Revert "Make scanr a good producer and consumer"

This reverts commit 4e1dfc37.

Due to #16943.

libraries/base/GHC/List.hs

@@ -385,29 +385,11 @@ foldr1 f = go
 -- Note that
 --
 -- > head (scanr f z xs) == foldr f z xs.
-{-# NOINLINE [1] scanr #-}
 scanr                   :: (a -> b -> b) -> b -> [a] -> [b]
 scanr _ q0 []           =  [q0]
 scanr f q0 (x:xs)       =  f x q : qs
                            where qs@(q:_) = scanr f q0 xs
 
-{-# INLINE [0] strictUncurryScanr #-}
-strictUncurryScanr :: (a -> b -> c) -> (a, b) -> c
-strictUncurryScanr f pair = case pair of
-                              (x, y) -> f x y
-
-{-# INLINE [0] scanrFB #-} -- See Note [Inline FB functions]
-scanrFB :: (a -> b -> b) -> (b -> c -> c) -> a -> (b, c) -> (b, c)
-scanrFB f c = \x (r, est) -> (f x r, r `c` est)
-
-{-# RULES
-"scanr" [~1] forall f q0 ls . scanr f q0 ls =
-  build (\c n -> strictUncurryScanr c (foldr (scanrFB f c) (q0,n) ls))
-"scanrList" [1] forall f q0 ls .
-               strictUncurryScanr (:) (foldr (scanrFB f (:)) (q0,[]) ls) =
-                 scanr f q0 ls
- #-}
-
 -- | \(\mathcal{O}(n)\). 'scanr1' is a variant of 'scanr' that has no starting
 -- value argument.
 scanr1                  :: (a -> a -> a) -> [a] -> [a]

libraries/base/changelog.md

@@ -57,6 +57,8 @@
     `Word`, and `WordN` now throw an overflow exception for negative shift
     values (instead of being undefined behaviour).
 
+  * `scanr` no longer participates in list fusion (due #16943)
+
 ## 4.12.0.0 *21 September 2018*
   * Bundled with GHC 8.6.1