I'm working on a universal tic-tac-toe backend that can be communicated with in any language via RPC. Should I chose Rust, or a higher level functional language like Haskell?
I don't think it's possible to give an universal answer for that. Can you do it with Haskell? You bet. Can you do it with Rust? Sure thing.
There are plenty of other considerations, such as are you doing it by yourself, are you hoping for a community, for work, how much effort are you willing to put etc.