I don't know a great deal about scala or f#, but I know scala has higher kinded types as well as implicit function arguments which are things that rust does not

I don't know a great deal about scala or f#, but I know scala has higher kinded types as well as implicit function arguments which are things that rust does not have. OCaml had parameterizable modules and functors, tail call optimization, and presumably other important features. I'm much more familiar with Haskell, which definitely has a lot of things that rust does not, not just higher kinded types but sophisticated type level programming, type families, functional dependencies, existential types, etc etc etc. Additionally it has lazy evaluation, a very sophisticated green threading system, software transactional memory, etc. Not to mention of course the purity. Haskell has a bunch that rust does not -- of course the reverse is true as well, but it's worth mentioning. At the end of the day, though, rust is meant to address use cases that no mainstream functional language does, and it is very successful at it, while also having a ton of really cool features which makes it very exciting for FP enthusiasts.