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.

AVR-Rust author here

The backend itself is working very well - there are only a handful of known bugs at this point, and many projects can be compiled with no issues.

The issue you linked also links to (https://github.com/rust-lang/rust/issues/44052), which shows that once these bugs are fixed, we can start working on merging avr-rust upstream.

A number of projects/libraries have also been developed recently

* https://github.com/avr-rust/blink * https://github.com/avr-rust/avrd * https://github.com/avr-rust/arduino * https://github.com/gergoerdi/rust-avr-chip8-avr