[GHC] #12511: template-haskell's Language.Haskell.Syntax module should be Trustworthy

[GHC]

#12511: template-haskell's Language.Haskell.Syntax module should be Trustworthy
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           Reporter:  int-e          |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  libraries      |           Version:  8.0.1
  (other)                            |
           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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 The reason why this module isn't inferred safe is that it uses
 `unsafePerformIO` to create a global `IORef`.

 From a SafeHaskell perspective, this module isn't very useful in itself,
 but its unsafety prevents other modules that import it to provide class
 instances from being inferred as safe. A concrete example is
 https://hackage.haskell.org/package/exceptions-0.8.3/docs/src/Control-
 Monad-Catch.html#line-186

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12511>
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