Embracing a mechanized formalization gap

If a code base is so big and complicated that complete mechanical verification is intractable, can we still apply and benefit from verification methods? We show that by allowing a deliberate mechanized formalization gap we can shrink and simplify the model until it is manageable, while still retaining a meaningful, declaratively documented connection to the original, unmodified source code. Concretely, we translate core parts of the Haskell compiler GHC into Coq, using hs-to-coq, and verify invariants related to the use of term variables.