> A good (trivial) example was a great command parsing library just doesn’t exist yet.

There is a Docopt implementation in Rust[1], which is used by Cargo. It tracks master and is regularly updated.

Interestingly, I've found people either love or hate Docopt, so maybe you knew about it but don't like it. :P

/shameless plug

[1] - https://github.com/docopt/docopt.rs