I love the concept and see the value for refactoring. There is tremendous value in knowing that two implementations are run-time equivalent.

I just wanted to share for others that there is a configuration language call Dhall (https://dhall-lang.org/) that has this kind of guarantee as one of its nicest features. See the "Refactor Fearlessly" section. In Dhall, most behavior preserving refactors produce the exact same hash code so you can know - for an absolute fact - that a refactor is behavior neutral. The point of sharing that other language is just to legitimize that "this is a thing" i.e. it has value recognized by others.