Google chose C++ 17 as the implementation language of the Zircon kernel for the fuchsia OS.
https://fuchsia.dev/fuchsia-src/development/languages/c-cpp/...
PS: C++ compile times are actually shorter than rust compile times.
For this kind of project its more important to have people with pratical experience in kernels, than experience in a particular language.
These people will have a experience in languages they used before, so even if the decision was taken today, you wouldn't find enough people with experience in kernels AND Rust, but will have much more people with experience in kernels C and/or C++, so C++ would still be a good default in comparison to C.
It would be pretty amateurish to get the kernel expert folks to learn Rust while implementing it, as it would probably mean that the end result in Rust would be worse than the end result in C++ as they were more comfortable and experienced in that language.
It will take more time to form specialists in some key areas so that decision can be made with less side-effects.