Redundant list allocations when working with `bindersOf`

Currently when we write code like:

addInScopeSet (bindersOf bndr) this results in optimized code of this general shape:

case bndr of
   NonRec b _ -> extendInScopeSetList [b] -- This allocates a redundant cons cell.
   Rec pairs -> extendInScopeSetList $ map fst pairs

But the ideal would be to generate code of this form instead:

case bndr of
   NonRec b _ -> extendInScope b
   Rec pairs -> extendInScopeSetList $ map fst pairs

One way to achieve the optimal version is to provide a custom folding function for binders. Something like this:

{-# INLINE foldBindersOfStrict #-}
foldBindersOfStrict :: (a -> b -> a) -> a -> Bind b -> a
foldBindersOfStrict f = \z bndr ->
  case bndr of
    (NonRec binder _) -> f z binder
    (Rec pairs) -> foldl' f z $ map fst pairs

Which avoids the intermediate list cell. I did write up a patch that provides folds as above and uses them in GHC and it indeed reduces allocations. By -0.1% on average and -0.5% in some specific cases.

The other option would be to force extendInScopeSetList to always inline. I don't think this is desireable.