Off the top of my head:  

*   Espressif has a team
*   Infineon has a team, or at least a couple of engineers in both automotive and consumer I think
*   STM32 has 1-2 engineers at least publicly working on sensor (not MCU) drivers
*   NXP has folks publicly working on embassy-nxp and job postings like this one

I've heard some rumors/had some chats with other vendors interested over the last couple years, not sure I've seen public action. I've also heard from some other vendors that have started using Rust for internal tooling/test work, not necessarily MCU code tho