custom-setup should respect hs-source-dirs

[ezyang]

Otherwise, we can't support Cabal self-bootstrapping now that the source files have gone into src.

[ezyang]

Chatting with @dcoutts, it seems that we want to keep custom-setup stanzas as simple as possible, because any tools which want to use them need to know how to build them and we want to keep this simple. (I'm a little skeptical; maybe this just means that there should be a way for a Cabal project to refer to an external Cabal project to supply the setup script...) Which means that we don't want to do this.

But then how do we bootstrap Cabal now? A thorny issue.

[23Skidoo]

Now that Cabal uses build-type: Simple, maybe bootstrapping should be something that is only done manually? This will mean that Cabal.cabal will only be able to use Cabal features that are available in supported GHC versions. Not sure how bad of a limitation this is.

[ezyang]

The obvious way to manually bootstrap (cabal install Cabal; cabal install cabal-install) is fairly annoying when you are refactoring data structures like LocalBuildInfo. (You may not want to "just" build a Setup executable because you want cabal-install to take care of things for you, e.g. cabal new-build.) It's possible that cabal-install could be adjusted to make this case easier ("Yes, I know it's a Simple build type, but please use this setup executable anyway").

[hvr]

@ezyang are you considering cabal.project-based development of Cabal+cabal-install, or is this still about the pre-nix workflows?

[ezyang]

This is in both worlds, but I'm thinking about Nix workflow.

[ezyang]

Cabal got moved out of src back to the base directory so this problem is no longer a blocker for Cabal. So I guess close for now. See also #3003