Kernels are big pieces of software. Rust is used for device drivers mainly, right? So in that case you write an idiomatic rust lib and wrap it in a C interface and load it in.
Actually interfacing with idiomatic C APIs provided by an OS is something else entirely. You can see this is when you compare the Rust ecosystem to Zig; ie Zig has a fantastic io-uring library in the std lib, where as rust has a few scattered crates none of which come close the Zig's ease of use and integration.
One thing I'd like to see is an OS built with rust that could provide its own rusty interface to kernel stuff.
