When you do fully value-oriented programming in Rust (i.e. no interior mutability involved) that's essentially functional programming. There's mutable, ephemeral data involved, but it's always confined to a single well-defined context and never escapes from it. You can even have most of your code base be sans-IO, which is the exact same pattern you'd use in Haskell.

I wouldn’t because idiomatic Haskell is way slower than idiomatic Rust.

Isn’t Rust a pretty good functional language? It has most of the features that enable safe, correct code without being anal about immutability and laziness that make performance difficult to predict.