Actually, there are people who have already written firmware in Rust for their DIY/kit keyboards. Nothing yet mainstream like QMK. Here's a write up of one of them: https://josh.robsonchase.com/rest-of-the-keyboard/

Meet the Ferris keeb that was just recently developed. Currently built using QMK but will eventually have Rust firmware. https://github.com/pierrechevalier83/ferris