Well, the AVR backend is being actively developed, I have no idea how good it is. (Although some tooling for embedded systems is rather poor, so one would imagine that LLVM with the right heuristics couldn't do badly).
rustc is apparently (https://github.com/rust-embedded/rfcs/issues/3) not far of it being usable.
However, I don't regularly write code for uC-ers - and when I do, it's usually simple enough to be in assembly - so I can't really say anymore than that.