diff --git a/rust/hg-cpython/src/dagops.rs b/rust/hg-cpython/src/dagops.rs
--- a/rust/hg-cpython/src/dagops.rs
+++ b/rust/hg-cpython/src/dagops.rs
@@ -15,10 +15,9 @@ use cpython::{PyDict, PyModule, PyObject
 use hg::dagops;
 use hg::Revision;
 use std::collections::HashSet;
-use vcsgraph::ancestors::node_rank;
-use vcsgraph::graph::{Parents, Rank};
+use vcsgraph::graph::Rank;
 
-use crate::revlog::pyindex_to_graph;
+use crate::revlog::py_rust_index_to_graph;
 
 /// Using the the `index`, return heads out of any Python iterable of Revisions
 ///
@@ -28,23 +27,33 @@ pub fn headrevs(
     index: PyObject,
     revs: PyObject,
 ) -> PyResult<HashSet<PyRevision>> {
-    let index = pyindex_to_graph(py, index)?;
-    let mut as_set: HashSet<Revision> = rev_pyiter_collect(py, &revs, &index)?;
-    dagops::retain_heads(&index, &mut as_set)
+    let py_leaked = py_rust_index_to_graph(py, index)?;
+    let index = &*unsafe { py_leaked.try_borrow(py)? };
+    let mut as_set: HashSet<Revision> = rev_pyiter_collect(py, &revs, index)?;
+    dagops::retain_heads(index, &mut as_set)
         .map_err(|e| GraphError::pynew(py, e))?;
     Ok(as_set.into_iter().map(Into::into).collect())
 }
 
 /// Computes the rank, i.e. the number of ancestors including itself,
 /// of a node represented by its parents.
+///
+/// Currently, the pure Rust index supports only the REVLOGV1 format, hence
+/// the only possible return value is that the rank is unknown.
+///
+/// References:
+/// - C implementation, function `index_fast_rank()`.
+/// - `impl vcsgraph::graph::RankedGraph for Index` in `crate::cindex`.
 pub fn rank(
     py: Python,
-    index: PyObject,
-    p1r: PyRevision,
-    p2r: PyRevision,
+    _index: PyObject,
+    _p1r: PyRevision,
+    _p2r: PyRevision,
 ) -> PyResult<Rank> {
-    node_rank(&pyindex_to_graph(py, index)?, &Parents([p1r.0, p2r.0]))
-        .map_err(|e| GraphError::pynew_from_vcsgraph(py, e))
+    Err(GraphError::pynew_from_vcsgraph(
+        py,
+        vcsgraph::graph::GraphReadError::InconsistentGraphData,
+    ))
 }
 
 /// Create the module, with `__package__` given from parent
diff --git a/tests/test-rust-ancestor.py b/tests/test-rust-ancestor.py
--- a/tests/test-rust-ancestor.py
+++ b/tests/test-rust-ancestor.py
@@ -157,7 +157,7 @@ class rustancestorstest(revlogtesting.Re
         self.assertEqual(exc.args, ('InvalidRevision', wdirrev))
 
     def testheadrevs(self):
-        idx = self.parseindex()
+        idx = self.parserustindex()
         self.assertEqual(dagop.headrevs(idx, [1, 2, 3]), {3})