--- a/mercurial/localrepo.py	Wed Dec 11 18:10:20 2019 +0100
+++ b/mercurial/localrepo.py	Thu Dec 12 18:31:17 2019 +0100
@@ -927,6 +927,9 @@
     if repository.NARROW_REQUIREMENT in requirements:
         options[b'enableellipsis'] = True
 
+    if ui.configbool('experimental', 'rust.index'):
+        options[b'rust.index'] = True
+
     return options