how can we use cabal.projects but avoid having huge rebuilds for testing the primitive package?

https://github.com/haskell/primitive/commit/85d6c2f07b88d6e97f08dc84e4beca4a0f77c377 is a stable link to the tip of current master as of this point in question

https://github.com/haskell/primitive/blob/85d6c2f07b88d6e97f08dc84e4beca4a0f77c377/cabal.project is the current flavor of cabal.project

we'd actually be totally ok with vector/statistics/the various testing libs packages to privately depend on older/released primitive... but at the moment we can't do that and thus any changes to primitive trigger a massive rebuild on a whole bunch of stuff. And we can't make use of cabal test/new-test or cabal benchmark/new-benchmark :(

Start with the linked primitive commit and its cabal.project, then inspect Cabal's cabal test/new-test and cabal benchmark/new-benchmark behavior. Determine how private dependencies and project changes affect rebuilds, and define done as a documented or implemented way to test and benchmark primitive without triggering the described mass rebuild.

Written by the indexing model from the issue text.