diff -r 49b00a04028f -r 633408a0f2e2 mercurial/revlog.py
--- a/mercurial/revlog.py	Sun Oct 29 18:35:32 2023 +0100
+++ b/mercurial/revlog.py	Fri Oct 27 23:29:29 2023 +0200
@@ -1683,7 +1683,10 @@
             if self._nodemap_file is not None:
                 use_rust_index = True
             else:
-                use_rust_index = self.opener.options.get(b'rust.index')
+                # Using the CIndex is not longer possible, as the
+                # `AncestorsIterator` and `LazyAncestors` classes now require
+                # a Rust index for instantiation.
+                use_rust_index = True
 
         self._parse_index = parse_index_v1
         if self._format_version == REVLOGV0: