Disclosure: I am a grad student, recently started working on RDL.
We did use your standard library annotations for Sorbet and have many fixes to them sitting in your pull request queue: https://github.com/plum-umd/rdl/pull/68 https://github.com/plum-umd/rdl/pull/72 https://github.com/plum-umd/rdl/pull/57 . We stopped submitting more since these weren't being upstreamed.
Before we started our project, I evaluated rolling out RDL instead of building our own typechecker. Trust me, I would MUCH rather use an existing project than have to build our own, but sadly it just didn't scale to our millions of lines of code.
I'm more than happy to chat about the details of why we didn't use RDL if you'd like to email me at sorbet@stripe.com. Thanks again for your great project, we're standing on the shoulders of giants.
Have you considered the property-based QuickCheck approach? Instead of building a type checker, you just annotate your code with properties (preconditions, postconditions and class invariants). You can generate tests to verify your code and also inject runtime checks to fail early in case of violations.
In my experience, this tends to work much better on dynamic languages and it also scales to larger codebases with heavy usage of dynamic idioms.
Will shoot you an email to discuss more.
Truthfully, these 'prestige' projects are done for reasons other than actually improving the development standards. They probably didn't use RDL because it was more fun to write their own then figure out someone else's thing.
For all we know they may not have even had a real need for a new typechecker. Stripe could have a valued engineer who seemed bored so engineering management gave him or her an intellectualy interesting project to do - even if it wasn't that important to the business.
It seems like Hummingbird is much more powerful than Sorbet, but it's hard to tell without them publishing a paper yet.