Unnecessary rebuilds

[edsko]

I'm still seeing unnecessary rebuilds occasionally, but I have not yet been able to pinpoint under which circumstances this happens (and of course it's always too late to run with -v3 when it happens). I think adding some kind of on-by-default information why something is being rebuilt is a priority to track this one down.

[ezyang]

I see this too, relatively frequently in fact (invariably, GHC concludes that there's nothing to rebuild). I'll try to see what is going on.

[edsko]

Hmm, I'm also seeing actual compiles.

[dcoutts]

I just fixed one source of these. When you do:

-- make source changes
cabal new-build the-exe
cabal new-build the-lib
-- everything should be up to date now
cabal new-build the-exe
-- oh no, it tries to rebuild the exe

It accumulates a set of the components that have not changed since config/files changed.

Also, I eliminated another case of flip-flopping on enabling/disabling optional stanzas (testsuites etc), so that should also reduce spurious rebuilds.