[GHC] #14776: Add equality rule for `eqString`

[GHC]

#14776: Add equality rule for `eqString`
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           Reporter:  mpickering     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           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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 Locally I sometimes have to define a rule

 {{{
 {-# RULES "eqStringEQ" forall x . eqString x x = True #-}
 }}}

 in order for "stuck" expressions to evaluate to `True` and for a case
 expression to be eliminated.

 Is there is a reason not to include this RULE directly in GHC.Base?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14776>
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