Provide warning for when RecordWildCards LHS {..} doesn't bind anything.

Given simple code like

{-# LANGUAGE RecordWildCards #-}
module M (foo) where

data D = D { d1 :: Int, d2 :: Int }

foo :: D -> Int -> Int
foo D{..} x = x + 5

There is no way that I know of to get GHC to warn us that the D{..} in foo is not binding anything at all.

This is very common after refactoring (perhaps foo looked like foo D{..} x = x + d1 + 5 before) and would be great to be able to get a warning about.

Of course it's not dead code as it still forces D to WHNF but if we're using RecordWildCards syntax then it's probably not what we were going for.

Trac metadata

Trac field	Value
Version	8.6.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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