Tock
A microcontroller operating system written in Rust, where drivers are isolated by the type system and applications by the hardware.
Tock began around 2015 at Stanford, and its 2017 research paper carried the admirably blunt title "Multiprogramming a 64 kB Computer Safely and Efficiently". The starting point was that classic RTOSes run everything in one address space, so a bug in any driver can scribble over anything.
Tock splits the world in two. Drivers, called capsules, are written in safe Rust and simply cannot touch memory that isn't theirs — the compiler won't allow it. Applications run as separate processes, in C or Rust, kept apart by the memory protection unit, and a crashed app can be restarted without the kernel noticing. The cost is more RAM and a steeper learning curve than FreeRTOS. Google's OpenSK security-key firmware runs on Tock, which tells you what sort of user thinks that trade is worth it.