Derived enum instances should support list fusion.

Motivation

List fusion improves performance so is desireable.

However it's currently not supported by Enum instances when derived by GHC.

Consider this case:

data T = A | B | C | D | E | F | G | H | I | J | K deriving (Enum)

bar = [A .. E]

If a use site maps over bar we would hope this would fuse with the enumeration.

In practice however we end up with Core like this:

Rec {
-- RHS size: {terms: 34, types: 21, coercions: 0, joins: 0/0}
MyEnum.$wgo [InlPrag=NOUSERINLINE[2], Occ=LoopBreaker]
  :: GHC.Prim.Int# -> (# T, [T] #)
[GblId, Arity=1, Str=<L,U>, Unf=OtherCon []]
MyEnum.$wgo
  = \ (w_s2SI :: GHC.Prim.Int#) ->
      (# case GHC.Prim.>=# w_s2SI 0# of {
           __DEFAULT -> MyEnum.$wlvl w_s2SI;
           1# ->
             case GHC.Prim.<=# w_s2SI 10# of {
               __DEFAULT -> MyEnum.$wlvl w_s2SI;
               1# -> GHC.Prim.tagToEnum# @ T w_s2SI
             }
         },
         case w_s2SI of wild_X13 {
           __DEFAULT ->
             case MyEnum.$wgo (GHC.Prim.+# wild_X13 1#) of
             { (# ww1_s2SN, ww2_s2SO #) ->
             GHC.Types.: @ T ww1_s2SN ww2_s2SO
             };
           4# -> GHC.Types.[] @ T
         } #)
end Rec }

-- RHS size: {terms: 7, types: 10, coercions: 0, joins: 0/0}
bar :: [T]
[GblId,
 Unf=Unf{Src=<vanilla>, TopLvl=True, Value=False, ConLike=False,
         WorkFree=False, Expandable=False, Guidance=IF_ARGS [] 40 30}]
bar
  = case MyEnum.$wgo 0# of { (# ww1_s2SN, ww2_s2SO #) ->
    GHC.Types.: @ T ww1_s2SN ww2_s2SO
    }

Which won't support list fusion. The recursive go function doesn't get a unfolding at all.

Even if it did it would need to be implemented differently to support fusion.

Proposal

The implementation should be changed to make bar a good producer for list fusion.