The Xous operating system
Xous is a microkernel operating system designed for medium embedded systems with clear separation of processes. Nearly everything is implemented in userspace, where message passing forms the basic communications primitive.
↫ Xous website
It's written in Rust, and it's been around for a while - so much so it's sponsored by NLnet and the EU. The Xous Book provides a ton more details a
https://www.osnews.com/story/144230/the-xous-operating-system/
