diff -r 992f0d6e7f33 -r 8042856c90b6 mercurial/revlog.py
--- a/mercurial/revlog.py	Wed Dec 11 18:10:20 2019 +0100
+++ b/mercurial/revlog.py	Thu Dec 12 18:31:17 2019 +0100
@@ -106,6 +106,7 @@
 parsers = policy.importmod('parsers')
 rustancestor = policy.importrust('ancestor')
 rustdagop = policy.importrust('dagop')
+rustrevlog = policy.importrust('revlog')
 
 # Aliased for performance.
 _zlibdecompress = zlib.decompress
@@ -351,6 +352,12 @@
         return p
 
 
+class rustrevlogio(revlogio):
+    def parseindex(self, data, inline):
+        index, cache = super(rustrevlogio, self).parseindex(data, inline)
+        return rustrevlog.MixedIndex(index), cache
+
+
 class revlog(object):
     """
     the underlying revision storage object
@@ -585,6 +592,8 @@
         self._storedeltachains = True
 
         self._io = revlogio()
+        if rustrevlog is not None and self.opener.options.get('rust.index'):
+            self._io = rustrevlogio()
         if self.version == REVLOGV0:
             self._io = revlogoldio()
         try: