There aren't really any "systems programming" platforms anywhere near production that doesn't have a workable rust target.

It's "embedded programming" where you often start to run into weird platforms (or sub-platforms) that only have a c compiler, or the rust compiler that does exist is somewhat borderline. We are sometimes talking about devices which don't even have a gcc port (or the port is based on a very old version of gcc). Which is a shame, because IMO, rust actually excels as an embedded programming language.

Linux is a bit marginal, as it crosses the boundary and is often used as a kernel for embedded devices (especially ones that need to do networking). The 68k people have been hit quite hard by this, linux on 68k is still a semi-common usecase, and while there is a prototype rust back end, it's still not production ready.

There’s also I believe an effort to target C as the mid level although I don’t know the state / how well it’ll work in an embedded space anyway where performance really matters and these compilers have super old optimizers that haven’t been updated in 3 decades.