[nix-local-build] Recompiling packages with many components is still pretty slow

[ezyang]

nix-local-build has to attempt to recompile EVERY component, even if files from only one component has changed. There seems to be some non-zero latency associated with thinking about a file, so when nix-local-build ends up hitting every component, this combined with GHC's recompilation avoidance latency makes the latency pretty bad.

[dcoutts]

To really do this we would need to be able to Setup build (or equiv) a component without it doing its own checks for dependent components. Probably the best approach to that is to go directly to the Cabal lib, bypassing the Setup build CLI API. Alternatively it needs some flag to Setup build to build exactly the given components without also rebuilding their deps.

[ezyang]

Will be fixed by #3662