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My understanding: As Rust is built on LLVM and not GCC, it is also limited to operating systems supporting LLVM.

GCC simply supports more platforms.

Rust has a GCC backend as well, rustc_codegen_gcc. However, the NonStop platform just has a proprietary C compiler.

MSVC is also proprietary. However LLVM is supported by Microsoft. The developer of Nonstop is apparently not doing that.