Nimrod is a great language, but it has different goals to Rust. You get better expressiveness, and cleaner code, but you don't get the huge benefits of Rust's static type system. It depends on which you value more - I think there is a place for them both though.
Nimrod's static type system looks comparable to Rust's; are you referring to Rust's safety guarantees?
If so, I believe Nimrod's support for immutability gets you pretty far, but I have not looked very deeply at it. For example, implements an explicit "IO taint" mechanism reminiscent of Haskell.
