revlog: do not call Rust code if the index is not compatible with it

This will avoid hitting the TypeError we defined in the previous changesets.
This is the simplest fix but not the most elegant.

Ideally we would teach the Rust code to use any kind of revlog. However this is
an adventure for another time.

Differential Revision: https://phab.mercurial-scm.org/D10666

mercurial/revlog.py

@@ -1063,7 +1063,7 b' class revlog(object):'
             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 b' class revlog(object):'
         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 b' class revlog(object):'
                 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)
 

mercurial/setdiscovery.py

@@ -274,6 +274,8 b' class partialdiscovery(object):'
         return sample
 
 
+pure_partialdiscovery = partialdiscovery
+
 partialdiscovery = policy.importrust(
     'discovery', member='PartialDiscovery', default=partialdiscovery
 )
@@ -434,9 +436,11 b' def findcommonheads('
     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