But at least a lot of tasks in a kernel would require something else than reference counting, unless it can be guaranteed that the reference counting is optimized away or something, right?
There are some allocations where it doesn't make sense for them to have multiple owners (strong references) but I wouldn't say it makes sense to think about it as being optimized away or not.
https://www.swift.org/get-started/embedded/
Rust's approach is overkill, I think. A lot of reference counting and stuff is just fine in a kernel.