[GHC] #10752: Print which warning-flag controls/enabled an emitted warning

[GHC]

#10752: Print which warning-flag controls/enabled an emitted warning
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        Reporter:  hvr               |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:  8.0.1
       Component:  Compiler          |                 Version:  7.10.2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by osa1):

 This is a great idea and it should be just a straightforward refactoring:

 - Modify `ErrUtils.makeIntoWarning` to get a `Maybe WarningFlag` as
 argument.
 - Modify `ErrUtils.ErrMsg` to record a `WarningFlag`.
 - Since we want to show flags uniformly in messages, maybe modify
 `LogAction`
   to pass this `Maybe WarningFlag`. Then modify
 `DynFlags.defaultLogAction` to
   print these flags.
 - Fix all the compile errors by passing warning flags to
 `makeIntoWarning`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10752#comment:3>
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