<!--READ THIS FIRST: If the feature you are proposing changes the language that GHC accepts or adds any warnings to `-Wall`, it should be written as a [GHC Proposal](https://github.com/ghc-proposals/ghc-proposals/). Other features, appropriate for a GitLab feature request, include GHC API/plugin innovations, new low-impact compiler flags, or other similar additions to GHC.--> ## Motivation I am trying to implement a GitHub Actions step that is expensive to build (90 minutes). When I use the standard `cache` action, GHC sees the old files but recompiles everything due to `[flags changed]`. I am not changing any flags between runs, so something is weird here. I want to understand what flags changed, so I pass `-ddump-hi-diffs`. The resulting output is useless:  The code here only checks the hash: https://gitlab.haskell.org/ghc/ghc/-/blob/wip/fromListN/compiler/GHC/Iface/Recomp.hs?ref_type=heads#L356-367 I am completely unable to diagnose what flags actually changed! ## Proposal We should store the actual flags we're fingerprinting in the interface file, and `fingerprintDynFlags` should return `(Fingerprint, InterfaceDynFlags)`. When this case gets tripped, we should `diffInterfaceDynFlags :: InterfaceDynFlags -> InterfaceDynFlags -> [String]` and render out exactly what changed.