There are some platforms which linux supports that LLVM does not (and GCC does). There is quite a lot of effort in making a decent LLVM backend, and these older systems tend to have relatively few maintainers, so there may not be the resources to make it happen.

Aren't those already situations we use cross compiler for?

A cross compiler is just compiler backend for machine X running on machine Y. You still need the backend.

> There is quite a lot of effort in making a decent LLVM backend, and these older systems tend to have relatively few maintainers

Well, it also takes effort to be held back with outdated tools. Also, the LLVM backend doesn't have to be top-notch, just runnable. If they want to run legacy hardware they should be okay with running a legacy or taking the performance hit of a weaker LLVM back-end.

Realistically

At version 16[1], LLVM supports: * IA-32 * x86-64 * ARM * Qualcomm Hexagon * LoongArch * M68K * MIPS * PowerPC * SPARC * z/Architecture * XCore * others

in the past it had support for Cell and Alpha, but I'm sure that the old code could be revived if needed, so how many users are effected here? Lets not forget the Linux dropped Itanium support and I'm sure someone is still running that somewhere.

Looking through this list [2], what I see missing is Elbrus, PA-RISC, OpenRisc, and SuperH. So pretty niche stuff.

[1] https://en.wikipedia.org/wiki/LLVM#Backends

[2] https://en.wikipedia.org/wiki/List_of_Linux-supported_comput...

Then use the GCC backend. https://github.com/rust-lang/rustc_codegen_gcc