It is well worth learning an ML-family language. In addition to Rust, OCaml or Haskell as njs12345 mentioned, F# or Scala might be options. OCaml is probably more mature than Rust, has more libraries available, and has automatic memory management which takes some of the load off you (especially if you're writing a GUI application, IME). But it's not got Rust's "hot right now" factor, it doesn't have Rust's ease of embedding in other languages, and it doesn't have quite the same focus on high-quality documentation.

If you already know Rust I wouldn't bother learning both - they're very similar languages.