--- a/mercurial/setdiscovery.py
+++ b/mercurial/setdiscovery.py
@@ -274,6 +274,8 @@
         return sample
 
 
+pure_partialdiscovery = partialdiscovery
+
 partialdiscovery = policy.importrust(
     'discovery', member='PartialDiscovery', default=partialdiscovery
 )
@@ -434,9 +436,11 @@
     hard_limit_sample = not (dynamic_sample or remote.limitedarguments)
 
     randomize = ui.configbool(b'devel', b'discovery.randomize')
-    disco = partialdiscovery(
-        local, ownheads, hard_limit_sample, randomize=randomize
-    )
+    if cl.index.rust_ext_compat:
+        pd = partialdiscovery
+    else:
+        pd = pure_partialdiscovery
+    disco = pd(local, ownheads, hard_limit_sample, randomize=randomize)
     if initial_head_exchange:
         # treat remote heads (and maybe own heads) as a first implicit sample
         # response