I want to encourage formulation of more concrete plans to fold https://github.com/NixOS/nixpkgs/pull/167670 and Add `haskell.lib.incremental` utility by Gabriella439 · Pull Request #204020 · NixOS/nixpkgs · GitHub into a joint effort.
Is that likely? Or even already underway?
“snapping” the date
The cool thing about this change is that it doesn’t compromise the purity of builtins.fetchGit. If a given fetchGit specification was pure then adding a date specification preserves that purity.
I think readers who indulge in this motion might benefit from conceptual argumentative proof short of auditing the code (which I still.will do now 
).