Overall, I think the big win for Rust is when you need low-level, low-cost, low-memory, high-performance applications, at the cost of anything else. Rust is a great replacement for low-level C.

Unfortunately, those are not the features that you care about most in a compiler. In a compiler, your priorities are ease of extension, ease of specification, and ease of transformation. The last thing you want in a compiler language is to worry about ownership when trying to implement expression-rewriting optimizations. Conversely, OCaml is excellent at these things.