If you want a single byte and `s` is a `&str`, then `s.as_bytes()[i]` returns a `u8` in `s` at index `i`. If the index `i` is out of bounds, then it panics, but no other UTF-8 checking is performed.
You do not need to do this if you're slicing. For example, if you know that `i..j` indexes a valid UTF-8 subslice of `s`, then `&s[i..j]` returns a subslice of `s` with type `&str`.
The only reason to subslice `s.as_bytes()` is if you want the raw bytes which may or may not be valid UTF-8. And in this case, it is a good thing that it is not automatic to convert that back to a `&str` since it may not be valid UTF-8.
You do not need to do this if you're slicing. For example, if you know that `i..j` indexes a valid UTF-8 subslice of `s`, then `&s[i..j]` returns a subslice of `s` with type `&str`.
The only reason to subslice `s.as_bytes()` is if you want the raw bytes which may or may not be valid UTF-8. And in this case, it is a good thing that it is not automatic to convert that back to a `&str` since it may not be valid UTF-8.