rust-pyo3-index: _index_headrevs This one demonstrates that why the `with_index_read` and similar helpers are useful and was actually the main motivation for doing them: if we kept the borrow used to grab the index before updating the caches, there would be a panic when calling `borrow_mut`. This was confirmed with an earlier version by the Python test. There are perhaps some internal API clarifications to be made, as the method updating the cache does a seemingly useless return), but we are keeping it as it was in `hg-cpython`.

--- a/rust/hg-pyo3/src/exceptions.rs	Wed Dec 25 19:05:27 2024 +0100
+++ b/rust/hg-pyo3/src/exceptions.rs	Wed Dec 25 19:06:59 2024 +0100
@@ -78,3 +78,9 @@
         }
     }
 }
+
+pub fn graph_error(_err: hg::GraphError) -> PyErr {
+    // ParentOutOfRange is currently the only alternative
+    // in `hg::GraphError`. The C index always raises this simple ValueError.
+    PyValueError::new_err("parent out of range")
+}