I made this mistake for ages because Haskell is so popular and it's functional and pure, but it's not actually a requirement for functional languages to be pure. OCaml isn't.

I didn't say that FP languages have to necessarily be pure, just that FP languages tackle the problem of mutation by arranging programs such that most things are typically pure and side effects typically happen at the periphery (logging is probably the one exception, though). This is true even in FP languages that allow arbitrary side effects in functions, it's just not enforced by a compiler.

I agree Rust code has a different feel to OCaml code but that's because it makes some things easier (e.g. mutation, vectors). You still could write Rust as if it was OCaml (except for the lack of currying), it's just that nobody does that because it sucks.

That's the entire point, though. It's all about what the language emphasizes and makes easy to do. If it's unnatural to write a functional program in Rust and no one does it, then it's not really reasonable to call it a functional language. Writing functional programs is not idiomatic Rust, and that's okay.