I would like to see that avr-unknown-gnu-atmega328 works with the latest compiler. Some bug in LLVM broke it[1], because it didn't work starting with versions after nightly-2021-01-07[2]. I know that rust team has nothing to do with this, but they should improve the gcc codegen[3] to be able to run rust on more embedded devices than LLVM has support for. Someone wanted to port his libc written in rust to ia64 and the gcc codegen broke and couldn't compile that.
[1] https://reviews.llvm.org/D114611
[2] https://github.com/Rahix/avr-hal/issues/124
[3] https://github.com/rust-lang/rustc_codegen_gcc