[GHC] #13253: Exponential compilation time with RWST & ReaderT stack with `-02`

[GHC]

#13253: Exponential compilation time with RWST & ReaderT stack with `-02`
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           Reporter:  phadej         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 See the attached file. The compilation time grows exponentially with
 additional fields in `HuteStruct`. Tried with GHC-8.0.1 and quite recent
 HEAD (8.2)

 adding a field doubles the compilation time with `-O2`.
 - 9: 17s
 - 10: 34s
 - 11: 83s
 on my machine. With `-01` around 3sec, independently of the struct size.

 The original issue: https://github.com/yesodweb/yesod/issues/1348

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13253>
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