With Rust it's the same as in needing to return either a String or None, but it works fine for this kata.

I have no idea about what you're talking about (I don't do much Swift), but whatever, the fact is that, with a very few exceptions, all the published solutions do it, that's what the reference solution does, I did it too, and it works.

You need to change the return type to String?. I think it's a mistake from the translator that should be fixed (?).