[GHC] #4879: Deprecate exports

[GHC]

#4879: Deprecate exports
-------------------------------------+------------------------------------
        Reporter:  basvandijk        |            Owner:
            Type:  feature request   |           Status:  new
        Priority:  high              |        Milestone:  7.8.1
       Component:  Compiler          |          Version:  7.0.1
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by igloo):

 I've added the beginnings of a patch for this. It needs some polishing,
 but basically works.

 In essence, warnings are parsed in export lists, and kept in `AvailInfo`
 and then `Provenance`. When giving deprecation warnings, we check to see
 whether any provenance is warning-free, and if not then we print all the
 warnings.

 I think we probably want to parameterise `AvailInfo`; in particular,
 rather than
 {{{
 addAvailInfoWarnings :: Map Name WarningTxt -> [AvailInfo] -> [AvailInfo]
 }}}
 we should have
 {{{
 addAvailInfoWarnings :: Map Name WarningTxt -> [AvailInfo Name] ->
 [AvailInfo NameWarn]
 }}}

 `bestImport` in `extendImportMap` should probably be trying to find an
 import without a warning, too.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4879#comment:12>
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