I had a case where tar was too slow for my needs. Rust did let me cobble the right syscalls and threads together to make it more HDD-friendly and faster while forcing me to handle all the gritty filesystem error cases from the start. Lo and behold, about 4 times faster on an idle system and orders of magnitude faster on a busy system. It just would not have been possible to do it by combining shell utilities and too finicky in C.