[GHC] #14899: Significant compilation time regression between 8.4 and HEAD due to coverage checking

[GHC]

#14899: Significant compilation time regression between 8.4 and HEAD due to
coverage checking
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by bgamari):

 * milestone:  8.6.1 => 8.8.1


Comment:

 Unfortunately the summary here suggests that the status quo isn't
 acceptable for release but the future direction is rather unclear. I can
 see a few possibilities:

  1. Teach the coverage checker to bail out (with a warning) if it sees a
 "large" pattern of this form
  2. Allow a TH splice to explicitly request that coverage checking be
 disabled. This seems like a
  3. Add a new `FromTH` `Origin` and a flag allowing the user to choose
 whether to run the coverage checker on TH splices.
  4. Advise users to disable coverage checking in any module affected by
 the bug
  5. Revert ffb2738f86c4e4c3f0eaacf0a95d7326fdd2e383 for 8.6

 Frankly, all of these options seem pretty terrible given that this pattern
 is problematic only due to a (hopefully temporary) limitation of the
 coverage checker. That being said, we clearly need to do something for
 8.6.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14899#comment:7>
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