I made a small example to show the "issue". It is available here: https://github.com/mikeycgto/ruru-rstring-encoding#example.

While this isn't incorrect it does feel a little odd since in modern Ruby and Rust both default to UTF-8 encoding.

Perhaps ruby-sys should expose rb_utf8_str_new as well? The RString struct could then maybe have a new_from_utf8 function which will ultimately call this Ruby C function. I can make the necessary PRs but I wanted some feedback first.