This is maybe true, given that Rust is intentionally multi-paradigm, but it's a pretty unfair comparison: My FP-style version deliberately made almost no change

This is maybe true, given that Rust is intentionally multi-paradigm, but it's a pretty unfair comparison: My FP-style version deliberately made almost no changes to the original code aside from adding names - I even stated in the comment that I don't even know Rust (haven't touched it beyond knowing what "Affine types" means, and once having gotten someone else's "hello world" to compile for ARM). The imperative version you're comparing it to completely changes the semantics of the program, and is written by someone very familiar with the language.