diff --git a/mercurial/ancestor.py b/mercurial/ancestor.py
--- a/mercurial/ancestor.py
+++ b/mercurial/ancestor.py
@@ -11,9 +11,12 @@ import heapq
 
 from .node import nullrev
 from . import (
+    policy,
     pycompat,
 )
 
+parsers = policy.importmod(r'parsers')
+
 def commonancestorsheads(pfunc, *nodes):
     """Returns a set with the heads of all common ancestors of all nodes,
     heads(::nodes[0] and ::nodes[1] and ...) .
@@ -379,3 +382,25 @@ class lazyancestors(object):
             # free up memory.
             self._containsiter = None
             return False
+
+class rustlazyancestors(lazyancestors):
+
+    def __init__(self, index, revs, stoprev=0, inclusive=False):
+        self._index = index
+        self._stoprev = stoprev
+        self._inclusive = inclusive
+        # no need to prefilter out init revs that are smaller than stoprev,
+        # it's done by rustlazyancestors constructor.
+        # we need to convert to a list, because our ruslazyancestors
+        # constructor (from C code) doesn't understand anything else yet
+        self._initrevs = initrevs = list(revs)
+
+        self._containsseen = set()
+        self._containsiter = parsers.rustlazyancestors(
+            index, initrevs, stoprev, inclusive)
+
+    def __iter__(self):
+        return parsers.rustlazyancestors(self._index,
+                                         self._initrevs,
+                                         self._stoprev,
+                                         self._inclusive)
diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -763,6 +763,10 @@ class revlog(object):
         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)