As a member of the development team for a C static analyzer, I use OCaml, which is also my favorite programming language, but that is because I'm from the generation in which it was the new thing (I learnt it when it had the same level of maturity as Rust, at a time when Rust didn't exist). It helps that it's perfect for writing compilers and static analyzers.

There are a lot of problems that seem a good match for Rust, and Rust is first in my list of programming languages I will never find the time to learn but wish I could.