[GHC] #9723: Give Tab warning only once per file

[GHC]

#9723: Give Tab warning only once per file
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        Reporter:  nomeata           |                   Owner:  dalaing
            Type:  feature request   |                  Status:  patch
        Priority:  low               |               Milestone:
       Component:  Compiler          |                 Version:  7.9
      Resolution:                    |                Keywords:  newcomer
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:  Phab:D760
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Comment (by hvr):

 I'm a bit dubious about this change. While I understand the intent it
 seems to the first warning which is not emitted at all occurrences. So
 tooling can't highlight all offending lines at once, and would need to be
 made aware that this warning actually affects more lines.

 So maybe the compromise would be to have a flag to enable warning *all*
 tab-polluted lines, but have it default to warn only about the first
 occurence

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9723#comment:4>
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