Speaking of operating systems written in Rust – a popular activity as of late – one of the SoC contributors to Redox is also writing their own operating system in rust, called Aero.
Aero is a new modern, experimental, unix-like operating system written in Rust. Aero follows the monolithic kernel design and it is inspired by the Linux Kernel. Aero supports modern PC features such as Long Mode, 5-level paging, and SMP (multicore), to name a few.
Open source, of course, licensed under the GPL, version 3.