[wyldfire]

> how this bug got through.

Simplest answer is that there's no representative test case in the test suite yet.

Unfortunately the problem of compiler testing is a very challenging one IMO. You can't test it exhaustively, or perhaps if you did write such a suite it would take longer to execute than is feasible.

> we all are just lucky enough that most messed up optimizations will break _something somewhere_ early enough to not get merged

Your compiler is only as good as the set of code people routinely use it on. Rust has exploded in popularity but it's still much less popular than C, C++.