rust-pyo3: exposition of AncestorsIterator Compared to the early experiments, we have one less `RwLock` in the wrapping. Still that is somewhat redundant with `UnsafePyLeaked` being itself some kind of lock. In the original rust-cpython code, we were borrowing the `RefCell` with a method that can panic. Instead we are now converting the `PoisonError` that unlocking a `RwLock` can produce. Since all methods acquiring the `RwLock` are themselves protected by the GIL and do not release it before returning, nor do they leak RwLock guards, there is no risk of contention on these locks themselves.

--- a/rust/hg-pyo3/src/convert_cpython.rs	Sat Dec 07 18:24:24 2024 +0100
+++ b/rust/hg-pyo3/src/convert_cpython.rs	Sat Dec 07 18:38:37 2024 +0100
@@ -263,7 +263,6 @@
 /// This would spare users of the `cpython` crate the additional `unsafe` deref
 /// to inspect the error and return it outside `UnsafePyLeaked`, and the
 /// subsequent unwrapping that this function performs.
-#[allow(dead_code)]
 pub(crate) fn py_leaked_or_map_err<T, E: std::fmt::Debug + Copy>(
     py: cpython::Python,
     leaked: cpython::UnsafePyLeaked<Result<T, E>>,