Optimise nullary type constructor usage

During the compilation of programs GHC very frequently deals with
the `Type` type, which is a synonym of `TYPE 'LiftedRep`. This patch
teaches GHC to avoid expanding the `Type` synonym (and other nullary
type synonyms) during type comparisons, saving a good amount of work.
This optimisation is described in `Note [Comparing nullary type
synonyms]`.

To maximize the impact of this optimisation, we introduce a few
special-cases to reduce `TYPE 'LiftedRep` to `Type`. See
`Note [Prefer Type over TYPE 'LiftedPtrRep]`.

Closes #17958.

Metric Decrease:
   T18698b
   T1969
   T12227
   T12545
   T12707
   T14683
   T3064
   T5631
   T5642
   T9020
   T9630
   T9872a
   T13035
   haddock.Cabal
   haddock.base

11 files changed, 20 insertions, 24 deletions

diff --git a/testsuite/tests/deSugar/should_compile/T2431.stderr b/testsuite/tests/deSugar/should_compile/T2431.stderr
index 86d74c2d35..6e7df6c5de 100644
--- a/testsuite/tests/deSugar/should_compile/T2431.stderr
+++ b/testsuite/tests/deSugar/should_compile/T2431.stderr
@@ -1,9 +1,9 @@
 
 ==================== Tidy Core ====================
 Result size of Tidy Core
-  = {terms: 63, types: 43, coercions: 1, joins: 0/0}
+  = {terms: 63, types: 39, coercions: 1, joins: 0/0}
 
--- RHS size: {terms: 2, types: 4, coercions: 1, joins: 0/0}
+-- RHS size: {terms: 2, types: 3, coercions: 1, joins: 0/0}
 T2431.$WRefl [InlPrag=INLINE[final] CONLIKE] :: forall a. a :~: a
 [GblId[DataConWrapper],
  Caf=NoCafRefs,
@@ -15,7 +15,7 @@ T2431.$WRefl [InlPrag=INLINE[final] CONLIKE] :: forall a. a :~: a
 T2431.$WRefl
   = \ (@a) -> T2431.Refl @a @a @~(<a>_N :: a GHC.Prim.~# a)
 
--- RHS size: {terms: 4, types: 8, coercions: 0, joins: 0/0}
+-- RHS size: {terms: 4, types: 7, coercions: 0, joins: 0/0}
 absurd :: forall a. (Int :~: Bool) -> a
 [GblId, Arity=1, Str=<U>b, Cpr=b, Unf=OtherCon []]
 absurd = \ (@a) (x :: Int :~: Bool) -> case x of { }
diff --git a/testsuite/tests/deriving/should_compile/T14578.stderr b/testsuite/tests/deriving/should_compile/T14578.stderr
index 0018ebe569..d93f12c34c 100644
--- a/testsuite/tests/deriving/should_compile/T14578.stderr
+++ b/testsuite/tests/deriving/should_compile/T14578.stderr
@@ -16,13 +16,12 @@ Derived class instances:
       = GHC.Prim.coerce
           @(T14578.App (Data.Functor.Compose.Compose f g) a
             -> T14578.App (Data.Functor.Compose.Compose f g) a
-            -> T14578.App (Data.Functor.Compose.Compose f g) a)
+               -> T14578.App (Data.Functor.Compose.Compose f g) a)
           @(T14578.Wat f g a -> T14578.Wat f g a -> T14578.Wat f g a)
           ((GHC.Base.<>) @(T14578.App (Data.Functor.Compose.Compose f g) a))
     GHC.Base.sconcat
       = GHC.Prim.coerce
-          @(GHC.Base.NonEmpty
-              (T14578.App (Data.Functor.Compose.Compose f g) a)
+          @(GHC.Base.NonEmpty (T14578.App (Data.Functor.Compose.Compose f g) a)
             -> T14578.App (Data.Functor.Compose.Compose f g) a)
           @(GHC.Base.NonEmpty (T14578.Wat f g a) -> T14578.Wat f g a)
           (GHC.Base.sconcat
@@ -31,7 +30,7 @@ Derived class instances:
       = GHC.Prim.coerce
           @(b
             -> T14578.App (Data.Functor.Compose.Compose f g) a
-            -> T14578.App (Data.Functor.Compose.Compose f g) a)
+               -> T14578.App (Data.Functor.Compose.Compose f g) a)
           @(b -> T14578.Wat f g a -> T14578.Wat f g a)
           (GHC.Base.stimes
              @(T14578.App (Data.Functor.Compose.Compose f g) a))
diff --git a/testsuite/tests/plugins/plugins09.stdout b/testsuite/tests/plugins/plugins09.stdout
index 0401941734..61f96283ff 100644
--- a/testsuite/tests/plugins/plugins09.stdout
+++ b/testsuite/tests/plugins/plugins09.stdout
@@ -3,6 +3,5 @@ interfacePlugin: Prelude
 interfacePlugin: GHC.Float
 interfacePlugin: GHC.Base
 typeCheckPlugin (rn)
-interfacePlugin: GHC.Types
 typeCheckPlugin (tc)
 interfacePlugin: GHC.Num.BigNat
diff --git a/testsuite/tests/plugins/plugins10.stdout b/testsuite/tests/plugins/plugins10.stdout
index ed31df86f1..37f424b076 100644
--- a/testsuite/tests/plugins/plugins10.stdout
+++ b/testsuite/tests/plugins/plugins10.stdout
@@ -6,7 +6,6 @@ interfacePlugin: GHC.Float
 interfacePlugin: GHC.Base
 interfacePlugin: Language.Haskell.TH.Syntax
 typeCheckPlugin (rn)
-interfacePlugin: GHC.Types
 typeCheckPlugin (tc)
 interfacePlugin: GHC.Num.BigNat
 parsePlugin(a)
diff --git a/testsuite/tests/plugins/plugins11.stdout b/testsuite/tests/plugins/plugins11.stdout
index b273bc7a10..6bab3559b1 100644
--- a/testsuite/tests/plugins/plugins11.stdout
+++ b/testsuite/tests/plugins/plugins11.stdout
@@ -3,6 +3,5 @@ interfacePlugin: Prelude
 interfacePlugin: GHC.Float
 interfacePlugin: GHC.Base
 typeCheckPlugin (rn)
-interfacePlugin: GHC.Types
 typeCheckPlugin (tc)
 interfacePlugin: GHC.Num.BigNat
diff --git a/testsuite/tests/plugins/static-plugins.stdout b/testsuite/tests/plugins/static-plugins.stdout
index 632af0076c..032992824f 100644
--- a/testsuite/tests/plugins/static-plugins.stdout
+++ b/testsuite/tests/plugins/static-plugins.stdout
@@ -5,11 +5,11 @@ interfacePlugin: GHC.Float
 interfacePlugin: GHC.Base
 interfacePlugin: System.IO
 typeCheckPlugin (rn)
-interfacePlugin: GHC.Prim
-interfacePlugin: GHC.Show
 interfacePlugin: GHC.Types
+interfacePlugin: GHC.Show
 interfacePlugin: GHC.TopHandler
 typeCheckPlugin (tc)
+interfacePlugin: GHC.Prim
 interfacePlugin: GHC.CString
 interfacePlugin: GHC.Num.BigNat
 ==pure.1
diff --git a/testsuite/tests/printer/T18052a.stderr b/testsuite/tests/printer/T18052a.stderr
index 582a14a32c..28c96670cd 100644
--- a/testsuite/tests/printer/T18052a.stderr
+++ b/testsuite/tests/printer/T18052a.stderr
@@ -10,9 +10,9 @@ Dependent packages: [base-4.16.0.0, ghc-bignum-1.0, ghc-prim-0.8.0]
 
 ==================== Tidy Core ====================
 Result size of Tidy Core
-  = {terms: 24, types: 61, coercions: 0, joins: 0/0}
+  = {terms: 24, types: 52, coercions: 0, joins: 0/0}
 
--- RHS size: {terms: 7, types: 8, coercions: 0, joins: 0/0}
+-- RHS size: {terms: 7, types: 6, coercions: 0, joins: 0/0}
 T18052a.$b:||: :: forall {a} {b}. a -> b -> (a, b)
 [GblId, Arity=2, Unf=OtherCon []]
 T18052a.$b:||: = \ (@a) (@b) (x :: a) (y :: b) -> (x, y)
@@ -22,7 +22,7 @@ T18052a.$b:||: = \ (@a) (@b) (x :: a) (y :: b) -> (x, y)
 [GblId]
 (+++) = ++
 
--- RHS size: {terms: 13, types: 20, coercions: 0, joins: 0/0}
+-- RHS size: {terms: 13, types: 18, coercions: 0, joins: 0/0}
 T18052a.$m:||:
   :: forall {rep :: GHC.Types.RuntimeRep} {r :: TYPE rep} {a} {b}.
      (a, b) -> (a -> b -> r) -> ((# #) -> r) -> r
diff --git a/testsuite/tests/simplCore/should_compile/T13143.stderr b/testsuite/tests/simplCore/should_compile/T13143.stderr
index 44ab565425..c2bc42a872 100644
--- a/testsuite/tests/simplCore/should_compile/T13143.stderr
+++ b/testsuite/tests/simplCore/should_compile/T13143.stderr
@@ -1,17 +1,17 @@
 
 ==================== Tidy Core ====================
 Result size of Tidy Core
-  = {terms: 71, types: 44, coercions: 0, joins: 0/0}
+  = {terms: 71, types: 40, coercions: 0, joins: 0/0}
 
 Rec {
--- RHS size: {terms: 4, types: 4, coercions: 0, joins: 0/0}
+-- RHS size: {terms: 4, types: 3, coercions: 0, joins: 0/0}
 T13143.$wf [InlPrag=NOINLINE, Occ=LoopBreaker]
   :: forall {a}. (# #) -> a
 [GblId, Arity=1, Str=<B>b, Cpr=b, Unf=OtherCon []]
 T13143.$wf = \ (@a) _ [Occ=Dead] -> T13143.$wf @a GHC.Prim.(##)
 end Rec }
 
--- RHS size: {terms: 4, types: 4, coercions: 0, joins: 0/0}
+-- RHS size: {terms: 4, types: 3, coercions: 0, joins: 0/0}
 f [InlPrag=[final]] :: forall a. Int -> a
 [GblId,
  Arity=1,
diff --git a/testsuite/tests/simplCore/should_compile/T18013.stderr b/testsuite/tests/simplCore/should_compile/T18013.stderr
index 51e30a9f75..20cb606cb4 100644
--- a/testsuite/tests/simplCore/should_compile/T18013.stderr
+++ b/testsuite/tests/simplCore/should_compile/T18013.stderr
@@ -129,9 +129,9 @@ Rule fired: Class op fmap (BUILTIN)
 
 ==================== Tidy Core ====================
 Result size of Tidy Core
-  = {terms: 52, types: 106, coercions: 17, joins: 0/1}
+  = {terms: 52, types: 101, coercions: 17, joins: 0/1}
 
--- RHS size: {terms: 37, types: 87, coercions: 17, joins: 0/1}
+-- RHS size: {terms: 37, types: 84, coercions: 17, joins: 0/1}
 mapMaybeRule
   :: forall a b. Rule IO a b -> Rule IO (Maybe a) (Maybe b)
 [GblId,
diff --git a/testsuite/tests/simplCore/should_compile/T7360.stderr b/testsuite/tests/simplCore/should_compile/T7360.stderr
index 73bafb04f6..21fe15d4f5 100644
--- a/testsuite/tests/simplCore/should_compile/T7360.stderr
+++ b/testsuite/tests/simplCore/should_compile/T7360.stderr
@@ -1,7 +1,7 @@
 
 ==================== Tidy Core ====================
 Result size of Tidy Core
-  = {terms: 106, types: 47, coercions: 0, joins: 0/0}
+  = {terms: 106, types: 45, coercions: 0, joins: 0/0}
 
 -- RHS size: {terms: 6, types: 3, coercions: 0, joins: 0/0}
 T7360.$WFoo3 [InlPrag=INLINE[final] CONLIKE] :: Int %1 -> Foo
@@ -31,7 +31,7 @@ T7360.fun4 :: ()
          WorkFree=False, Expandable=False, Guidance=IF_ARGS [] 20 0}]
 T7360.fun4 = fun1 T7360.Foo1
 
--- RHS size: {terms: 11, types: 8, coercions: 0, joins: 0/0}
+-- RHS size: {terms: 11, types: 7, coercions: 0, joins: 0/0}
 fun2 :: forall {a}. [a] -> ((), Int)
 [GblId,
  Arity=1,
diff --git a/testsuite/tests/typecheck/should_compile/T13032.stderr b/testsuite/tests/typecheck/should_compile/T13032.stderr
index 596d09a927..3855f728c5 100644
--- a/testsuite/tests/typecheck/should_compile/T13032.stderr
+++ b/testsuite/tests/typecheck/should_compile/T13032.stderr
@@ -1,9 +1,9 @@
 
 ==================== Desugar (after optimization) ====================
 Result size of Desugar (after optimization)
-  = {terms: 13, types: 24, coercions: 0, joins: 0/0}
+  = {terms: 13, types: 18, coercions: 0, joins: 0/0}
 
--- RHS size: {terms: 6, types: 11, coercions: 0, joins: 0/0}
+-- RHS size: {terms: 6, types: 8, coercions: 0, joins: 0/0}
 f :: forall a b. (a ~ b) => a -> b -> Bool
 [LclIdX,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,