The crate uses a lot of unsafe, although the conditions for safety seem to have been taken in consideration carefully.
The use of unsafe consists of: - Handling len/capacity where bounds are checked manually. This looks correct. - Handling unitialized memory for indices >= len. I don't know enough about it to judge correctness here - Handling utf8 conversions/assumptions. For example, the conversion from char to bytes. Correctness is checked for every byte here, so this is likely correct, too.