rust-index: expose a method to retrieve the C index

The code for `shortesthexnodeidprefix` need to access the actual C index. For
now we grant its wish and expose a method to do so. Once we have the nodemap in
Rust, we will be able to implement the same feature from rust and we will be
able to drop this method.

Differential Revision: https://phab.mercurial-scm.org/D7658

rust/hg-cpython/src/revlog.rs

@@ -29,6 +29,17 b' py_class!(pub class MixedIndex |py| {'
             cindex::Index::new(py, cindex)?))
     }
 
+    /// Compatibility layer used for Python consumers needing access to the C index
+    ///
+    /// Only use case so far is `scmutil.shortesthexnodeidprefix`,
+    /// that may need to build a custom `nodetree`, based on a specified revset.
+    /// With a Rust implementation of the nodemap, we will be able to get rid of
+    /// this, by exposing our own standalone nodemap class,
+    /// ready to accept `MixedIndex`.
+    def get_cindex(&self) -> PyResult<PyObject> {
+        Ok(self.cindex(py).borrow().inner().clone_ref(py))
+    }
+
 
     // Reforwarded C index API
 

tests/test-rust-revlog.py

@@ -25,6 +25,13 b' class RustRevlogIndexTest(revlogtesting.'
         rustidx = revlog.MixedIndex(idx)
         self.assertEqual(rustidx.headrevs(), idx.headrevs())
 
+    def test_get_cindex(self):
+        # drop me once we no longer need the method for shortest node
+        idx = self.parseindex()
+        rustidx = revlog.MixedIndex(idx)
+        cidx = rustidx.get_cindex()
+        self.assertTrue(idx is cidx)
+
     def test_len(self):
         idx = self.parseindex()
         rustidx = revlog.MixedIndex(idx)