TTG: Rework and improve splices

This commit redefines the structure of Splices in the AST.

We get rid of `HsSplice` which used to represent typed and untyped
splices, quasi quotes, and the result of splicing either an expression,
a type or a pattern.

Instead we have `HsUntypedSplice` which models an untyped splice or a
quasi quoter, which works in practice just like untyped splices.

The `HsExpr` constructor `HsSpliceE` which used to be constructed with
an `HsSplice` is split into `HsTypedSplice` and `HsUntypedSplice`. The
former is directly constructed with an `HsExpr` and the latter now takes
an `HsUntypedSplice`.

Both `HsType` and `Pat` constructors `HsSpliceTy` and `SplicePat` now
take an `HsUntypedSplice` instead of a `HsSplice` (remember only
/untyped splices/ can be spliced as types or patterns).

The result of splicing an expression, type, or pattern is now
comfortably stored in the extension fields `XSpliceTy`, `XSplicePat`,
`XUntypedSplice` as, respectively, `HsUntypedSpliceResult (HsType
GhcRn)`, `HsUntypedSpliceResult (Pat GhcRn)`, and `HsUntypedSpliceResult
(HsExpr GhcRn)`

Overall the TTG extension points are now better used to
make invalid states unrepresentable and model the progression between
stages better.

See Note [Lifecycle of an untyped splice, and PendingRnSplice]
and Note [Lifecycle of an typed splice, and PendingTcSplice] for more
details.

Updates haddock submodule

Fixes #21263

-------------------------
Metric Decrease:
    hard_hole_fits
-------------------------

6 files changed, 47 insertions, 98 deletions

diff --git a/compiler/Language/Haskell/Syntax/Decls.hs b/compiler/Language/Haskell/Syntax/Decls.hs
index 026080d3f6..303105e3d4 100644
--- a/compiler/Language/Haskell/Syntax/Decls.hs
+++ b/compiler/Language/Haskell/Syntax/Decls.hs
@@ -66,7 +66,7 @@ module Language.Haskell.Syntax.Decls (
   -- ** @default@ declarations
   DefaultDecl(..), LDefaultDecl,
   -- ** Template haskell declaration splice
-  SpliceExplicitFlag(..),
+  SpliceDecoration(..),
   SpliceDecl(..), LSpliceDecl,
   -- ** Foreign function interface declarations
   ForeignDecl(..), LForeignDecl, ForeignImport(..), ForeignExport(..),
@@ -95,7 +95,7 @@ module Language.Haskell.Syntax.Decls (
 import GHC.Prelude
 
 import {-# SOURCE #-} Language.Haskell.Syntax.Expr
-  ( HsExpr, HsSplice )
+  ( HsExpr, HsUntypedSplice )
         -- Because Expr imports Decls via HsBracket
 
 import Language.Haskell.Syntax.Binds
@@ -245,10 +245,21 @@ type LSpliceDecl pass = XRec pass (SpliceDecl pass)
 data SpliceDecl p
   = SpliceDecl                  -- Top level splice
         (XSpliceDecl p)
-        (XRec p (HsSplice p))
-        SpliceExplicitFlag
+        (XRec p (HsUntypedSplice p))
+        SpliceDecoration -- Whether $( ) variant found, for pretty printing
   | XSpliceDecl !(XXSpliceDecl p)
 
+-- | A splice can appear with various decorations wrapped around it. This data
+-- type captures explicitly how it was originally written, for use in the pretty
+-- printer.
+data SpliceDecoration
+  = DollarSplice  -- ^ $splice
+  | BareSplice    -- ^ bare splice
+  deriving (Data, Eq, Show)
+
+instance Outputable SpliceDecoration where
+  ppr x = text $ show x
+
 {-
 ************************************************************************
 *                                                                      *
diff --git a/compiler/Language/Haskell/Syntax/Expr.hs b/compiler/Language/Haskell/Syntax/Expr.hs
index f964c5d3f6..051edda97f 100644
--- a/compiler/Language/Haskell/Syntax/Expr.hs
+++ b/compiler/Language/Haskell/Syntax/Expr.hs
@@ -46,13 +46,9 @@ import GHC.Data.FastString
 
 -- libraries:
 import Data.Data hiding (Fixity(..))
-import qualified Data.Data as Data (Fixity(..))
 
 import Data.List.NonEmpty ( NonEmpty )
 
-import GHCi.RemoteTypes ( ForeignRef )
-import qualified Language.Haskell.TH as TH (Q)
-
 {- Note [RecordDotSyntax field updates]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 The extensions @OverloadedRecordDot@ @OverloadedRecordUpdate@ together
@@ -603,7 +599,8 @@ data HsExpr p
   --         'GHC.Parser.Annotation.AnnClose'
 
   -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
-  | HsSpliceE  (XSpliceE p) (HsSplice p)
+  | HsTypedSplice    (XTypedSplice p)   (LHsExpr p) -- `$$z` or `$$(f 4)`
+  | HsUntypedSplice  (XUntypedSplice p) (HsUntypedSplice p)
 
   -----------------------------------------------------------
   -- Arrow notation extension
@@ -1542,87 +1539,29 @@ Workshop 2007).
 
 Briefly, one writes
         [p| stuff |]
-and the arbitrary string "stuff" gets parsed by the parser 'p', whose
-type should be Language.Haskell.TH.Quote.QuasiQuoter.  'p' must be
-defined in another module, because we are going to run it here.  It's
-a bit like a TH splice:
-        $(p "stuff")
-
-However, you can do this in patterns as well as terms.  Because of this,
-the splice is run by the *renamer* rather than the type checker.
+and the arbitrary string "stuff" gets parsed by the parser 'p', whose type
+should be Language.Haskell.TH.Quote.QuasiQuoter.  'p' must be defined in
+another module, because we are going to run it here.  It's a bit like an
+/untyped/ TH splice where the parser is applied the "stuff" as a string, thus:
+     $(p "stuff")
+
+Notice that it's an /untyped/ TH splice: it can occur in patterns and types, as well
+as in expressions; and it runs in the renamer.
 -}
 
 -- | Haskell Splice
-data HsSplice id
-   = HsTypedSplice       --  $$z  or $$(f 4)
-        (XTypedSplice id)
-        SpliceDecoration -- Whether $$( ) variant found, for pretty printing
-        (IdP id)         -- A unique name to identify this splice point
-        (LHsExpr id)     -- See Note [Pending Splices]
-
-   | HsUntypedSplice     --  $z  or $(f 4)
-        (XUntypedSplice id)
-        SpliceDecoration -- Whether $( ) variant found, for pretty printing
-        (IdP id)         -- A unique name to identify this splice point
-        (LHsExpr id)     -- See Note [Pending Splices]
-
-   | HsQuasiQuote        -- See Note [Quasi-quote overview]
-        (XQuasiQuote id)
-        (IdP id)         -- Splice point
-        (IdP id)         -- Quoter
-        SrcSpan          -- The span of the enclosed string
-        FastString       -- The enclosed string
-
-   -- AZ:TODO: use XSplice instead of HsSpliced
-   | HsSpliced  -- See Note [Delaying modFinalizers in untyped splices] in
-                -- GHC.Rename.Splice.
-                -- This is the result of splicing a splice. It is produced by
-                -- the renamer and consumed by the typechecker. It lives only
-                -- between the two.
-        (XSpliced id)
-        ThModFinalizers     -- TH finalizers produced by the splice.
-        (HsSplicedThing id) -- The result of splicing
-   | XSplice !(XXSplice id) -- Extension point; see Note [Trees That Grow]
-                            -- in Language.Haskell.Syntax.Extension
-
--- | A splice can appear with various decorations wrapped around it. This data
--- type captures explicitly how it was originally written, for use in the pretty
--- printer.
-data SpliceDecoration
-  = DollarSplice  -- ^ $splice or $$splice
-  | BareSplice    -- ^ bare splice
-  deriving (Data, Eq, Show)
-
-instance Outputable SpliceDecoration where
-  ppr x = text $ show x
-
-
-isTypedSplice :: HsSplice id -> Bool
-isTypedSplice (HsTypedSplice {}) = True
-isTypedSplice _                  = False   -- Quasi-quotes are untyped splices
-
--- | Finalizers produced by a splice with
--- 'Language.Haskell.TH.Syntax.addModFinalizer'
---
--- See Note [Delaying modFinalizers in untyped splices] in GHC.Rename.Splice. For how
--- this is used.
---
-newtype ThModFinalizers = ThModFinalizers [ForeignRef (TH.Q ())]
+data HsUntypedSplice id
+   = HsUntypedSpliceExpr --  $z  or $(f 4)
+        (XUntypedSpliceExpr id)
+        (LHsExpr id)
 
--- A Data instance which ignores the argument of 'ThModFinalizers'.
-instance Data ThModFinalizers where
-  gunfold _ z _ = z $ ThModFinalizers []
-  toConstr  a   = mkConstr (dataTypeOf a) "ThModFinalizers" [] Data.Prefix
-  dataTypeOf a  = mkDataType "HsExpr.ThModFinalizers" [toConstr a]
-
--- | Haskell Spliced Thing
---
--- Values that can result from running a splice.
-data HsSplicedThing id
-    = HsSplicedExpr (HsExpr id) -- ^ Haskell Spliced Expression
-    | HsSplicedTy   (HsType id) -- ^ Haskell Spliced Type
-    | HsSplicedPat  (Pat id)    -- ^ Haskell Spliced Pattern
+   | HsQuasiQuote            -- See Note [Quasi-quote overview]
+        (XQuasiQuote id)
+        (IdP id)             -- The quoter (the bit between `[` and `|`)
+        (XRec id FastString) -- The enclosed string
 
+   | XUntypedSplice !(XXUntypedSplice id) -- Extension point; see Note [Trees That Grow]
+                                          -- in Language.Haskell.Syntax.Extension
 
 -- | Haskell (Untyped) Quote = Expr + Pat + Type + Var
 data HsQuote p
diff --git a/compiler/Language/Haskell/Syntax/Expr.hs-boot b/compiler/Language/Haskell/Syntax/Expr.hs-boot
index 3ea7e32708..b6a0d79431 100644
--- a/compiler/Language/Haskell/Syntax/Expr.hs-boot
+++ b/compiler/Language/Haskell/Syntax/Expr.hs-boot
@@ -12,9 +12,9 @@ import Data.Kind  ( Type )
 type role HsExpr nominal
 type role MatchGroup nominal nominal
 type role GRHSs nominal nominal
-type role HsSplice nominal
+type role HsUntypedSplice nominal
 data HsExpr (i :: Type)
-data HsSplice (i :: Type)
+data HsUntypedSplice (i :: Type)
 data MatchGroup (a :: Type) (body :: Type)
 data GRHSs (a :: Type) (body :: Type)
 type family SyntaxExpr (i :: Type)
diff --git a/compiler/Language/Haskell/Syntax/Extension.hs b/compiler/Language/Haskell/Syntax/Extension.hs
index dc39d10c99..5cffd96690 100644
--- a/compiler/Language/Haskell/Syntax/Extension.hs
+++ b/compiler/Language/Haskell/Syntax/Extension.hs
@@ -431,7 +431,8 @@ type family XExprWithTySig  x
 type family XArithSeq       x
 type family XTypedBracket   x
 type family XUntypedBracket x
-type family XSpliceE        x
+type family XTypedSplice    x
+type family XUntypedSplice  x
 type family XProc           x
 type family XStatic         x
 type family XTick           x
@@ -463,12 +464,10 @@ type family XMissing  x
 type family XXTupArg  x
 
 -- -------------------------------------
--- HsSplice type families
-type family XTypedSplice   x
-type family XUntypedSplice x
-type family XQuasiQuote    x
-type family XSpliced       x
-type family XXSplice       x
+-- HsUntypedSplice type families
+type family XUntypedSpliceExpr x
+type family XQuasiQuote        x
+type family XXUntypedSplice    x
 
 -- -------------------------------------
 -- HsQuoteBracket type families
diff --git a/compiler/Language/Haskell/Syntax/Pat.hs b/compiler/Language/Haskell/Syntax/Pat.hs
index 2f96d107c0..0e9f11dc1b 100644
--- a/compiler/Language/Haskell/Syntax/Pat.hs
+++ b/compiler/Language/Haskell/Syntax/Pat.hs
@@ -31,7 +31,7 @@ module Language.Haskell.Syntax.Pat (
 
 import GHC.Prelude
 
-import {-# SOURCE #-} Language.Haskell.Syntax.Expr (SyntaxExpr, LHsExpr, HsSplice)
+import {-# SOURCE #-} Language.Haskell.Syntax.Expr (SyntaxExpr, LHsExpr, HsUntypedSplice)
 
 -- friends:
 import Language.Haskell.Syntax.Lit
@@ -161,7 +161,7 @@ data Pat p
 
   -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
   | SplicePat       (XSplicePat p)
-                    (HsSplice p)    -- ^ Splice Pattern (Includes quasi-quotes)
+                    (HsUntypedSplice p)    -- ^ Splice Pattern (Includes quasi-quotes)
 
         ------------ Literal and n+k patterns ---------------
   | LitPat          (XLitPat p)
diff --git a/compiler/Language/Haskell/Syntax/Type.hs b/compiler/Language/Haskell/Syntax/Type.hs
index 0c84e9faa6..e394628f25 100644
--- a/compiler/Language/Haskell/Syntax/Type.hs
+++ b/compiler/Language/Haskell/Syntax/Type.hs
@@ -58,7 +58,7 @@ module Language.Haskell.Syntax.Type (
 
 import GHC.Prelude
 
-import {-# SOURCE #-} Language.Haskell.Syntax.Expr ( HsSplice )
+import {-# SOURCE #-} Language.Haskell.Syntax.Expr ( HsUntypedSplice )
 
 import Language.Haskell.Syntax.Extension
 
@@ -836,7 +836,7 @@ data HsType pass
       -- For details on above see Note [exact print annotations] in GHC.Parser.Annotation
 
   | HsSpliceTy          (XSpliceTy pass)
-                        (HsSplice pass)   -- Includes quasi-quotes
+                        (HsUntypedSplice pass)   -- Includes quasi-quotes
       -- ^ - 'GHC.Parser.Annotation.AnnKeywordId' : 'GHC.Parser.Annotation.AnnOpen' @'$('@,
       --         'GHC.Parser.Annotation.AnnClose' @')'@