For other OS stuff in Rust, see Redox (a microkernel) and Robigalia (Rust on seL4).
Redox is cool but Unix has outlived it's usefulness at this point. Also. the Unix philosophy is the very anti-thesis of what I think is the Rust philosophy. I can imagine that a Rust port of QNX could be a big deal tho (QNX is/was only like 500KLOC of C, which could be like 100KLOC of Rust).
But I think that we need mode advanced systems, e.g. don't need as much virtual memory protection because that's handled by the language. There's actually quite a few things in the current OSs that seem to be pure anachronisms.
This is true at compile time. But what does it do for you at runtime when bad actors will be trying to break any and all assumptions? The guarantees in the kernel need to be vigilant against attackers, so while the kernel itself might be simpler, the boundary with processes still needs a lot of constraints at runtime.
https://github.com/ryanra/RustOS
RustOS is hobby OS project that's pretty bare bones, but has similar ideas to what you've mentioned. It's essentially a port of the Rust standard library to bare metal, relying on the safety of the language rather than hardware features. It's sort-of a unikernel in implementation if not intention.
https://github.com/helena-project/tock
Tock is an interesting academic project to create an embedded OS. It still uses more traditional hardware protection for user processes, since these can be written in any language. The kernel, however, is composed of cooperatively-scheduled components called capsules, which rely on the language for isolation and don't have the overhead associated with real processes. See this doc for more details: https://www.tockos.org/documentation/design/