IMO, Rust makes the most sense on multi-threaded OS's. The strictness may help single-core systems but I doubt the learning curve is worth it on a single-threaded system.

Haiku has some of the Rust runtimes ported but not the whole thing, by any stretch.

I've liked what can be done with Rust but by the time it's ready for mainstream app programming, we'd be better off with a WebAssembly runtime via W2C2 instead and cross-develop from there.