I am assuming they're talking about https://github.com/thepowersgang/mrustc

Others claim mrustc is the second, but has anyone actually tried to use it? the readme does not inspire too much confidence, and looks to (currently) be a C transpiler:

"mrustc's primary goal is bootstrapping rustc, and as such it tends to assume that the code it's compiling is valid (and any errors in the generated code are mrustc bugs). Code generation is done by emitting a high-level assembly (currently very ugly C, but LLVM/cretone/GIMPLE/... could work) and getting an external tool (i.e. gcc) to do the heavy-lifting of optimising and machine code generation."

At least it can be used to bootstrap the actual rust compiler. It's a bit crazy to me how popular this language appears to be, but doesn't have a proper second compiler yet.

mrustc[0] I think.

[0](https://github.com/thepowersgang/mrustc/)