Well it's less that they rewrote it in Rust for the heck of it and more that they wrote it in Rust for the heck of it. Seems like it's going OK for them, though.
Well no, actually they explain their reasons for doing it in the article others linked here (https://asahilinux.org/2022/11/tales-of-the-m1-gpu/, section "A new language for the Linux kernel"), and it sounds pretty convincing - if you start a basically new and very complex driver project, and if there is a language more modern than C that might help you and is officially supported by the kernel, then why not use it?
Well for one it won't be heading upstream anytime soon, so they could have really picked any language they wanted. Rust definitely wasn't a bad choice but they did actively choose it because they wanted to.
