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

1 files changed, 65 insertions, 20 deletions

diff --git a/compiler/GHC/Tc/Utils/Zonk.hs b/compiler/GHC/Tc/Utils/Zonk.hs
index 4fb5286c70..aad5299bbf 100644
--- a/compiler/GHC/Tc/Utils/Zonk.hs
+++ b/compiler/GHC/Tc/Utils/Zonk.hs
@@ -20,7 +20,7 @@ module GHC.Tc.Utils.Zonk (
         -- * Other HsSyn functions
         mkHsDictLet, mkHsApp,
         mkHsAppTy, mkHsCaseAlt,
-        shortCutLit, hsOverLitName,
+        tcShortCutLit, shortCutLit, hsOverLitName,
         conLikeResTy,
 
         -- * re-exported from TcMonad
@@ -90,6 +90,7 @@ import GHC.Types.Basic
 import GHC.Types.SrcLoc
 import GHC.Types.Unique.FM
 import GHC.Types.TyThing
+import GHC.Driver.Session( getDynFlags, targetPlatform )
 
 import GHC.Data.Maybe
 import GHC.Data.Bag
@@ -151,28 +152,75 @@ hsLitType (HsRat _ _ ty)     = ty
 hsLitType (HsFloatPrim _ _)  = floatPrimTy
 hsLitType (HsDoublePrim _ _) = doublePrimTy
 
+{- *********************************************************************
+*                                                                      *
+         Short-cuts for overloaded numeric literals
+*                                                                      *
+********************************************************************* -}
+
 -- Overloaded literals. Here mainly because it uses isIntTy etc
 
+{- Note [Short cut for overloaded literals]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A literal like "3" means (fromInteger @ty (dNum :: Num ty) (3::Integer)).
+But if we have a list like
+  [4,2,3,2,4,4,2]::[Int]
+we use a lot of compile time and space generating and solving all those Num
+constraints, and generating calls to fromInteger etc.  Better just to cut to
+the chase, and cough up an Int literal. Large collections of literals like this
+sometimes appear in source files, so it's quite a worthwhile fix.
+
+So we try to take advantage of whatever nearby type information we have,
+to short-cut the process for built-in types.  We can do this in two places;
+
+* In the typechecker, when we are about to typecheck the literal.
+* If that fails, in the desugarer, once we know the final type.
+-}
+
+tcShortCutLit :: HsOverLit GhcRn -> ExpRhoType -> TcM (Maybe (HsOverLit GhcTc))
+tcShortCutLit lit@(OverLit { ol_val = val, ol_ext = rebindable }) exp_res_ty
+  | not rebindable
+  , Just res_ty <- checkingExpType_maybe exp_res_ty
+  = do { dflags <- getDynFlags
+       ; let platform = targetPlatform dflags
+       ; case shortCutLit platform val res_ty of
+            Just expr -> return $ Just $
+                         lit { ol_witness = expr
+                             , ol_ext = OverLitTc False res_ty }
+            Nothing   -> return Nothing }
+  | otherwise
+  = return Nothing
+
 shortCutLit :: Platform -> OverLitVal -> TcType -> Maybe (HsExpr GhcTc)
-shortCutLit platform (HsIntegral int@(IL src neg i)) ty
-  | isIntTy ty  && platformInIntRange  platform i = Just (HsLit noExtField (HsInt noExtField int))
-  | isWordTy ty && platformInWordRange platform i = Just (mkLit wordDataCon (HsWordPrim src i))
-  | isIntegerTy ty = Just (HsLit noExtField (HsInteger src i ty))
-  | otherwise = shortCutLit platform (HsFractional (integralFractionalLit neg i)) ty
+shortCutLit platform val res_ty
+  = case val of
+      HsIntegral int_lit    -> go_integral int_lit
+      HsFractional frac_lit -> go_fractional frac_lit
+      HsIsString s src      -> go_string   s src
+  where
+    go_integral int@(IL src neg i)
+      | isIntTy res_ty  && platformInIntRange  platform i
+      = Just (HsLit noExtField (HsInt noExtField int))
+      | isWordTy res_ty && platformInWordRange platform i
+      = Just (mkLit wordDataCon (HsWordPrim src i))
+      | isIntegerTy res_ty
+      = Just (HsLit noExtField (HsInteger src i res_ty))
+      | otherwise
+      = go_fractional (integralFractionalLit neg i)
         -- The 'otherwise' case is important
         -- Consider (3 :: Float).  Syntactically it looks like an IntLit,
         -- so we'll call shortCutIntLit, but of course it's a float
         -- This can make a big difference for programs with a lot of
         -- literals, compiled without -O
 
-shortCutLit _ (HsFractional f) ty
-  | isFloatTy ty  = Just (mkLit floatDataCon  (HsFloatPrim noExtField f))
-  | isDoubleTy ty = Just (mkLit doubleDataCon (HsDoublePrim noExtField f))
-  | otherwise     = Nothing
+    go_fractional f
+      | isFloatTy res_ty  = Just (mkLit floatDataCon  (HsFloatPrim noExtField f))
+      | isDoubleTy res_ty = Just (mkLit doubleDataCon (HsDoublePrim noExtField f))
+      | otherwise         = Nothing
 
-shortCutLit _ (HsIsString src s) ty
-  | isStringTy ty = Just (HsLit noExtField (HsString src s))
-  | otherwise     = Nothing
+    go_string src s
+      | isStringTy res_ty = Just (HsLit noExtField (HsString src s))
+      | otherwise         = Nothing
 
 mkLit :: DataCon -> HsLit GhcTc -> HsExpr GhcTc
 mkLit con lit = HsApp noExtField (nlHsDataCon con) (nlHsLit lit)
@@ -881,13 +929,10 @@ zonkExpr env (HsDo ty do_or_lc (L l stmts))
        new_ty <- zonkTcTypeToTypeX env ty
        return (HsDo new_ty do_or_lc (L l new_stmts))
 
-zonkExpr env (ExplicitList ty wit exprs)
-  = do (env1, new_wit) <- zonkWit env wit
-       new_ty <- zonkTcTypeToTypeX env1 ty
-       new_exprs <- zonkLExprs env1 exprs
-       return (ExplicitList new_ty new_wit new_exprs)
-   where zonkWit env Nothing    = return (env, Nothing)
-         zonkWit env (Just fln) = second Just <$> zonkSyntaxExpr env fln
+zonkExpr env (ExplicitList ty exprs)
+  = do new_ty <- zonkTcTypeToTypeX env ty
+       new_exprs <- zonkLExprs env exprs
+       return (ExplicitList new_ty new_exprs)
 
 zonkExpr env expr@(RecordCon { rcon_ext = con_expr, rcon_flds = rbinds })
   = do  { new_con_expr <- zonkExpr env con_expr