Anything that can run C++ should be able to run Rust if you use the LLVM backend, perhaps except when a watch uses musl libc and you rely on some glibc-only call in your program. afaik the only physical devices shipping with musl are car infotainment systems based on l4 kernels and networking hardware (OpenWRT).

Do you mean compiler running on the Watch? Because the compiled code is no longer Rust and therefore there is no reason why it should not run on the Watch. Or do you mean specifically applications with the Rust library Egui?