Use per-component cabal_macros.h

[bennofs]

I have a project that builds a library and has some benchmarks. Now, everytime I switch the benchmarks from disabled to enabled (I don't enable them all the time to make building faster when only working on the library), cabal_macros.h changes because it has different dependencies than the library. This causes parts of the library to be recompiled. Using a per-component cabal_macros.h would solve the issue.

[ezyang]

I think in the course of #3022 I will fix this bug.

[ezyang]

OK, here's the problem: if we have separate cabal_macros.h per library, the preprocessors need to know about what component they are building. Currently, the Preprocessor interfaces does not get this information, so we'd need to extend this interface. Unfortunately, the preprocessor hooks are a very commonly used interface in Custom setup files, so we need to decide if we're going to break it for more recent Cabal, or try to support this in a backwards compatible way.

But actually, I think this is OK, because with custom-setup, people who depend on old Cabal won't pick up a breaking version of Cabal.

By the way, extension point is probably extending

type PPSuffixHandler
    = (String, BuildInfo -> LocalBuildInfo -> PreProcessor)

to also get a ComponentLocalBuildInfo.

[23Skidoo]

But actually, I think this is OK, because with custom-setup, people who depend on old Cabal won't pick up a breaking version of Cabal.

You mean setup-depends? Yes, that's what it's for. We should start taking advantage of it!