--- a/mercurial/revlog.py	Mon Jan 14 17:07:39 2019 +0100
+++ b/mercurial/revlog.py	Fri Nov 30 14:35:57 2018 +0100
@@ -893,6 +893,8 @@
         if common is None:
             common = [nullrev]
 
+        if rustext is not None:
+            return rustext.ancestor.MissingAncestors(self.index, common)
         return ancestor.incrementalmissingancestors(self.parentrevs, common)
 
     def findmissingrevs(self, common=None, heads=None):