Rust support in the kernel is still optional; it's currently only allowed in drivers, and drivers only use Rust if they can accept running only on current Rust targets (which is not a substantive limitation).

I expect Rust to eventually get used in the core kernel, or in drivers that everyone wants to use (e.g. some new bus or device on most new hardware), but I expect that by the time that happens the set of targets supported by the kernel and the set of targets supported by Rust (including through things like crustc and codegen_gcc) will have converged sufficiently.