Is there anything about Rust that would make it hard to use existing python extensions that are written in C? I see the FFI docs, so it seems like it would be fine, but have no Rust experience.
Fwiw, nothing about rust (probably) but a whole lot about CPython's implementation. Extensions can and do poke around in object internals and will break if the internals don't work the way the extension expects them to (that is, the way CPython works). This has long been PyPy's big stumbling block (though it seems to have gotten better).