diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -97,6 +97,10 @@
 REVIDX_RAWTEXT_CHANGING_FLAGS
 
 parsers = policy.importmod(r'parsers')
+try:
+    from . import rustext
+except ImportError:
+    rustext = None
 
 # Aliased for performance.
 _zlibdecompress = zlib.decompress
@@ -779,12 +783,28 @@
         for r in revs:
             checkrev(r)
         # and we're sure ancestors aren't filtered as well
-        if util.safehasattr(parsers, 'rustlazyancestors'):
-            return ancestor.rustlazyancestors(
-                self.index, revs,
-                stoprev=stoprev, inclusive=inclusive)
-        return ancestor.lazyancestors(self._uncheckedparentrevs, revs,
-                                      stoprev=stoprev, inclusive=inclusive)
+
+        # rustext can be an unloaded module (see hgdemandimport)
+        # so we need to trigger an actual import to check for its presence,
+        # which is done by looking up any attribute.
+        # If in the future we reach the point where the ancestor module
+        # can be imported by policy, we will be able to get rid of this
+        global rustext
+        if rustext is not None:
+            try:
+                rustext.__name__
+            except ImportError:
+                rustext = None
+        if rustext is not None:
+            lazyancestors = rustext.ancestor.lazyancestors
+            arg = self.index
+        elif util.safehasattr(parsers, 'rustlazyancestors'):
+            lazyancestors = ancestor.rustlazyancestors
+            arg = self.index
+        else:
+            lazyancestors = ancestor.lazyancestors
+            arg = self._uncheckedparentrevs
+        return lazyancestors(arg, revs, stoprev=stoprev, inclusive=inclusive)
 
     def descendants(self, revs):
         return dagop.descendantrevs(revs, self.revs, self.parentrevs)