[GHC] #11437: Don't put static (version-based) feature gates in compilerInfo

[GHC]

#11437: Don't put static (version-based) feature gates in compilerInfo
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        Reporter:  ezyang            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.11
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ezyang):

 The feature gates which I want to remove are `Support parallel --make`,
 `Support reexported-modules`, `Support thinning and renaming package
 flags`, `Requires unified installed package IDs` and `Uses package keys`.

 I want to remove these because, if this is followed to its logical
 extreme, you'll end up putting lots of feature gates in `compilerInfo`,
 ballooning its size, slowing down Cabal (since it always has to parse this
 structure when it is configuring). What should you put in this structure?
 If we want to continue feature-testing, we should do it another way, like
 giving a list of supported flags, etc.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11437#comment:2>
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