--- a/rust/hg-pyo3/src/ancestors.rs	Sat Dec 07 14:55:42 2024 +0100
+++ b/rust/hg-pyo3/src/ancestors.rs	Sat Dec 07 16:43:30 2024 +0100
@@ -10,9 +10,12 @@
 //! and can be used as replacement for the the pure `ancestor` Python module.
 use cpython::UnsafePyLeaked;
 use pyo3::prelude::*;
+use pyo3::types::PyTuple;
 
+use std::collections::HashSet;
 use std::sync::RwLock;
 
+use hg::MissingAncestors as CoreMissing;
 use vcsgraph::lazy_ancestors::{
     AncestorsIterator as VCGAncestorsIterator,
     LazyAncestors as VCGLazyAncestors,
@@ -153,6 +156,130 @@
     }
 }
 
+#[pyclass]
+struct MissingAncestors {
+    inner: RwLock<UnsafePyLeaked<CoreMissing<PySharedIndex>>>,
+    proxy_index: PyObject,
+}
+
+#[pymethods]
+impl MissingAncestors {
+    #[new]
+    fn new(
+        index_proxy: &Bound<'_, PyAny>,
+        bases: &Bound<'_, PyAny>,
+    ) -> PyResult<Self> {
+        let cloned_proxy = index_proxy.clone().unbind();
+        let bases_vec: Vec<_> =
+            rev_pyiter_collect_with_py_index(bases, index_proxy)?;
+        let (py, leaked_idx) = proxy_index_py_leak(index_proxy)?;
+
+        // Safety: we don't leak the "faked" reference out of
+        // `UnsafePyLeaked`
+        let inner = unsafe {
+            leaked_idx.map(py, |idx| CoreMissing::new(idx, bases_vec))
+        };
+        Ok(Self {
+            inner: inner.into(),
+            proxy_index: cloned_proxy,
+        })
+    }
+
+    fn hasbases(slf: PyRef<'_, Self>) -> PyResult<bool> {
+        let leaked = slf.inner.read().map_err(map_lock_error)?;
+        // Safety: we don't leak the "faked" reference out of `UnsafePyLeaked`
+        let inner = unsafe { py_leaked_borrow(&slf, &leaked) }?;
+        Ok(inner.has_bases())
+    }
+
+    fn addbases(
+        slf: PyRefMut<'_, Self>,
+        bases: &Bound<'_, PyAny>,
+    ) -> PyResult<()> {
+        let index_proxy = slf.proxy_index.bind(slf.py());
+        let bases_vec: Vec<_> =
+            rev_pyiter_collect_with_py_index(bases, index_proxy)?;
+
+        let mut leaked = slf.inner.write().map_err(map_lock_error)?;
+        // Safety: we don't leak the "faked" reference out of `UnsafePyLeaked`
+        let mut inner = unsafe { py_leaked_borrow_mut(&slf, &mut leaked) }?;
+        inner.add_bases(bases_vec);
+        Ok(())
+    }
+
+    fn bases(slf: PyRef<'_, Self>) -> PyResult<HashSet<PyRevision>> {
+        let leaked = slf.inner.read().map_err(map_lock_error)?;
+        // Safety: we don't leak the "faked" reference out of `UnsafePyLeaked`
+        let inner = unsafe { py_leaked_borrow(&slf, &leaked) }?;
+        Ok(inner.get_bases().iter().map(|r| PyRevision(r.0)).collect())
+    }
+
+    fn basesheads(slf: PyRef<'_, Self>) -> PyResult<HashSet<PyRevision>> {
+        let leaked = slf.inner.read().map_err(map_lock_error)?;
+        // Safety: we don't leak the "faked" reference out of `UnsafePyLeaked`
+        let inner = unsafe { py_leaked_borrow(&slf, &leaked) }?;
+        Ok(inner
+            .bases_heads()
+            .map_err(GraphError::from_hg)?
+            .iter()
+            .map(|r| PyRevision(r.0))
+            .collect())
+    }
+
+    fn removeancestorsfrom(
+        slf: PyRef<'_, Self>,
+        revs: &Bound<'_, PyAny>,
+    ) -> PyResult<()> {
+        // Original comment from hg-cpython:
+        //   this is very lame: we convert to a Rust set, update it in place
+        //   and then convert back to Python, only to have Python remove the
+        //   excess (thankfully, Python is happy with a list or even an
+        //   iterator)
+        //   Leads to improve this:
+        //    - have the CoreMissing instead do something emit revisions to
+        //      discard
+        //    - define a trait for sets of revisions in the core and implement
+        //      it for a Python set rewrapped with the GIL marker
+        // PyO3 additional comment: the trait approach would probably be
+        // simpler because we can implement it without a Py wrappper, just
+        // on &Bound<'py, PySet>
+        let index_proxy = slf.proxy_index.bind(slf.py());
+        let mut revs_set: HashSet<_> =
+            rev_pyiter_collect_with_py_index(revs, index_proxy)?;
+
+        let mut leaked = slf.inner.write().map_err(map_lock_error)?;
+        // Safety: we don't leak the "faked" reference out of `UnsafePyLeaked`
+        let mut inner = unsafe { py_leaked_borrow_mut(&slf, &mut leaked) }?;
+
+        inner
+            .remove_ancestors_from(&mut revs_set)
+            .map_err(GraphError::from_hg)?;
+        // convert as Python tuple and discard from original `revs`
+        let remaining_tuple =
+            PyTuple::new(slf.py(), revs_set.iter().map(|r| PyRevision(r.0)))?;
+        revs.call_method("intersection_update", (remaining_tuple,), None)?;
+        Ok(())
+    }
+
+    fn missingancestors(
+        slf: PyRefMut<'_, Self>,
+        bases: &Bound<'_, PyAny>,
+    ) -> PyResult<Vec<PyRevision>> {
+        let index_proxy = slf.proxy_index.bind(slf.py());
+        let revs_vec: Vec<_> =
+            rev_pyiter_collect_with_py_index(bases, index_proxy)?;
+
+        let mut leaked = slf.inner.write().map_err(map_lock_error)?;
+        // Safety: we don't leak the "faked" reference out of `UnsafePyLeaked`
+        let mut inner = unsafe { py_leaked_borrow_mut(&slf, &mut leaked) }?;
+
+        let missing_vec = inner
+            .missing_ancestors(revs_vec)
+            .map_err(GraphError::from_hg)?;
+        Ok(missing_vec.iter().map(|r| PyRevision(r.0)).collect())
+    }
+}
+
 pub fn init_module<'py>(
     py: Python<'py>,
     package: &str,
@@ -160,5 +287,6 @@
     let m = new_submodule(py, package, "ancestor")?;
     m.add_class::<AncestorsIterator>()?;
     m.add_class::<LazyAncestors>()?;
+    m.add_class::<MissingAncestors>()?;
     Ok(m)
 }

--- a/tests/test-rust-ancestor.py	Sat Dec 07 14:55:42 2024 +0100
+++ b/tests/test-rust-ancestor.py	Sat Dec 07 16:43:30 2024 +0100
@@ -172,12 +172,6 @@
         idx = self.parserustindex()
         self.assertEqual(dagop.headrevs(idx, [1, 2, 3]), {3})
 
-
-class RustCPythonAncestorsTest(
-    revlogtesting.RustRevlogBasedTestBase, RustAncestorsTestMixin
-):
-    rustext_pkg = rustext
-
     def testmissingancestors(self):
         MissingAncestors = self.ancestors_mod().MissingAncestors
 
@@ -200,6 +194,12 @@
         self.assertEqual(revs, {2, 3})
 
 
+class RustCPythonAncestorsTest(
+    revlogtesting.RustRevlogBasedTestBase, RustAncestorsTestMixin
+):
+    rustext_pkg = rustext
+
+
 class PyO3AncestorsTest(
     revlogtesting.RustRevlogBasedTestBase, RustAncestorsTestMixin
 ):