I also found a Rust implementation which seems not to compile, but is useful as I'm trying to teach myself Rust https://github.com/kenpratt/rusty_torrent I think the work to get it to compile might be minimal.
There is also another project in Rust, it looks more active: https://github.com/GGist/bip-rs It is a collection of libraries.
If anyone who hasn't tried doing this before, the "official" BitTorrent spec docs, namely BEP-3 (http://bittorrent.org/beps/bep_0003.html), seem little more than a vague blog post turned in to a "spec".
Doesn't look vague at all. What do you think is missing from it?