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).
I did not really mean that out of the box there would be incompatibilities, more likely within unsafe blocks and external crates