Redox, a Unix-like operating system written in Rust, recently rewrote its kernel:
Since August 13, the kernel is going through a complete rewrite, which makes the kernel space ultra-small (about the size of L4). Everything which can run outside the kernel in practice, will do so.
It is almost complete and will likely be merged in the coming week.
The reasons cited for the rewrite include memory management, concurrent design, SMP support, and 64-bit by default.