--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hg-cpython/src/conversion.rs	Sat Jan 12 16:57:04 2019 +0100
@@ -0,0 +1,28 @@
+// conversion.rs
+//
+// Copyright 2019 Georges Racinet <georges.racinet@octobus.net>
+//
+// This software may be used and distributed according to the terms of the
+// GNU General Public License version 2 or any later version.
+
+//! Bindings for the hg::ancestors module provided by the
+//! `hg-core` crate. From Python, this will be seen as `rustext.ancestor`
+
+use cpython::{ObjectProtocol, PyObject, PyResult, Python};
+use hg::Revision;
+use std::iter::FromIterator;
+
+/// Utility function to convert a Python iterable into various collections
+///
+/// We need this in particular to feed to various methods of inner objects
+/// with `impl IntoIterator<Item=Revision>` arguments, because
+/// a `PyErr` can arise at each step of iteration, whereas these methods
+/// expect iterables over `Revision`, not over some `Result<Revision, PyErr>`
+pub fn rev_pyiter_collect<C>(py: Python, revs: &PyObject) -> PyResult<C>
+where
+    C: FromIterator<Revision>,
+{
+    revs.iter(py)?
+        .map(|r| r.and_then(|o| o.extract::<Revision>(py)))
+        .collect()
+}