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