In an attempt to ensure deforestation in Data.Map usage, toAscList was reimplemented as follows:

{-# INLINE toAscList #-}
-- | /O(n)/. Convert to an ascending list.
--
-- > toAscList (fromList [(5,"a"), (3,"b")]) == [(3,"b"), (5,"a")]
toAscList :: Map k a -> [(k,a)]
#ifdef __GLASGOW_HASKELL__
toAscList t = GHC.build (\ c n -> foldrWithKey (curry c) n t)
#else
toAscList t   = foldrWithKey (\k x xs -> (k,x):xs) [] t
#endif

As one might hope, ghc-core gives the following compilation result:

Data.Map.toAscList :: forall k_a8cF a_a8cG.
                      Data.Map.Map k_a8cF a_a8cG -> [(k_a8cF, a_a8cG)]
GblId

Data.Map.toAscList =
  __inline_me (\ (@ k_a8E0)
                 (@ a_a8E1)
                 (t_a8ot :: Data.Map.Map k_a8E0 a_a8E1) ->
                 build
                   @ (k_a8E0, a_a8E1)
                   (\ (@ b_a8E3)
                      (c_a8ou :: (k_a8E0, a_a8E1) -> b_a8E3 -> b_a8E3)
                      (n_a8ov :: b_a8E3) ->
                      Data.Map.foldrWithKey
                        @ k_a8E0
                        @ a_a8E1
                        @ b_a8E3
                        (Data.Tuple.curry @ k_a8E0 @ a_a8E1 @ (b_a8E3 -> b_a8E3) c_a8ou)
                        n_a8ov
                        t_a8ot))

But mapKeysWith is compiled -- in the very same file! -- to

Data.Map.mapKeysWith :: forall k2_a8dc a_a8dd k1_a8de.
                        (Ord k2_a8dc) =>
                        (a_a8dd -> a_a8dd -> a_a8dd)
                        -> (k1_a8de -> k2_a8dc)
                        -> Data.Map.Map k1_a8de a_a8dd
                        -> Data.Map.Map k2_a8dc a_a8dd
GblId

Data.Map.mapKeysWith =
  \ (@ k2_a8Qc)
    (@ a_a8Qd)
    (@ k1_a8Qe)
    ($dOrd_a8QE :: Ord k2_a8Qc)
    (c_a8nI :: a_a8Qd -> a_a8Qd -> a_a8Qd)
    (f_a8nJ :: k1_a8Qe -> k2_a8Qc)
    (eta_B1 :: Data.Map.Map k1_a8Qe a_a8Qd) ->
    Data.Map.fromListWithKey
      @ k2_a8Qc
      @ a_a8Qd
      $dOrd_a8QE
      (\ _ (x_a8ok :: a_a8Qd) (y_a8ol :: a_a8Qd) -> c_a8nI x_a8ok y_a8ol)
      (map
         @ (k1_a8Qe, a_a8Qd)
         @ (k2_a8Qc, a_a8Qd)
         (\ (ds_d9om :: (k1_a8Qe, a_a8Qd)) ->
            case ds_d9om of _ { (x_a8nL, y_a8nM) -> (f_a8nJ x_a8nL, y_a8nM) })
         (Data.Map.toAscList @ k1_a8Qe @ a_a8Qd eta_B1))