[GHC] #13160: Simplify CoreFV.FVAnn

[GHC]

#13160: Simplify CoreFV.FVAnn
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           Reporter:  simonpj        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           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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 When introducing type-in-type, Richard elaborated the free varaible finder
 so that it finds three things at once (in `CoreFV`):
 {{{
 data FVAnn = FVAnn { fva_fvs    :: DVarSet   -- free in expression
                    , fva_ty_fvs :: DVarSet   -- free only in expression's
 type
                    , fva_ty     :: Type      -- expression's type
                    }
 }}}
 I think `fva_ty` is needed only to support `fva_ty_fvs`.  And `fva_ty_fvs`
 seems to be used only to support the calls to `freeVarsOfType` in
 `FloatIn`.   And those calls in turn are only to suppor `used_in_ty` in
 `FloatIn.sepBindsByDropPoint`.

 In conversation yesterday, Richard and I agreed that all this is
 unnecessary.  Coercion bindings simply do not float inwards, so we do not
 need to take these precautions.  (We should add a Note to explain why they
 don't float, and what problem might arise if they did.)

 To confirm this I set `used_in_ty` to `False` and compiled from scratch;
 everything is fine.

 So I propose that we

 * get rid of the `ty_fvs` argument to `sepBindsByDropPoint`
 * simplfy `FVAnn` to just gather free variables (ie one field only)

 Result: it's all simpler.

 Richard will do this when he gets a moment.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13160>
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