Migrate errors to diagnostics in GHC.Tc.Module

This commit migrates the errors in GHC.Tc.Module to use the new
diagnostic infrastructure.

It required a significant overhaul of the compatibility checks between
an hs-boot or signature module and its implementation; we now use
a Writer monad to accumulate errors; see the BootMismatch datatype
in GHC.Tc.Errors.Types, with its panoply of subtypes.
For the sake of readability, several local functions inside the
'checkBootTyCon' function were split off into top-level functions.

We split off GHC.Types.HscSource into a "boot or sig" vs "normal hs file"
datatype, as this mirrors the logic in several other places where we
want to treat hs-boot and hsig files in a similar fashion.

This commit also refactors the Backpack checks for type synonyms
implementing abstract data, to correctly reject implementations that
contain qualified or quantified types (this fixes #23342 and #23344).

1 files changed, 5 insertions, 1 deletions

diff --git a/compiler/GHC/Types/Hint.hs b/compiler/GHC/Types/Hint.hs
index 7057925dea..69f98ba1da 100644
--- a/compiler/GHC/Types/Hint.hs
+++ b/compiler/GHC/Types/Hint.hs
@@ -27,11 +27,12 @@ import qualified Data.List.NonEmpty as NE
 
 import GHC.Utils.Outputable
 import qualified GHC.LanguageExtensions as LangExt
-import Data.Typeable
+import Data.Typeable (Typeable)
 import GHC.Unit.Module (ModuleName, Module)
 import GHC.Hs.Extension (GhcTc, GhcRn)
 import GHC.Core.Coercion
 import GHC.Core.FamInstEnv (FamFlavor)
+import GHC.Core.TyCon (TyCon)
 import GHC.Core.Type (PredType)
 import GHC.Types.Fixity (LexicalFixity(..))
 import GHC.Types.Name (Name, NameSpace, OccName (occNameFS), isSymOcc, nameOccName)
@@ -453,6 +454,9 @@ data GhcHint
     Name -- ^ method with non-canonical implementation
     Name -- ^ possible other method to use as the RHS instead
     String -- ^ Documentation URL
+  {-| Suggest eta-reducing a type synonym used in the implementation
+      of abstract data. -}
+  | SuggestEtaReduceAbsDataTySyn TyCon
 
 -- | An 'InstantiationSuggestion' for a '.hsig' file. This is generated
 -- by GHC in case of a 'DriverUnexpectedSignature' and suggests a way