[GHC] #11735: Optimize coercionKind

[GHC]

#11735: Optimize coercionKind
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 I still say that most uses of `mkNthCo` already know the right role --
 though perhaps this role may not be as obvious to someone who isn't me.
 Just supplying the role would be much better than computing it, of course.

 I don't agree with Simon's suggestion about `mkNthCoDirect` -- this is a
 first step toward invariants that are not upheld. Instead, I would have
 `mkNthCo` require a role and make a new `mkNthCoNoRole` that computes it.
 The naming of the functions should discourage the use of the second.

 The Note explaining why I originally bundled `coercionKind` and
 `coercionRole` points to a test case. Does that give a concrete testing
 ground? You might also look in the git history around that note to see if
 that gives you any pointers.

 Agreed about not cluttering `IfaceCoercion`.

 And, yes, I will take a look at `simplCast` tomorrow. (Though it's deeply
 scary when Simon says "My brain melted" and then asks me to take a look!)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11735#comment:29>
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