rust-index: handle `MixedIndex` in `pyindex_to_graph`

On the long run we will want to implement the Graph trait directly in Rust, but
for now we take the path with the least amount of change to focus on the coming
persistent NodeMap code.

We test this new code through with the lazy ancestors code.

Differential Revision: https://phab.mercurial-scm.org/D7657

rust/hg-cpython/src/revlog.rs

@@ -15,7 +15,10 b' use std::cell::RefCell;'
 
 /// Return a Struct implementing the Graph trait
 pub(crate) fn pyindex_to_graph(py: Python, index: PyObject) -> PyResult<cindex::Index> {
-    cindex::Index::new(py, index)
+    match index.extract::<MixedIndex>(py) {
+        Ok(midx) => Ok(midx.clone_cindex(py)),
+        Err(_) => cindex::Index::new(py, index),
+    }
 }
 
 py_class!(pub class MixedIndex |py| {

tests/test-rust-revlog.py

@@ -10,6 +10,9 b' except ImportError:'
 else:
     from mercurial.rustext import revlog
 
+    # this would fail already without appropriate ancestor.__package__
+    from mercurial.rustext.ancestor import LazyAncestors
+
 from mercurial.testing import revlog as revlogtesting
 
 
@@ -27,6 +30,22 b' class RustRevlogIndexTest(revlogtesting.'
         rustidx = revlog.MixedIndex(idx)
         self.assertEqual(len(rustidx), len(idx))
 
+    def test_ancestors(self):
+        idx = self.parseindex()
+        rustidx = revlog.MixedIndex(idx)
+        lazy = LazyAncestors(rustidx, [3], 0, True)
+        # we have two more references to the index:
+        # - in its inner iterator for __contains__ and __bool__
+        # - in the LazyAncestors instance itself (to spawn new iterators)
+        self.assertTrue(2 in lazy)
+        self.assertTrue(bool(lazy))
+        self.assertEqual(list(lazy), [3, 2, 1, 0])
+        # a second time to validate that we spawn new iterators
+        self.assertEqual(list(lazy), [3, 2, 1, 0])
+
+        # let's check bool for an empty one
+        self.assertFalse(LazyAncestors(idx, [0], 0, False))
+
 
 if __name__ == '__main__':
     import silenttestrunner