I'm using Rust at my startup. The prototype version was written in F#, but that means you're anyways fighting memory allocation, which was the single biggest performance issue in my program. My system works on network traffic at line rate, so every allocation per packet is noticeable.

C programs doing what I'm doing have traditionally had security issues. Since our code often runs in privileged locations, that's an unacceptable risk. Plus, Rust is vastly more expressive than C and not messed up like C++. And abstractions are cheaper, too. For instance, we use Generics + Traits as a way to keep flexibility and a plugin model per frame, while keeping the performance of static dispatch. C wouldn't be able to do that in an easy way.