Skip-less stream fusion: a missed opportunity

A simple stream chain

chain :: Int -> Int
chain = sum . filter even . enumFromTo 1

doesn't fuse under a Skip-less stream on GHC 8.2-rc3 -O2.

Benchmarked against a Skip stream (LLVM backend):

benchmarking Skip-less
time                 248.9 ms   (243.3 ms .. 257.3 ms)
                     0.998 R²   (0.995 R² .. 0.999 R²)
mean                 250.9 ms   (248.1 ms .. 254.7 ms)
std dev              5.985 ms   (4.831 ms .. 7.311 ms)

benchmarking Skip
time                 61.26 ms   (60.39 ms .. 62.44 ms)
                     0.998 R²   (0.997 R² .. 0.999 R²)
mean                 62.45 ms   (61.96 ms .. 62.91 ms)
std dev              1.387 ms   (1.190 ms .. 1.669 ms)

Relevant core (chain1 is Skip-less, chain2 has Skip):

-- RHS size: {terms: 51, types: 27, coercions: 0, joins: 1/2}
Main.$wchain1 [InlPrag=NOINLINE] :: Int# -> Int#
[GblId, Arity=1, Caf=NoCafRefs, Str=<S,U>]
Main.$wchain1
  = \ (ww_s9ep :: Int#) ->
      letrec {
        $wloop_s9ea [InlPrag=[0], Occ=LoopBreaker] :: Int# -> Step1 Int Int
        [LclId, Arity=1, Str=<S,U>, Unf=OtherCon []]
        $wloop_s9ea
          = \ (ww1_s9e8 :: Int#) ->
              case tagToEnum# @ Bool (># ww1_s9e8 ww_s9ep) of {
                False ->
                  case remInt# ww1_s9e8 2# of {
                    __DEFAULT -> $wloop_s9ea (+# ww1_s9e8 1#);
                    0# ->
                      Main.Yield1
                        @ Int @ Int (GHC.Types.I# (+# ww1_s9e8 1#)) (GHC.Types.I# ww1_s9e8)
                  };
                True -> Main.Done1 @ Int @ Int
              }; } in
      joinrec {
        $wloop1_s9el [InlPrag=[0], Occ=LoopBreaker] :: Int# -> Int# -> Int#
        [LclId[JoinId(2)], Arity=2, Str=<S,U><S,U>, Unf=OtherCon []]
        $wloop1_s9el (ww1_s9ef :: Int#) (ww2_s9ej :: Int#)
          = case $wloop_s9ea ww2_s9ej of {
              Done1 -> ww1_s9ef;
              Yield1 s'_a497 x_a498 ->
                case x_a498 of { GHC.Types.I# y_a66i ->
                case s'_a497 of { GHC.Types.I# ww4_X9hA ->
                jump $wloop1_s9el (+# ww1_s9ef y_a66i) ww4_X9hA
                }
                }
            }; } in
      jump $wloop1_s9el 0# 1#


-- RHS size: {terms: 33, types: 9, coercions: 0, joins: 1/1}
Main.$wchain2 [InlPrag=NOINLINE] :: Int# -> Int#
[GblId, Arity=1, Caf=NoCafRefs, Str=<S,U>]
Main.$wchain2
  = \ (ww_s9dZ :: Int#) ->
      joinrec {
        $wloop_s9dV [InlPrag=[0], Occ=LoopBreaker] :: Int# -> Int# -> Int#
        [LclId[JoinId(2)], Arity=2, Str=<S,U><S,U>, Unf=OtherCon []]
        $wloop_s9dV (ww1_s9dP :: Int#) (ww2_s9dT :: Int#)
          = case tagToEnum# @ Bool (># ww2_s9dT ww_s9dZ) of {
              False ->
                case remInt# ww2_s9dT 2# of {
                  __DEFAULT -> jump $wloop_s9dV ww1_s9dP (+# ww2_s9dT 1#);
                  0# -> jump $wloop_s9dV (+# ww1_s9dP ww2_s9dT) (+# ww2_s9dT 1#)
                };
              True -> ww1_s9dP
            }; } in
      jump $wloop_s9dV 0# 1#

The code was adapted from M. Snoyman's blog post "Iterators and Streams in Rust and Haskell".