There are patches that let Rust run on ESP32 systems, so think it's entirely suitable for tiny embedded things. What makes it bloaty is the linked in standard library, but it's not an unsolvable crisis; you can dynlink against glibc, there is a crate for core rust IIRC. That'll get you reasonably sized rust applications.

And for writing kernels the same applies; without the stdlib, it gets a lot smaller very fast. I've done it, so I think I can count myself on having some experience there. The biggest part of my kernel is a 128KB scratch space variable it uses during boot as temporary memory until it has read the memory mappings from the firmware and bootstrapped memory management on a basic level. The remainder of the kernel (minus initramfs) is then about 1MB large, the largest part after the 128KB scratch space using about 96KB.