Just to be a nitpicking little jerk, Rust’s design isn’t really based on a FP paradigm. It is imperative, allows for side-effects and borrows more from ML style languages than from pure functional languages. You CAN probably write Rust code that acts as a functional program using only «pure» functions though.

I know that many say that various ML descendants are functional, but if it allows for side-effects, it’s not really functional per definition. Functional style is different from true functional behavior.