There are many, here's a simple Google search:

https://github.com/jyn514/saltwater

https://github.com/ClementTsang/rustcc

https://github.com/maekawatoshiki/rucc

Did you actually look at these?

> https://github.com/jyn514/saltwater

This is just a frontend. It uses Cranelift as the backend. It's missing some fairly basic language features like bitfields and variadic functions. And if I'm reading the documentation right, it requires all the source code to be in a single file...

> https://github.com/ClementTsang/rustcc

This will compile basically no real-world code. The only supported data type is "int".

> https://github.com/maekawatoshiki/rucc

This is just a frontend. It uses LLVM as the backend.

Look at what those compilers are capable of compiling and to which targets, and compare it to what this compiler can do. Those are wonderful, and I have nothing but respect for them, but they aren't going to be compiling the Linux kernel.

I found this one too: https://github.com/PhilippRados/wrecc