For some of the unsupported targets there is the Rust->C compiler. Haven't used it myself but I've seen that, for example, the ESP32/ESP8266/Xtensa is supported by it.
That's true, but the issue with it as a real development option is that its job was to bootstrap the compiler, and so it implements Rust 1.19 (I believe...), and nothing further. So you're missing out on a lot of useful stuff.
steveklabnik|7 years ago