[GHC] #12610: Emit tab warning promptly

[GHC]

#12610: Emit tab warning promptly
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        Reporter:  dfeuer            |                Owner:  dalaing
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.3
       Component:  Compiler          |              Version:  8.0.1
  (Parser)                           |
      Resolution:                    |             Keywords:
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 dalaing):

 It looks like these are the various warnings that can be generated by the
 parser:

 If `Opt_WarnTabs` is set:
 * the tab warnings are collected and summarised

 If `Opt_WarnUnrecognisedPragmas` is set:
 * "Unrecognised pragma"

 If `Opt_WarnAlternativeLayoutRuleTransitional` is set:
 * "`where' clause at the same depth as implicit layout block"
 * "`|' at the same depth as implicit layout block"

 I think it's probably reasonable to print all of the warnings generated
 during parsing in the event of an unsuccessful parse.

 The other option would be to print the tab warning and suppress the
 others.

 I'm not sure we have enough structure in the warnings at the moment to do
 anything more fine grained unless we filtered them based on their warning
 strings.

 Adding an ADT for the warnings would help with that, and could be a
 positive step for later, but it's not something we need right now if folks
 agree that we always want to print all of the warnings on an unsuccessful
 parse.

 I'm going to start working on a change that prints warnings on
 unsuccessful parses (except perhaps for HeaderInfo.getImports, which has a
 note in it about not logging warning) and see how that goes.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12610#comment:5>
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