It is a rustc backend, ie an alternative to llvm, gcc, or the cranelift backends. It started as a .NET backend but they found that their approach could easily

It is a rustc backend, ie an alternative to llvm, gcc, or the cranelift backends. It started as a .NET backend but they found that their approach could easily support C code generation as well so they added that. They do this by turning what rustc gives them into their own IR.