[GHC] #10117: Change the scheme for reporting redundant imports

[GHC]

#10117: Change the scheme for reporting redundant imports
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        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.8.4
      Resolution:                    |             Keywords:  deprecate
                                     |  warning
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ezyang):

 I don't think so, but it depends on how #12067 is going to be fixed. If
 it's just that `addUsedDataCons` is not being called, or we are running
 the warning code too early, the changes should not conflict (since you'll
 be modifying the collection code, whereas this change modifies the
 reporting code.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10117#comment:11>
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