Implement unboxed sum primitive type

Summary:
This patch implements primitive unboxed sum types, as described in
https://ghc.haskell.org/trac/ghc/wiki/UnpackedSumTypes.

Main changes are:

- Add new syntax for unboxed sums types, terms and patterns. Hidden
  behind `-XUnboxedSums`.

- Add unlifted unboxed sum type constructors and data constructors,
  extend type and pattern checkers and desugarer.

- Add new RuntimeRep for unboxed sums.

- Extend unarise pass to translate unboxed sums to unboxed tuples right
  before code generation.

- Add `StgRubbishArg` to `StgArg`, and a new type `CmmArg` for better
  code generation when sum values are involved.

- Add user manual section for unboxed sums.

Some other changes:

- Generalize `UbxTupleRep` to `MultiRep` and `UbxTupAlt` to
  `MultiValAlt` to be able to use those with both sums and tuples.

- Don't use `tyConPrimRep` in `isVoidTy`: `tyConPrimRep` is really
  wrong, given an `Any` `TyCon`, there's no way to tell what its kind
  is, but `kindPrimRep` and in turn `tyConPrimRep` returns `PtrRep`.

- Fix some bugs on the way: #12375.

Not included in this patch:

- Update Haddock for new the new unboxed sum syntax.

- `TemplateHaskell` support is left as future work.

For reviewers:

- Front-end code is mostly trivial and adapted from unboxed tuple code
  for type checking, pattern checking, renaming, desugaring etc.

- Main translation routines are in `RepType` and `UnariseStg`.
  Documentation in `UnariseStg` should be enough for understanding
  what's going on.

Credits:

- Johan Tibell wrote the initial front-end and interface file
  extensions.

- Simon Peyton Jones reviewed this patch many times, wrote some code,
  and helped with debugging.

Reviewers: bgamari, alanz, goldfire, RyanGlScott, simonpj, austin,
           simonmar, hvr, erikd

Reviewed By: simonpj

Subscribers: Iceland_jack, ggreif, ezyang, RyanGlScott, goldfire,
             thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D2259

1 files changed, 17 insertions, 0 deletions

diff --git a/testsuite/tests/unboxedsums/T12375.hs b/testsuite/tests/unboxedsums/T12375.hs
new file mode 100644
index 0000000000..62b6094e64
--- /dev/null
+++ b/testsuite/tests/unboxedsums/T12375.hs
@@ -0,0 +1,17 @@
+{-# LANGUAGE UnboxedTuples #-}
+
+module Main where
+
+type Null = (# #)
+
+{-# NOINLINE showNull #-}
+showNull :: Null -> String
+showNull (# #) = "(# #)"
+
+{-# NOINLINE showNullPair #-}
+showNullPair :: (# Null, Null #) -> String
+showNullPair (# n1, n2 #) = "(# " ++ showNull n1 ++ ", " ++ showNull n2 ++ " #)"
+
+main :: IO ()
+main = do
+    putStrLn (showNullPair (# (# #), (# #) #))