I could not imagine running rustc on a Rust program, let alone building a 200-crate dependency graph or all of rustc, on a 2000s car entertainment system microcontroller.
What we'd program if we only have microcontrollers available, would be much smaller in scope, maybe a lot of focus on controlling physical infrastructure for agriculture and such.
That may not actually be a problem though, since the current compiler executable is already good enough for almost any project.