[GHC] #14295: tagToEnum# leads to some silly closures

[GHC]

#14295: tagToEnum# leads to some silly closures
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  datacon-tags
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 I think we can do better.  Add a `BuiltInRule` for `tagToEnum#`.
 {{{
 tagToEnum# Bool
 ===>
   \x -> case x of
     0# -> False
     1# -> True
     DEFAULT -> error "blah"
 }}}
 The rule can fire whenever `tagToEnum#` is applied to a known type, albeit
 perhaps one without too many constructors.  It's a bit like inlining
 `tagToEnum#`.
 }}}
 The built-in rule mechanism works well; we can use it here too.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14295#comment:5>
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