Based on [1], also full support for s390x (IBM mainframes), as well as support for SPARC, WebAssembly, and asm.js as targets only (you can't run rustc itself on them, but can cross-compile to them). RISC-V support was recently merged into master (not sure why it's not on that page). AVR and m68k (seriously) both have actively developed ports which are living in forks until they're ready to be merged.

Anyway, most of the work of porting Rust to new platforms is not in rustc itself, but rather porting LLVM to target that platform. This is bad in a way and good in a way. It's true that LLVM doesn't have as extensive a list of supported targets as GCC. However, for people developing new architectures and OSes, LLVM is often the go-to choice as the first compiler to port. For example, WebAssembly, eBPF, and AMDGPU are all supported in upstream LLVM at present, but not in GCC. On the other hand, RISC-V got GCC support first – but lowRISC also actively developed an LLVM backend, which is now upstream. [2]

Of course, C benefits from having multiple implementations; even on platforms that only LLVM has a backend for, you can just use LLVM's C compiler (Clang), whereas there's (currently) no Rust compiler with a GCC backend. I'd love to see that change in the future. Still, I'd say Rust target support is already pretty strong when it comes to the (embedded) platforms most people are doing new development for, and rapidly getting stronger.

[1] https://forge.rust-lang.org/platform-support.html

[2] https://www.lowrisc.org/llvm/status/