There are many Rust compilers in the bootstrap chain. The point is to make every step in the chain human-readable. Auto-generated C is not "source code" in the

There are many Rust compilers in the bootstrap chain. The point is to make every step in the chain human-readable. Auto-generated C is not "source code" in the sense of "the preferred form of the work for making modifications to it" (the GPL's definition of source code). A malicious code generator could hide a trusting trust attack in the generated code in such a way that it would be difficult to find. True source code is easier to audit.