Think it's pretty dubious to call Rust a functional language at all: most Rust is written in an almost imperative style, but with some extra immutability sprinkled in when easy.

Coming from a functional background, when I first started programming Rust I really tried to leverage FP concepts as much as possible. However, I slowly began to realize that the FP version of the code I wanted to write was more complicated, more lines of code, and slower than the naive imperative version.

Clean FP without GC seems really difficult, and I think Rust team did as good a job as they could given the current state of research.