cabal.project ghc-options are not in plan.json

[hamishmack]

Describe the bug
It would be useful to have the ghc-options that are specified in a cabal.project file included in the plan.json. We use the plan.json to build nix expressions for building with nix and currently users who specify ghc-options for packages in the cabal.project file must also specify them in their nix expressions to get the same results. This is in contrast to flags specified in the cabal.project file which do wind up in the plan.json file.

To Reproduce
Add something like this to cabal.project

package test-ghc-options
  ghc-options: -DTEST_GHC_OPTION

Run

$ cabal v2-configure

Observe that the ghc-options are not in the plan.json generated, though they will be used when cabal v2-build test-ghc-options is run.

Example code here and the nix based work around we currently use is here.

[phadej]

If we are going this way, we should specify all data which may affect ~/.cabal/store hashes in plan.json.

That will be IMHO very welcome.

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FWIW: ghc-options: -DTEST_GHC_OPTIONS is wrong, you should use cpp-options: -DTEST_GHC_OPTIONS e.g. so only providing ghc-options in plan.json would be incomplete solution.

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EDIT: until then cabal.project is an input to v2-build etc. So simply copying data from cabal.project to plan.json doesn't make sense to me.