Introduce `hsExprType :: HsExpr GhcTc -> Type` in the new modulewip/hsExprType

`GHC.Hs.Syn.Type`

The existing `hsPatType`, `hsLPatType` and `hsLitType` functions have also been
moved to this module

This is a less ambitious take on the same problem that !2182 and !3866
attempt to solve. Rather than have the `hsExprType` function attempt to
efficiently compute the `Type` of every subexpression in an `HsExpr`, this
simply computes the overall `Type` of a single `HsExpr`.

- Explicitly forbids the `SplicePat` `HsIPVar`, `HsBracket`, `HsRnBracketOut`
  and `HsTcBracketOut` constructors during the typechecking phase by using
  `Void` as the TTG extension field
- Also introduces `dataConCantHappen` as a domain specific alternative to `absurd`
  to handle cases where the TTG extension points forbid a constructor.
- Turns HIE file generation into a pure function that doesn't need access to the
  `DsM` monad to compute types, but uses `hsExprType` instead.
- Computes a few more types during HIE file generation
- Makes GHCi's `:set +c` command also use `hsExprType` instead of going through
  the desugarer to compute types.

Updates haddock submodule

Co-authored-by: Zubin Duggal <zubin.duggal@gmail.com>

1 files changed, 29 insertions, 8 deletions

diff --git a/compiler/GHC/Hs/Expr.hs b/compiler/GHC/Hs/Expr.hs
index 006c8a2e8e..72ac021e45 100644
--- a/compiler/GHC/Hs/Expr.hs
+++ b/compiler/GHC/Hs/Expr.hs
@@ -196,13 +196,25 @@ type instance PendingTcSplice' (GhcPass _) = PendingTcSplice
 {- Note [Constructor cannot occur]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Some data constructors can't occur in certain phases; e.g. the output
-of the type checker never has OverLabel. We signal this by setting
-the extension field to Void. For example:
+of the type checker never has OverLabel. We signal this by
+* setting the extension field to Void
+* using dataConCantHappen in the cases that can't happen
+
+For example:
+
    type instance XOverLabel GhcTc = Void
-   dsExpr (HsOverLabel x _) = absurd x
+
+   dsExpr :: HsExpr GhcTc -> blah
+   dsExpr (HsOverLabel x _) = dataConCantHappen x
+
+The function dataConCantHappen is defined thus:
+   dataConCantHappen :: Void -> a
+   dataConCantHappen x = case x of {}
+(i.e. identically to Data.Void.absurd, but more helpfully named).
+Remember Void is a type whose only element is bottom.
 
 It would be better to omit the pattern match altogether, but we
-could only do that if the extension field was strict (#18764)
+could only do that if the extension field was strict (#18764).
 -}
 
 -- API Annotations types
@@ -246,7 +258,9 @@ type instance XUnboundVar    GhcTc = HoleExprRef
   -- Much, much easier just to define HoleExprRef with a Data instance and
   -- store the whole structure.
 
-type instance XIPVar         (GhcPass _) = EpAnnCO
+type instance XIPVar         GhcPs = EpAnnCO
+type instance XIPVar         GhcRn = EpAnnCO
+type instance XIPVar         GhcTc = Void -- See Note [Constructor cannot occur]
 type instance XOverLitE      (GhcPass _) = EpAnnCO
 type instance XLitE          (GhcPass _) = EpAnnCO
 
@@ -348,10 +362,17 @@ type instance XArithSeq      GhcPs = EpAnn [AddEpAnn]
 type instance XArithSeq      GhcRn = NoExtField
 type instance XArithSeq      GhcTc = PostTcExpr
 
-type instance XBracket       (GhcPass _) = EpAnn [AddEpAnn]
+type instance XBracket       GhcPs = EpAnn [AddEpAnn]
+type instance XBracket       GhcRn = EpAnn [AddEpAnn]
+type instance XBracket       GhcTc = Void -- See Note [Constructor cannot occur]
+
+type instance XRnBracketOut  GhcPs = Void -- See Note [Constructor cannot occur]
+type instance XRnBracketOut  GhcRn = NoExtField
+type instance XRnBracketOut  GhcTc = Void -- See Note [Constructor cannot occur]
 
-type instance XRnBracketOut  (GhcPass _) = NoExtField
-type instance XTcBracketOut  (GhcPass _) = NoExtField
+type instance XTcBracketOut  GhcPs = Void -- See Note [Constructor cannot occur]
+type instance XTcBracketOut  GhcRn = Void -- See Note [Constructor cannot occur]
+type instance XTcBracketOut  GhcTc = Type -- Type of the TcBracketOut
 
 type instance XSpliceE       (GhcPass _) = EpAnnCO
 type instance XProc          (GhcPass _) = EpAnn [AddEpAnn]