If you’d asked me five years ago how to safely deploy a change of this size in Ruby, I’d have a very simple answer: you can’t.
Ruby’s parser is no help in detecting regressions – it’ll notice invalid syntax, and that’s about it. A good test suite helps, but it’s never complete enough – the happy path and a few error cases are tested, but in production all kinds of alternate data tends to be flowing through. A similar story for staging environments – activity there isn’t of high enough fidelity to catch problems. Even a test suite with 100% line coverage (a rare thing anyway) is no sure fire bet because even covered lines may still have alternate branches which aren’t.
