Interestingly, Resea Kernel is used to be written in Rust. However, it soon be rewrote in C because of the following drawbacks:

• It tends to consume too much kernel stack (especially core::fmt).
• It implicitly employs panics to prevent undefined behaviors.
• libcore is too large for the microkernel (occupies approximately 30% of .text).
• Writing intrusive data structures (e.g., LinkedList, SpinLock) is painful.

I do think C is better for a minimalistic microkernels as of this writing, that said, I also believe that the problems might be addressed someday.