I disagree that the language of the compiler doesn't matter. It absolutely matters in some cases, all of which are cases where (ANSI) C is pretty much the only option because of the inherent difficulty of C++ compilers.
The available compilers targeting your microcontroller certainly matter, though. You certainly still find lots of options that aren't Rust-compatible, but a non-trivial number microcontrollers are ARM or RISC-V based now, and can be targeted by LLVM/Rust.