The compiler toolchain can cross compile to esp8266, but I think the pure Rust HAL is less developed there. If you use Rust to write components/code that link against the ESP-IDF, I’d imagine that would work?

I have the toolchain in a Docker container if you want to try it out.

Yes, though IIRC a recent release broke esp8266 support accidentally. I haven't checked back to see if it's working again.