Improve handling of overloaded labels, literals, lists etcwip/T19154

When implementing Quick Look I'd failed to remember that overloaded
labels, like #foo, should be treated as a "head", so that they can be
instantiated with Visible Type Application. This caused #19154.

A very similar ticket covers overloaded literals: #19167.

This patch fixes both problems, but (annoyingly, albeit temporarily)
in two different ways.

Overloaded labels

I dealt with overloaded labels by buying fully into the
Rebindable Syntax approach described in GHC.Hs.Expr
Note [Rebindable syntax and HsExpansion].

There is a good overview in GHC.Rename.Expr
Note [Handling overloaded and rebindable constructs].
That module contains much of the payload for this patch.

Specifically:

* Overloaded labels are expanded in the renamer, fixing #19154.
  See Note [Overloaded labels] in GHC.Rename.Expr.

* Left and right sections used to have special code paths in the
  typechecker and desugarer.  Now we just expand them in the
  renamer. This is harder than it sounds.  See GHC.Rename.Expr
  Note [Left and right sections].

* Infix operator applications are expanded in the typechecker,
  specifically in GHC.Tc.Gen.App.splitHsApps.  See
  Note [Desugar OpApp in the typechecker] in that module

* ExplicitLists are expanded in the renamer, when (and only when)
  OverloadedLists is on.

* HsIf is expanded in the renamer when (and only when) RebindableSyntax
  is on.  Reason: the coverage checker treats HsIf specially.  Maybe
  we could instead expand it unconditionally, and fix up the coverage
  checker, but I did not attempt that.

Overloaded literals

Overloaded literals, like numbers (3, 4.2) and strings with
OverloadedStrings, were not working correctly with explicit type
applications (see #19167).  Ideally I'd also expand them in the
renamer, like the stuff above, but I drew back on that because they
can occur in HsPat as well, and I did not want to to do the HsExpanded
thing for patterns.

But they *can* now be the "head" of an application in the typechecker,
and hence something like ("foo" @T) works now.  See
GHC.Tc.Gen.Head.tcInferOverLit.  It's also done a bit more elegantly,
rather than by constructing a new HsExpr and re-invoking the
typechecker. There is some refactoring around tcShortCutLit.

Ultimately there is more to do here, following the Rebindable Syntax
story.

There are a lot of knock-on effects:

* HsOverLabel and ExplicitList no longer need funny (Maybe SyntaxExpr)
  fields to support rebindable syntax -- good!

* HsOverLabel, OpApp, SectionL, SectionR all become impossible in the
  output of the typecheker, GhcTc; so we set their extension fields to
  Void. See GHC.Hs.Expr Note [Constructor cannot occur]

* Template Haskell quotes for HsExpanded is a bit tricky.  See
  Note [Quotation and rebindable syntax] in GHC.HsToCore.Quote.

* In GHC.HsToCore.Match.viewLExprEq, which groups equal HsExprs for the
  purpose of pattern-match overlap checking, I found that dictionary
  evidence for the same type could have two different names.  Easily
  fixed by comparing types not names.

* I did quite a bit of annoying fiddling around in GHC.Tc.Gen.Head and
  GHC.Tc.Gen.App to get error message locations and contexts right,
  esp in splitHsApps, and the HsExprArg type.  Tiresome and not very
  illuminating.  But at least the tricky, higher order, Rebuilder
  function is gone.

* Some refactoring in GHC.Tc.Utils.Monad around contexts and locations
  for rebindable syntax.

* Incidentally fixes #19346, because we now print renamed, rather than
  typechecked, syntax in error mesages about applications.

The commit removes the vestigial module GHC.Builtin.RebindableNames,
and thus triggers a 2.4% metric decrease for test MultiLayerModules
(#19293).

Metric Decrease:
    MultiLayerModules
    T12545

3 files changed, 32 insertions, 26 deletions

diff --git a/compiler/GHC/Builtin/Names.hs b/compiler/GHC/Builtin/Names.hs
index 0764e5c536..fc0589730a 100644
--- a/compiler/GHC/Builtin/Names.hs
+++ b/compiler/GHC/Builtin/Names.hs
@@ -443,7 +443,7 @@ basicKnownKeyNames
         knownNatClassName, knownSymbolClassName, knownCharClassName,
 
         -- Overloaded labels
-        isLabelClassName,
+        fromLabelClassOpName,
 
         -- Implicit Parameters
         ipClassName,
@@ -1626,9 +1626,9 @@ knownCharClassName :: Name
 knownCharClassName  = clsQual gHC_TYPELITS (fsLit "KnownChar") knownCharClassNameKey
 
 -- Overloaded labels
-isLabelClassName :: Name
-isLabelClassName
- = clsQual gHC_OVER_LABELS (fsLit "IsLabel") isLabelClassNameKey
+fromLabelClassOpName :: Name
+fromLabelClassOpName
+ = varQual gHC_OVER_LABELS (fsLit "fromLabel") fromLabelClassOpKey
 
 -- Implicit Parameters
 ipClassName :: Name
@@ -1786,9 +1786,6 @@ knownCharClassNameKey = mkPreludeClassUnique 44
 ghciIoClassKey :: Unique
 ghciIoClassKey = mkPreludeClassUnique 45
 
-isLabelClassNameKey :: Unique
-isLabelClassNameKey = mkPreludeClassUnique 46
-
 semigroupClassKey, monoidClassKey :: Unique
 semigroupClassKey = mkPreludeClassUnique 47
 monoidClassKey    = mkPreludeClassUnique 48
@@ -2332,6 +2329,9 @@ sndIdKey                      = mkPreludeMiscIdUnique 42
 otherwiseIdKey                = mkPreludeMiscIdUnique 43
 assertIdKey                   = mkPreludeMiscIdUnique 44
 
+leftSectionKey, rightSectionKey :: Unique
+leftSectionKey                = mkPreludeMiscIdUnique 45
+rightSectionKey               = mkPreludeMiscIdUnique 46
 
 rootMainKey, runMainKey :: Unique
 rootMainKey                   = mkPreludeMiscIdUnique 101
@@ -2413,6 +2413,10 @@ mfixIdKey       = mkPreludeMiscIdUnique 175
 failMClassOpKey :: Unique
 failMClassOpKey = mkPreludeMiscIdUnique 176
 
+-- fromLabel
+fromLabelClassOpKey :: Unique
+fromLabelClassOpKey = mkPreludeMiscIdUnique 177
+
 -- Arrow notation
 arrAIdKey, composeAIdKey, firstAIdKey, appAIdKey, choiceAIdKey,
     loopAIdKey :: Unique
diff --git a/compiler/GHC/Builtin/RebindableNames.hs b/compiler/GHC/Builtin/RebindableNames.hs
deleted file mode 100644
index 0a07224b15..0000000000
--- a/compiler/GHC/Builtin/RebindableNames.hs
+++ /dev/null
@@ -1,6 +0,0 @@
-module GHC.Builtin.RebindableNames where
-
-import GHC.Data.FastString
-
-reboundIfSymbol :: FastString
-reboundIfSymbol = fsLit "ifThenElse"
diff --git a/compiler/GHC/Builtin/Types/Prim.hs b/compiler/GHC/Builtin/Types/Prim.hs
index 89093a2350..27df5236a3 100644
--- a/compiler/GHC/Builtin/Types/Prim.hs
+++ b/compiler/GHC/Builtin/Types/Prim.hs
@@ -23,10 +23,13 @@ module GHC.Builtin.Types.Prim(
         alphaTys, alphaTy, betaTy, gammaTy, deltaTy,
         alphaTyVarsUnliftedRep, alphaTyVarUnliftedRep,
         alphaTysUnliftedRep, alphaTyUnliftedRep,
-        runtimeRep1TyVar, runtimeRep2TyVar, runtimeRep1Ty, runtimeRep2Ty,
-        openAlphaTy, openBetaTy, openAlphaTyVar, openBetaTyVar,
+        runtimeRep1TyVar, runtimeRep2TyVar, runtimeRep3TyVar,
+        runtimeRep1Ty, runtimeRep2Ty, runtimeRep3Ty,
 
-        multiplicityTyVar,
+        openAlphaTyVar, openBetaTyVar, openGammaTyVar,
+        openAlphaTy, openBetaTy, openGammaTy,
+
+        multiplicityTyVar1, multiplicityTyVar2,
 
         -- Kind constructors...
         tYPETyCon, tYPETyConName,
@@ -375,26 +378,31 @@ alphaTysUnliftedRep = mkTyVarTys alphaTyVarsUnliftedRep
 alphaTyUnliftedRep :: Type
 (alphaTyUnliftedRep:_) = alphaTysUnliftedRep
 
-runtimeRep1TyVar, runtimeRep2TyVar :: TyVar
-(runtimeRep1TyVar : runtimeRep2TyVar : _)
+runtimeRep1TyVar, runtimeRep2TyVar, runtimeRep3TyVar :: TyVar
+(runtimeRep1TyVar : runtimeRep2TyVar : runtimeRep3TyVar : _)
   = drop 16 (mkTemplateTyVars (repeat runtimeRepTy))  -- selects 'q','r'
 
-runtimeRep1Ty, runtimeRep2Ty :: Type
+runtimeRep1Ty, runtimeRep2Ty, runtimeRep3Ty :: Type
 runtimeRep1Ty = mkTyVarTy runtimeRep1TyVar
 runtimeRep2Ty = mkTyVarTy runtimeRep2TyVar
+runtimeRep3Ty = mkTyVarTy runtimeRep3TyVar
 
-openAlphaTyVar, openBetaTyVar :: TyVar
+openAlphaTyVar, openBetaTyVar, openGammaTyVar :: TyVar
 -- alpha :: TYPE r1
 -- beta  :: TYPE r2
-[openAlphaTyVar,openBetaTyVar]
-  = mkTemplateTyVars [tYPE runtimeRep1Ty, tYPE runtimeRep2Ty]
+-- gamma :: TYPE r3
+[openAlphaTyVar,openBetaTyVar,openGammaTyVar]
+  = mkTemplateTyVars [tYPE runtimeRep1Ty, tYPE runtimeRep2Ty, tYPE runtimeRep3Ty]
 
-openAlphaTy, openBetaTy :: Type
+openAlphaTy, openBetaTy, openGammaTy :: Type
 openAlphaTy = mkTyVarTy openAlphaTyVar
 openBetaTy  = mkTyVarTy openBetaTyVar
+openGammaTy = mkTyVarTy openGammaTyVar
+
+multiplicityTyVar1, multiplicityTyVar2  :: TyVar
+(multiplicityTyVar1 : multiplicityTyVar2 : _)
+   = drop 13 (mkTemplateTyVars (repeat multiplicityTy))  -- selects 'n', 'm'
 
-multiplicityTyVar :: TyVar
-multiplicityTyVar = mkTemplateTyVars (repeat multiplicityTy) !! 13  -- selects 'n'
 
 {-
 ************************************************************************
@@ -432,7 +440,7 @@ funTyCon :: TyCon
 funTyCon = mkFunTyCon funTyConName tc_bndrs tc_rep_nm
   where
     -- See also unrestrictedFunTyCon
-    tc_bndrs = [ mkNamedTyConBinder Required multiplicityTyVar
+    tc_bndrs = [ mkNamedTyConBinder Required multiplicityTyVar1
                , mkNamedTyConBinder Inferred runtimeRep1TyVar
                , mkNamedTyConBinder Inferred runtimeRep2TyVar ]
                ++ mkTemplateAnonTyConBinders [ tYPE runtimeRep1Ty