The kernel is required to be compilable with very old versions of GCC. So unless all users/companies agree on lifting that restriction, there is not much to do. rustc is not stable nor old enough.
So it's currently GCC 4.6+, with patches queued to bump that to 4.8+. Also, you can use Clang. So I'm not sure how rustc being used for some parts of the kernel has anything to do with developers "restriction?"
Via LWN, looks like the GCC 4.8+ patches got merged into the kernel's 5.8 merge window recently, with a footnote suggesting this be bumped to GCC 4.9+.
