In order to make stack ghci reliable / consistent, the cabal_macros.h and Paths_* files need to be generated. Currently, this means that we do a full build of the targets (unless --no-build is passed).

Really all that needs to happen is to run initialBuildSteps before running ghci. Running this function requires LocalBuildInfo, which gets stored in dist/setup-config. Unfortunately, we can't call it directly from stack - it uses Binary / Generic, so we can't rely on it being compatible between cabal versions (and stack may be linked with a different version).

I think a reasonable solution to this is to have a separate program which runs initialBuildSteps on the specified packages. This separate program could either be a script that gets runhaskell-ed, or something that gets build and installed to the snapshot (like the default Setup.hs).

This solution also has relevance to other issues:

• It's also the information necessary to allow Setup.hs to modify the cabal info (see "stack ghci" doesn't honor configured cpp flags #970). Doing this would be more complicated. I think the best way to do this would be to serialize localPkgDescr as a .cabal file (I really hope this is possible). Then, we'd would parse the resulting cabal file, and use that description to generate the arguments passed to ghci.
• This may also allow us to avoid doing full rebuilds for haddock - I noted that in cabal this function is also called by the haddock stuff.