[GHC] #11735: Optimize coercionKind

[GHC]

#11735: Optimize coercionKind
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           Reporter:  goldfire       |             Owner:
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           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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 Currently, the `ForAllCo` case of `coercionKind` does an inefficient one-
 variable substitution. This could be improved by looking for nested
 `ForAllCo`s.

 Furthermore, perhaps we don't need an in-scope set and the full
 substitution machinery here. The subst is simply propagating the update of
 a tyvar's kind. No structural changes at all. No need for smart coercion
 constructors or other processing. So if we're going to optimize this, it
 might be worth making a specialized version of `subst_ty` and `subst_co`
 that operate over a `VarEnv Var` instead of a full substitution.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11735>
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