I've queued the first 5 patches, thanks. Please send a follow up to fix nits
and minor issues.

+py_module_initializer!(rustext, initrustext, PyInit_rustext, |py, m| {
+ m.add(
+ py,
+ "doc",
+ "Mercurial core concepts - Rust implementation",
+ )?;
+
+ let dotted_name: String = m.get(py, "name")?.extract(py)?;
+ m.add(py, "package", "mercurial")?;

IIUC, package should be set by the importer, not by the module. And if
set, I think it should be "mercurial.rustext" since this is a package module.

https://stackoverflow.com/a/21233334/10435339

+impl GraphError {
+ pub fn pynew(py: Python, inner: hg::GraphError) -> PyErr {
+ match inner {
+ hg::GraphError::ParentOutOfRange(r) => {
+ GraphError::new(py, ("ParentOutOfRange", r))
+ }
+ }
+ }
+}

Maybe this can be a trait impl or helper function that maps hg::GraphError
to PyErr::new::<exc::ValueType, _>() ?

+ let sys = PyModule::import(py, "sys")?;
+ let sys_modules: PyDict = sys.get(py, "modules")?.extract(py)?;
+ sys_modules.set_item(py, dotted_name, &m)?;

We'll probably want to move this to the caller to avoid dups. The init
function can have the same signature as the lambda of
py_module_initializer!().

+[lib]
+name='rusthg'

Perhaps this is an old name.