Upstream? Likely never. Out of tree? https://github.com/tsgates/rust.ko/blob/master/README.md

I recall that Linus once said on the LKML that he could see himself accepting Rust code into the kernel. But I cannot find the source right now.

The problem isn't technical rather political.

If you want to fix UNIX security issues related to C, first you need to convince kernel developers across all UNIX variants to move away from C.

Then even a fully modernized UNIX kernel needs to provide support to unsafe POSIX APIs.

As we all know, Windows is totally secure, GUIs are awesome and the Burroughs architecture is unique.

See you already know, improving the world one person at a time.

One issue that would make Rust more likely in Linux would be a GCC toolchain for Rust. I personally don't want to see fragmented development of the language, though. Perhaps a backend for MIR for GCC could be done?

But, Linus et al still love C...

No way will Torvalds allow modifying the build process to include Rust.

Write your own out-of-tree Rust modules if you like, but they'll never be merged in.