The majority of Linux Rust work is in individual drivers, there is almost no work for it in the Kernel itself.
Mixed languages on their own are a problem too. You can exploit a bug that would usually be covered by an exploit protection in one language (i.e. CFI for C), by taking advantage of another binary in another language (like Rust).
https://www.cs.ucy.ac.cy/~eliasathan/papers/tops20.pdf
