I don't know LISP9, and I don't know Rust. So, what did I decide to do? Implement a language interpreter for a language I don't know in a language I don't know.
I have not yet decided whether this is sane or insane, but it seems like a good exercise to me. I'll get to refresh my C too, since that's what the original interpreter was written in.
(I'm currently leaning more towards insane.)