My understanding is that Ruby's strings are sequences of arbitrary bytes, even if the associated encoding is UTF-8. So the natural mapping to Rust would be as [u8] and Vec<u8> instead of str and String for most purposes. A couple of helpers using str should be fine (e.g. the from_utf8 function).
Functions which convert a ruby string to &str or String without verifying UTF-8 validity must be marked unsafe, otherwise they're unsound.