I don't even understand why you'd want to remove functional programming from a low level language like Rust. Its focus on zero cost abstractions and LLVM's optimizations mean that functional patterns like map or filter are most often inlined with your function at the call site, combining their overhead with the caller's stack and optimizing the code down to a single loop that is far less likely to have a cache miss or branch misprediction. This gives you complete control over performance and it's really easy to write your own iterators/operators to tweak the final result.

Functional programming gets a bad rap for performance and clarity (of the generated assembly) because most of them have gigantic runtimes like the JVM or Haskell but the way it is implemented in Rust is quite a bit different.