[GHC] #15957: Provide warning for when RecordWildCards LHS {..} doesn't bind anything.

[GHC]

#15957: Provide warning for when RecordWildCards LHS {..} doesn't bind anything.
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        Reporter:  Fuuzetsu          |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Fuuzetsu):

 To be clear, the behaviour we would like to see is a warning whenever the
 {..} adds no new used bindings into the environment.

 {{{#!haskell
 data P = P { x :: Int, y :: Int }

 f1 P{..} = 1 + 3 -- nothing bound is used
 f2 P{x, ..} = x + 3 -- y bound but not used
 f3 P{x, y, ..} = x + y -- no bindings left, i.e. no new useful bindings
 introduced

 g1 P{..} = x + 3 -- x from .. is used
 g2 P{x, ..} = x + y -- y from .. is used, even if it's in a weird style
 }}}

 f1, f2 and f3 should warn, g1 and g2 should not.

 I actually gave this a go myself but the way it was programmed was a bit
 too much for me for a quick hack. The way it is done now is that {{{..}}}
 is expanded into the bindings and those bindings are tagged with "don't
 warn if unusued" flag. Instead it should warn, but only if none of the
 {{{..}}}-bound names is used. Depending on how it's done, we should be
 careful to produce only a single warning &c.

 I imagine any solution is not too hard for anyone used to the codebase.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15957#comment:4>
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