You're splitting hairs in a weird way. rustc cannot compile C code. zig can. That they delegate to LLVM is immaterial.
"This crate calls out to the most relevant compiler for a platform, for example using cl on MSVC."
"This crate calls out to the most relevant compiler for a platform, for example using cl on MSVC."
But why do I care? I don't use rustc directly, the build system of choice does. And very few of the major build systems have an issue handling multiple languages.
Cargo (rust's build system) supports build scripts and the community has already created C/C++ compiler hooks such as https://github.com/alexcrichton/cc-rs
rustup and cargo also provide easy cross-compilation support, too.
I quoted from cc’s readme above.
I haven't had this experience with any other language. When a package manager lands, you won't even have to install any SDKs by hand anymore.