If you are going on proprietary tool chains... most of those are moving to llvm which rust is based on. In theory any proprietary toolchain based on llvm
could provide rustc given incentives to do so.
If you are speaking to missing a rust compiler built on gcc, that seems to be an ongoing project with some momentum.
Realistically the most widely used architectures are now supported by rustc through llvm... x86, arm, riscv, and even to some extent xtensa now.
Power, arc, mips, sparc, and some others aren't too far away if someone cared enough.
If Linux can support Rust, I'd think that's a good sign most project can use Rust.
