Tock
https://github.com/helena-project/tock

Tock is an embedded operating system designed for running multiple concurrent, mutually distrustful applications on Cortex-M based embedded platforms. Tock's design centers around protection, both from potentially malicious applications and from device drivers. Tock uses two mechanisms to protect different components of the operating system. First, the kernel and device drivers are written in Rust, a systems programming language that provides compile-time memory safety, type safety and strict aliasing. Tock uses Rust to protect the kernel (e.g. the scheduler and hardware abstraction layer) from platform specific device drivers as well as isolate device drivers from each other. Second, Tock uses memory protection units to isolate applications from each other and the kernel.

Redox
https://github.com/redox-os/redox

Redox is an operating system written in Rust, a language with focus on safety and high performance. Redox, following the microkernel design, aims to be secure, usable, and free. Redox is inspired by previous kernels and operating systems, such as SeL4, Minix, Plan 9, and BSD.

Redox is not just a kernel, it's a full-featured Operating System, providing packages (memory allocator, file system, display manager, core utilities, etc.) that together make up a functional and convenient operating system. You can loosely think of it as the GNU or BSD ecosystem, but in a memory safe language and with modern technology. See this list for overview of the ecosystem.