Too many compilers
I honestly don’t think it’s a good idea to require multiple compilers/languages to build a kernel. That’s going to be confusing for any humans whether they like/can program in both (or more) languages. It’s just an extra, unnecessary, cognitive load.
I recall, back in the day, Windows 95 use to take multiple compilers, assemblers and other tools to build - admittedly this is a full OS and not just a kernel but even so, it’s a bit OTT.
As other have suggested a new kernel written in Rust seems like an okay idea, and certainly user space tools in Rust should also be hunky-dory.
As a side note: I can see there are certain higher level aspects of a kernel where memory safety is a good thing BUT all of the kernel code is doing some fairly unsafe and “hacky” things, C is good at that I wonder if Rust would just get in the way (or just require lots of “unsafe” sections which would kind of defeat the purpose)