I believe the best way to do this is `set fish_user_paths $fish_user_paths ~/.rbenv/bin`, which will prepend `~/.rbenv/bin` to your $PATH and will be persistent. Specifying the universal flag (`-U`) as seen in the diff shouldn't be necessary because [the variable `$fish_user_paths` is universal](https://github.com/fish-shell/fish-shell/issues/527#issuecomment-13315434) and in fish [such settings are honoured](https://github.com/fish-shell/fish-shell/blob/master/doc_src/set.txt#L52).