Correct – but that’s exactly why you can’t just say “all strings are UTF-8, we’re done here”. Because Windows filenames can contain unpaired surrogates, which don’t have a sensible UTF-8 translation. (Unix has a similar problem with filenames being arbitrary bytes that may not be valid UTF-8.)

Note that Rust has OsString to handle both of these cases, but it’s somewhat hard to use.

With networking code you sometimes need an OsString for the platform you're not on. Rust doesn't seem to support this, other than by writing your own.