So again your choices in practice are assembly, C, a subset of C++ (essentially C + classes, but no standard library and practically no templates due to stack limitation), maybe rust or some language you design yourself.
Here's a blog post about kernel drivers on Windows in Rust, turns out that just like C++ you can't use the standard library in rust:
https://not-matthias.github.io/posts/kernel-driver-with-rust/
"Rust provides lots of abstractions in
the standard library which
cannot be used in the kernel because it uses the Windows API
behind the scenes. Thanks to the awesome language design, we can
remove the standard library by specifying the #![no_std] attribute
in main.rs."
The same thing would apply to linux,
as there are almost certainly usermode/(g)libc calls in the rust
standard library that you can't make from the kernel.
Bijan