[Manishearth]

DDC is irrelevant here, DDC is an argument to not write the second compiler in C++ and write it in Rust too.

Having a Rust compiler in C++ is a mitigation to the trusting trust attack, period. You don't need DDC for this.

DDC is necessary when you have two self hosted compilers (e.g. GCC and clang). Here we have one self-hosted compiler (rustc), and one in another language (C++). To mitigate trusting trust in rustc, use mrustc to compile rustc, and then use that rustc to compile itself, and now you have a trusted binary (provided you trust your C++ compiler. you can fix this by DDCing the C++ compilers)