Extensible Hints for diagnostic messages

This commit extends the GHC diagnostic hierarchy with a `GhcHint` type,
modelling helpful suggestions emitted by GHC which can be used to deal
with a particular warning or error.

As a direct consequence of this, the `Diagnostic` typeclass has been extended
with a `diagnosticHints` method, which returns a `[GhcHint]`. This means
that now we can clearly separate out the printing of the diagnostic
message with the suggested fixes.

This is done by extending the `printMessages` function in
`GHC.Driver.Errors`.

On top of that, the old `PsHint` type has been superseded by the new `GhcHint`
type, which de-duplicates some hints in favour of a general `SuggestExtension`
constructor that takes a `GHC.LanguageExtensions.Extension`.

3 files changed, 20 insertions, 5 deletions

diff --git a/compiler/GHC/Tc/Errors/Ppr.hs b/compiler/GHC/Tc/Errors/Ppr.hs
index ffabf0f69c..650befdd8f 100644
--- a/compiler/GHC/Tc/Errors/Ppr.hs
+++ b/compiler/GHC/Tc/Errors/Ppr.hs
@@ -31,6 +31,7 @@ instance Diagnostic TcRnMessage where
       -> mkDecorated [ text "The import item" <+> quotes (ppr ie) <+>
                        text "does not have an explicit import list"
                      ]
+
   diagnosticReason = \case
     TcRnUnknownMessage m
       -> diagnosticReason m
@@ -45,6 +46,20 @@ instance Diagnostic TcRnMessage where
     TcRnMissingImportList{}
       -> WarningWithFlag Opt_WarnMissingImportList
 
+  diagnosticHints = \case
+    TcRnUnknownMessage m
+      -> diagnosticHints m
+    TcRnImplicitLift{}
+      -> noHints
+    TcRnUnusedPatternBinds{}
+      -> noHints
+    TcRnDodgyImports{}
+      -> noHints
+    TcRnDodgyExports{}
+      -> noHints
+    TcRnMissingImportList{}
+      -> noHints
+
 dodgy_msg :: (Outputable a, Outputable b) => SDoc -> a -> b -> SDoc
 dodgy_msg kind tc ie
   = sep [ text "The" <+> kind <+> text "item"
diff --git a/compiler/GHC/Tc/Module.hs b/compiler/GHC/Tc/Module.hs
index 50b832ed49..450e97819a 100644
--- a/compiler/GHC/Tc/Module.hs
+++ b/compiler/GHC/Tc/Module.hs
@@ -214,7 +214,7 @@ tcRnModule hsc_env mod_sum save_rn_syntax
     logger  = hsc_logger hsc_env
     home_unit = hsc_home_unit hsc_env
     err_msg = mkPlainErrorMsgEnvelope loc $
-              TcRnUnknownMessage $ mkPlainError $
+              TcRnUnknownMessage $ mkPlainError noHints $
                 text "Module does not have a RealSrcSpan:" <+> ppr this_mod
 
     pair :: (Module, SrcSpan)
@@ -3152,5 +3152,5 @@ mark_plugin_unsafe dflags = unless (gopt Opt_PluginTrustworthy dflags) $
       singleMessage $
       mkPlainMsgEnvelope dflags noSrcSpan $
       TcRnUnknownMessage $
-      mkPlainDiagnostic WarningWithoutFlag $
+      mkPlainDiagnostic WarningWithoutFlag noHints $
       Outputable.text unsafeText
diff --git a/compiler/GHC/Tc/Utils/Monad.hs b/compiler/GHC/Tc/Utils/Monad.hs
index 730e666a2a..2d9298e12b 100644
--- a/compiler/GHC/Tc/Utils/Monad.hs
+++ b/compiler/GHC/Tc/Utils/Monad.hs
@@ -1038,7 +1038,7 @@ mkLongErrAt loc msg extra
          let msg' = pprWithUnitState unit_state msg in
          return $ mkErrorMsgEnvelope loc printer
                 $ TcRnUnknownMessage
-                $ mkDecoratedError [msg', extra] }
+                $ mkDecoratedError noHints [msg', extra] }
 
 mkTcRnMessage :: DiagnosticReason
               -> SrcSpan
@@ -1058,7 +1058,7 @@ mkTcRnMessage reason loc important context extra
          in
          return $ mkMsgEnvelope dflags loc printer
                 $ TcRnUnknownMessage
-                $ mkDecoratedDiagnostic reason errDocs }
+                $ mkDecoratedDiagnostic reason noHints errDocs }
 
 addLongErrAt :: SrcSpan -> SDoc -> SDoc -> TcRn ()
 addLongErrAt loc msg extra = mkLongErrAt loc msg extra >>= reportDiagnostic
@@ -1585,7 +1585,7 @@ add_diagnostic_at reason loc msg extra_info
          dflags  <- getDynFlags ;
          let { dia = mkMsgEnvelope dflags loc printer $
                      TcRnUnknownMessage $
-                     mkDecoratedDiagnostic reason [msg, extra_info] } ;
+                     mkDecoratedDiagnostic reason noHints [msg, extra_info] } ;
          reportDiagnostic dia }