Add some DriverMessage type constructors

This commit expands the DriverMessage type with new type constructors,
making the number of diagnostics GHC can emit richer. In particular:

* Add DriverMissingHomeModules message
* Add DriverUnusedPackage message
* Add DriverUnnecessarySourceImports message

    This commit adds the `DriverUnnecessarySourceImports` message and
    fixes a small bug in its reporting: inside
    `warnUnnecessarySourceImports` we were checking for
    `Opt_WarnUnusedSourceImports` to be set, but we were emitting the
    diagnostic with `WarningWithoutFlag`. This also adjusts the T10637 test to reflect that.

* Add DriverDuplicatedModuleDeclaration message
* Add DriverModuleNotFound message
* Add DriverFileModuleNameMismatch message
* Add DriverUnexpectedSignature message
* Add DriverFileNotFound message
* Add DriverStaticPointersNotSupported message
* Add DriverBackpackModuleNotFound message

8 files changed, 270 insertions, 106 deletions

diff --git a/compiler/GHC.hs b/compiler/GHC.hs
index 2a75c2b840..9f64e5b02c 100644
--- a/compiler/GHC.hs
+++ b/compiler/GHC.hs
@@ -310,7 +310,6 @@ import GHC.Driver.Errors
 import GHC.Driver.Errors.Types
 import GHC.Driver.CmdLine
 import GHC.Driver.Session
-import qualified GHC.Driver.Session as Session
 import GHC.Driver.Backend
 import GHC.Driver.Config
 import GHC.Driver.Main
@@ -913,10 +912,8 @@ checkNewInteractiveDynFlags logger dflags0 = do
   -- the REPL. See #12356.
   if xopt LangExt.StaticPointers dflags0
   then do liftIO $ printOrThrowDiagnostics logger dflags0 $ singleMessage
-            $ mkPlainMsgEnvelope dflags0 interactiveSrcSpan
-               $ GhcDriverMessage $ DriverUnknownMessage
-               $ mkPlainDiagnostic Session.WarningWithoutFlag
-               $ text "StaticPointers is not supported in GHCi interactive expressions."
+            $ fmap GhcDriverMessage
+            $ mkPlainMsgEnvelope dflags0 interactiveSrcSpan DriverStaticPointersNotSupported
           return $ xopt_unset dflags0 LangExt.StaticPointers
   else return dflags0
 
diff --git a/compiler/GHC/Driver/Backpack.hs b/compiler/GHC/Driver/Backpack.hs
index 30289129c4..e0e5b183ab 100644
--- a/compiler/GHC/Driver/Backpack.hs
+++ b/compiler/GHC/Driver/Backpack.hs
@@ -809,9 +809,8 @@ summariseDecl _pn hsc_src lmodname@(L loc modname) Nothing
                          Nothing -- GHC API buffer support not supported
                          [] -- No exclusions
          case r of
-            Nothing -> throwOneError $ mkPlainErrorMsgEnvelope loc $
-                                       GhcDriverMessage $ DriverUnknownMessage $ mkPlainError $
-                                       (text "module" <+> ppr modname <+> text "was not found")
+            Nothing -> throwOneError $ fmap GhcDriverMessage
+                                     $ mkPlainErrorMsgEnvelope loc (DriverBackpackModuleNotFound modname)
             Just (Left err) -> throwErrors (fmap GhcDriverMessage err)
             Just (Right summary) -> return summary
 
diff --git a/compiler/GHC/Driver/Errors/Ppr.hs b/compiler/GHC/Driver/Errors/Ppr.hs
index 06ebe0be96..74c3d4bf21 100644
--- a/compiler/GHC/Driver/Errors/Ppr.hs
+++ b/compiler/GHC/Driver/Errors/Ppr.hs
@@ -5,11 +5,26 @@ module GHC.Driver.Errors.Ppr where
 
 import GHC.Prelude
 
-import GHC.Types.Error
 import GHC.Driver.Errors.Types
-import GHC.Parser.Errors.Ppr
-import GHC.Tc.Errors.Ppr ()
+import GHC.Driver.Flags
+import GHC.Driver.Session
 import GHC.HsToCore.Errors.Ppr ()
+import GHC.Parser.Errors.Ppr (pprPsError)
+import GHC.Tc.Errors.Ppr ()
+import GHC.Types.Error
+import GHC.Unit.Types
+import GHC.Utils.Outputable
+import GHC.Unit.Module
+
+--
+-- Suggestions
+--
+
+-- | Suggests a list of 'InstantiationSuggestion' for the '.hsig' file to the user.
+suggestInstantiatedWith :: ModuleName -> GenInstantiations UnitId -> [InstantiationSuggestion]
+suggestInstantiatedWith pi_mod_name insts =
+  [ InstantiationSuggestion k v | (k,v) <- ((pi_mod_name, mkHoleModule pi_mod_name) : insts) ]
+
 
 instance Diagnostic GhcMessage where
   diagnosticMessage = \case
@@ -37,9 +52,98 @@ instance Diagnostic GhcMessage where
       -> diagnosticReason m
 
 instance Diagnostic DriverMessage where
-  diagnosticMessage (DriverUnknownMessage m)  = diagnosticMessage m
-  diagnosticMessage (DriverPsHeaderMessage desc hints)
-    = mkSimpleDecorated $ pprPsError desc hints
+  diagnosticMessage = \case
+    DriverUnknownMessage m
+      -> diagnosticMessage m
+    DriverPsHeaderMessage desc hints
+      -> mkSimpleDecorated $ pprPsError desc hints
+    DriverMissingHomeModules missing buildingCabalPackage
+      -> let msg | buildingCabalPackage == YesBuildingCabalPackage
+                 = hang
+                     (text "These modules are needed for compilation but not listed in your .cabal file's other-modules: ")
+                     4
+                     (sep (map ppr missing))
+                 | otherwise
+                 =
+                   hang
+                     (text "Modules are not listed in command line but needed for compilation: ")
+                     4
+                     (sep (map ppr missing))
+         in mkSimpleDecorated msg
+    DriverUnusedPackages unusedArgs
+      -> let msg = vcat [ text "The following packages were specified" <+>
+                          text "via -package or -package-id flags,"
+                        , text "but were not needed for compilation:"
+                        , nest 2 (vcat (map (withDash . pprUnusedArg) unusedArgs))
+                        ]
+         in mkSimpleDecorated msg
+         where
+            withDash :: SDoc -> SDoc
+            withDash = (<+>) (text "-")
+
+            pprUnusedArg :: PackageArg -> SDoc
+            pprUnusedArg (PackageArg str) = text str
+            pprUnusedArg (UnitIdArg uid) = ppr uid
+    DriverUnnecessarySourceImports mod
+      -> mkSimpleDecorated (text "{-# SOURCE #-} unnecessary in import of " <+> quotes (ppr mod))
+    DriverDuplicatedModuleDeclaration mod files
+      -> mkSimpleDecorated $
+           text "module" <+> quotes (ppr mod) <+>
+           text "is defined in multiple files:" <+>
+           sep (map text files)
+    DriverModuleNotFound mod
+      -> mkSimpleDecorated (text "module" <+> quotes (ppr mod) <+> text "cannot be found locally")
+    DriverFileModuleNameMismatch actual expected
+      -> mkSimpleDecorated $
+           text "File name does not match module name:"
+           $$ text "Saw     :" <+> quotes (ppr actual)
+           $$ text "Expected:" <+> quotes (ppr expected)
 
-  diagnosticReason (DriverUnknownMessage m)   = diagnosticReason m
-  diagnosticReason (DriverPsHeaderMessage {}) = ErrorWithoutFlag
+    DriverUnexpectedSignature pi_mod_name buildingCabalPackage suggestions
+      -> let suggested_instantiated_with =
+               hcat (punctuate comma $
+                   [ ppr k <> text "=" <> ppr v
+                   | InstantiationSuggestion k v <- suggestions
+                   ])
+             msg = text "Unexpected signature:" <+> quotes (ppr pi_mod_name)
+                   $$ if buildingCabalPackage == YesBuildingCabalPackage
+                       then parens (text "Try adding" <+> quotes (ppr pi_mod_name)
+                               <+> text "to the"
+                               <+> quotes (text "signatures")
+                               <+> text "field in your Cabal file.")
+                       else parens (text "Try passing -instantiated-with=\"" <>
+                                    suggested_instantiated_with <> text "\"" $$
+                                       text "replacing <" <> ppr pi_mod_name <> text "> as necessary.")
+         in mkSimpleDecorated msg
+    DriverFileNotFound hsFilePath
+      -> mkSimpleDecorated (text "Can't find" <+> text hsFilePath)
+    DriverStaticPointersNotSupported
+      -> mkSimpleDecorated (text "StaticPointers is not supported in GHCi interactive expressions.")
+    DriverBackpackModuleNotFound modname
+      -> mkSimpleDecorated (text "module" <+> ppr modname <+> text "was not found")
+
+  diagnosticReason = \case
+    DriverUnknownMessage m
+      -> diagnosticReason m
+    DriverPsHeaderMessage {}
+      -> ErrorWithoutFlag
+    DriverMissingHomeModules{}
+      -> WarningWithFlag Opt_WarnMissingHomeModules
+    DriverUnusedPackages{}
+      -> WarningWithFlag Opt_WarnUnusedPackages
+    DriverUnnecessarySourceImports{}
+      -> WarningWithFlag Opt_WarnUnusedImports
+    DriverDuplicatedModuleDeclaration{}
+      -> ErrorWithoutFlag
+    DriverModuleNotFound{}
+      -> ErrorWithoutFlag
+    DriverFileModuleNameMismatch{}
+      -> ErrorWithoutFlag
+    DriverUnexpectedSignature{}
+      -> ErrorWithoutFlag
+    DriverFileNotFound{}
+      -> ErrorWithoutFlag
+    DriverStaticPointersNotSupported
+      -> WarningWithoutFlag
+    DriverBackpackModuleNotFound{}
+      -> ErrorWithoutFlag
diff --git a/compiler/GHC/Driver/Errors/Types.hs b/compiler/GHC/Driver/Errors/Types.hs
index 017852fcbb..6f1684d789 100644
--- a/compiler/GHC/Driver/Errors/Types.hs
+++ b/compiler/GHC/Driver/Errors/Types.hs
@@ -3,6 +3,8 @@
 module GHC.Driver.Errors.Types (
     GhcMessage(..)
   , DriverMessage(..), DriverMessages
+  , BuildingCabalPackage(..)
+  , InstantiationSuggestion(..)
   , WarningMessages
   , ErrorMessages
   , WarnMsg
@@ -12,18 +14,22 @@ module GHC.Driver.Errors.Types (
   , hoistTcRnMessage
   , hoistDsMessage
   , foldPsMessages
+  , checkBuildingCabalPackage
   ) where
 
 import GHC.Prelude
 
+import Data.Bifunctor
 import Data.Typeable
+
+import GHC.Driver.Session
 import GHC.Types.Error
+import GHC.Unit.Module
 
 import GHC.Parser.Errors       ( PsErrorDesc, PsHint )
 import GHC.Parser.Errors.Types ( PsMessage )
 import GHC.Tc.Errors.Types     ( TcRnMessage )
 import GHC.HsToCore.Errors.Types ( DsMessage )
-import Data.Bifunctor
 
 -- | A collection of warning messages.
 -- /INVARIANT/: Each 'GhcMessage' in the collection should have 'SevWarning' severity.
@@ -105,16 +111,129 @@ hoistTcRnMessage = fmap (first (fmap GhcTcRnMessage))
 hoistDsMessage :: Monad m => m (Messages DsMessage, a) -> m (Messages GhcMessage, a)
 hoistDsMessage = fmap (first (fmap GhcDsMessage))
 
+-- | A collection of driver messages
+type DriverMessages = Messages DriverMessage
+
 -- | A message from the driver.
-data DriverMessage
-  = DriverUnknownMessage !DiagnosticMessage
-  -- ^ Simply rewraps a generic 'DiagnosticMessage'. More
-  -- constructors will be added in the future (#18516).
-  | DriverPsHeaderMessage !PsErrorDesc ![PsHint]
-  -- ^ A parse error in parsing a Haskell file header during dependency
+data DriverMessage where
+  -- | Simply wraps a generic 'DiagnosticMessage'.
+  DriverUnknownMessage :: (Diagnostic a, Typeable a) => a -> DriverMessage
+
+  -- | A parse error in parsing a Haskell file header during dependency
   -- analysis
+  DriverPsHeaderMessage :: !PsErrorDesc -> ![PsHint] -> DriverMessage
 
--- | A collection of driver messages
-type DriverMessages = Messages DriverMessage
+  {-| DriverMissingHomeModules is a warning (controlled with -Wmissing-home-modules) that
+      arises when running GHC in --make mode when some modules needed for compilation
+      are not included on the command line. For example, if A imports B, `ghc --make
+      A.hs` will cause this warning, while `ghc --make A.hs B.hs` will not.
+
+      Useful for cabal to ensure GHC won't pick up modules listed neither in
+      'exposed-modules' nor in 'other-modules'.
+
+      Test case: warnings/should_compile/MissingMod
+
+  -}
+  DriverMissingHomeModules :: [ModuleName] -> !BuildingCabalPackage -> DriverMessage
+
+  {-| DriverUnusedPackages occurs when when package is requested on command line,
+      but was never needed during compilation. Activated by -Wunused-packages.
+
+     Test cases: warnings/should_compile/UnusedPackages
+  -}
+  DriverUnusedPackages :: [PackageArg] -> DriverMessage
+
+  {-| DriverUnnecessarySourceImports (controlled with -Wunused-imports) occurs if there
+      are {-# SOURCE #-} imports which are not necessary. See 'warnUnnecessarySourceImports'
+      in 'GHC.Driver.Make'.
+
+     Test cases: warnings/should_compile/T10637
+  -}
+  DriverUnnecessarySourceImports :: !ModuleName -> DriverMessage
+
+  {-| DriverDuplicatedModuleDeclaration occurs if a module 'A' is declared in
+       multiple files.
+
+     Test cases: None.
+  -}
+  DriverDuplicatedModuleDeclaration :: !Module -> [FilePath] -> DriverMessage
+
+  {-| DriverModuleNotFound occurs if a module 'A' can't be found.
+
+     Test cases: None.
+  -}
+  DriverModuleNotFound :: !ModuleName -> DriverMessage
+
+  {-| DriverFileModuleNameMismatch occurs if a module 'A' is defined in a file with a different name.
+      The first field is the name written in the source code; the second argument is the name extracted
+      from the filename.
+
+     Test cases: module/mod178, /driver/bug1677
+  -}
+  DriverFileModuleNameMismatch :: !ModuleName -> !ModuleName -> DriverMessage
+
+  {-| DriverUnexpectedSignature occurs when GHC encounters a module 'A' that imports a signature
+      file which is neither in the 'signatures' section of a '.cabal' file nor in any package in
+      the home modules.
+
+      Example:
+
+      -- MyStr.hsig is defined, but not added to 'signatures' in the '.cabal' file.
+      signature MyStr where
+          data Str
+
+      -- A.hs, which tries to import the signature.
+      module A where
+      import MyStr
+
+
+     Test cases: driver/T12955
+  -}
+  DriverUnexpectedSignature :: !ModuleName -> !BuildingCabalPackage -> [InstantiationSuggestion] -> DriverMessage
+
+  {-| DriverFileNotFound occurs when the input file (e.g. given on the command line) can't be found.
+
+     Test cases: None.
+  -}
+  DriverFileNotFound :: !FilePath -> DriverMessage
+
+  {-| DriverStaticPointersNotSupported occurs when the 'StaticPointers' extension is used
+       in an interactive GHCi context.
+
+     Test cases: ghci/scripts/StaticPtr
+  -}
+  DriverStaticPointersNotSupported :: DriverMessage
+
+  {-| DriverBackpackModuleNotFound occurs when Backpack can't find a particular module
+      during its dependency analysis.
+
+     Test cases: -
+  -}
+  DriverBackpackModuleNotFound :: !ModuleName -> DriverMessage
+
+-- | An 'InstantiationSuggestion' for a '.hsig' file. This is generated
+-- by GHC in case of a 'DriverUnexpectedSignature' and suggests a way
+-- to instantiate a particular signature, where the first argument is
+-- the signature name and the second is the module where the signature
+-- was defined.
+-- Example:
+--
+-- src/MyStr.hsig:2:11: error:
+--     Unexpected signature: ‘MyStr’
+--     (Try passing -instantiated-with="MyStr=<MyStr>"
+--      replacing <MyStr> as necessary.)
+data InstantiationSuggestion = InstantiationSuggestion !ModuleName !Module
+
+-- | Pass to a 'DriverMessage' the information whether or not the
+-- '-fbuilding-cabal-package' flag is set.
+data BuildingCabalPackage
+  = YesBuildingCabalPackage
+  | NoBuildingCabalPackage
+  deriving Eq
 
--- | A message about Safe Haskell.
+-- | Checks if we are building a cabal package by consulting the 'DynFlags'.
+checkBuildingCabalPackage :: DynFlags -> BuildingCabalPackage
+checkBuildingCabalPackage dflags =
+  if gopt Opt_BuildingCabalPackage dflags
+     then YesBuildingCabalPackage
+     else NoBuildingCabalPackage
diff --git a/compiler/GHC/Driver/Make.hs b/compiler/GHC/Driver/Make.hs
index 767d0b884b..51110e4134 100644
--- a/compiler/GHC/Driver/Make.hs
+++ b/compiler/GHC/Driver/Make.hs
@@ -61,6 +61,7 @@ import GHC.Driver.Backend
 import GHC.Driver.Monad
 import GHC.Driver.Env
 import GHC.Driver.Errors
+import GHC.Driver.Errors.Ppr
 import GHC.Driver.Errors.Types
 import GHC.Driver.Main
 
@@ -271,7 +272,7 @@ instantiationNodes unit_state = InstantiationNode <$> iuids_to_check
 warnMissingHomeModules :: GhcMonad m => HscEnv -> ModuleGraph -> m ()
 warnMissingHomeModules hsc_env mod_graph =
     when (not (null missing)) $
-        logDiagnostics warn
+        logDiagnostics (GhcDriverMessage <$> warn)
   where
     dflags = hsc_dflags hsc_env
     targets = map targetId (hsc_targets hsc_env)
@@ -306,22 +307,8 @@ warnMissingHomeModules hsc_env mod_graph =
     missing = map (moduleName . ms_mod) $
       filter (not . is_known_module) (mgModSummaries mod_graph)
 
-    msg
-      | gopt Opt_BuildingCabalPackage dflags
-      = hang
-          (text "These modules are needed for compilation but not listed in your .cabal file's other-modules: ")
-          4
-          (sep (map ppr missing))
-      | otherwise
-      =
-        hang
-          (text "Modules are not listed in command line but needed for compilation: ")
-          4
-          (sep (map ppr missing))
-    warn = singleMessage $
-      mkPlainMsgEnvelope (hsc_dflags hsc_env) noSrcSpan $
-      GhcDriverMessage $ DriverUnknownMessage $
-      mkPlainDiagnostic (WarningWithFlag Opt_WarnMissingHomeModules) msg
+    warn = singleMessage $ mkPlainMsgEnvelope (hsc_dflags hsc_env) noSrcSpan
+                         $ DriverMissingHomeModules missing (checkBuildingCabalPackage dflags)
 
 -- | Describes which modules of the module graph need to be loaded.
 data LoadHowMuch
@@ -386,27 +373,15 @@ warnUnusedPackages = do
           = filter (\arg -> not $ any (matching state arg) loadedPackages)
                    requestedArgs
 
-    let warn = singleMessage $
-          mkPlainMsgEnvelope dflags noSrcSpan $
-          GhcDriverMessage $ DriverUnknownMessage $
-          mkPlainDiagnostic (WarningWithFlag Opt_WarnUnusedPackages) msg
-        msg = vcat [ text "The following packages were specified" <+>
-                     text "via -package or -package-id flags,"
-                   , text "but were not needed for compilation:"
-                   , nest 2 (vcat (map (withDash . pprUnusedArg) unusedArgs)) ]
+    let warn = singleMessage $ mkPlainMsgEnvelope dflags noSrcSpan (DriverUnusedPackages unusedArgs)
 
     when (not (null unusedArgs)) $
-      logDiagnostics warn
+      logDiagnostics (GhcDriverMessage <$> warn)
 
     where
         packageArg (ExposePackage _ arg _) = Just arg
         packageArg _ = Nothing
 
-        pprUnusedArg (PackageArg str) = text str
-        pprUnusedArg (UnitIdArg uid) = ppr uid
-
-        withDash = (<+>) (text "-")
-
         matchingStr :: String -> UnitInfo -> Bool
         matchingStr str p
                 =  str == unitPackageIdString p
@@ -2226,10 +2201,7 @@ warnUnnecessarySourceImports sccs = do
 
         warn :: DynFlags -> Located ModuleName -> MsgEnvelope GhcMessage
         warn dflags (L loc mod) =
-           mkPlainMsgEnvelope dflags loc $
-           GhcDriverMessage $ DriverUnknownMessage $
-           mkPlainDiagnostic WarningWithoutFlag $
-           text "{-# SOURCE #-} unnecessary in import of " <+> quotes (ppr mod)
+          GhcDriverMessage <$> mkPlainMsgEnvelope dflags loc (DriverUnnecessarySourceImports mod)
 
 
 -----------------------------------------------------------------------------
@@ -2300,10 +2272,8 @@ downsweep hsc_env old_summaries excl_mods allow_dup_roots
                 if exists || isJust maybe_buf
                     then summariseFile hsc_env old_summaries file mb_phase
                                        obj_allowed maybe_buf
-                    else return $ Left $ singleMessage $
-                           mkPlainErrorMsgEnvelope noSrcSpan $
-                           DriverUnknownMessage $ mkPlainError $
-                             text "can't find file:" <+> text file
+                    else return $ Left $ singleMessage
+                                $ mkPlainErrorMsgEnvelope noSrcSpan (DriverFileNotFound file)
         getRootSummary Target { targetId = TargetModule modl
                               , targetAllowObjCode = obj_allowed
                               , targetContents = maybe_buf
@@ -2737,32 +2707,13 @@ summariseModule hsc_env old_summary_map is_boot (L loc wanted_mod)
               | otherwise = HsSrcFile
 
         when (pi_mod_name /= wanted_mod) $
-                throwE $ singleMessage $
-                  mkPlainErrorMsgEnvelope pi_mod_name_loc $
-                  DriverUnknownMessage $ mkPlainError $
-                              text "File name does not match module name:"
-                              $$ text "Saw     :" <+> quotes (ppr pi_mod_name)
-                              $$ text "Expected:" <+> quotes (ppr wanted_mod)
+                throwE $ singleMessage $ mkPlainErrorMsgEnvelope pi_mod_name_loc
+                       $ DriverFileModuleNameMismatch pi_mod_name wanted_mod
 
         when (hsc_src == HsigFile && isNothing (lookup pi_mod_name (homeUnitInstantiations home_unit))) $
-            let suggested_instantiated_with =
-                    hcat (punctuate comma $
-                        [ ppr k <> text "=" <> ppr v
-                        | (k,v) <- ((pi_mod_name, mkHoleModule pi_mod_name)
-                                : homeUnitInstantiations home_unit)
-                        ])
-            in throwE $ singleMessage $
-                mkPlainErrorMsgEnvelope pi_mod_name_loc $
-                DriverUnknownMessage $ mkPlainError $
-                text "Unexpected signature:" <+> quotes (ppr pi_mod_name)
-                $$ if gopt Opt_BuildingCabalPackage dflags
-                    then parens (text "Try adding" <+> quotes (ppr pi_mod_name)
-                            <+> text "to the"
-                            <+> quotes (text "signatures")
-                            <+> text "field in your Cabal file.")
-                    else parens (text "Try passing -instantiated-with=\"" <>
-                                 suggested_instantiated_with <> text "\"" $$
-                                text "replacing <" <> ppr pi_mod_name <> text "> as necessary.")
+            let suggestions = suggestInstantiatedWith pi_mod_name (homeUnitInstantiations home_unit)
+            in throwE $ singleMessage $ mkPlainErrorMsgEnvelope pi_mod_name_loc
+                      $ DriverUnexpectedSignature pi_mod_name (checkBuildingCabalPackage dflags) suggestions
 
         liftIO $ makeNewModSummary hsc_env $ MakeNewModSummary
             { nms_src_fn = src_fn
@@ -2918,25 +2869,17 @@ noModError hsc_env loc wanted_mod err
 
 noHsFileErr :: SrcSpan -> String -> DriverMessages
 noHsFileErr loc path
-  = singleMessage $ mkPlainErrorMsgEnvelope loc $
-    DriverUnknownMessage $ mkPlainError $
-    text "Can't find" <+> text path
+  = singleMessage $ mkPlainErrorMsgEnvelope loc (DriverFileNotFound path)
 
 moduleNotFoundErr :: ModuleName -> DriverMessages
 moduleNotFoundErr mod
-  = singleMessage $ mkPlainErrorMsgEnvelope noSrcSpan $
-    DriverUnknownMessage $ mkPlainError $
-        text "module" <+> quotes (ppr mod) <+> text "cannot be found locally"
+  = singleMessage $ mkPlainErrorMsgEnvelope noSrcSpan (DriverModuleNotFound mod)
 
 multiRootsErr :: [ModSummary] -> IO ()
 multiRootsErr [] = panic "multiRootsErr"
 multiRootsErr summs@(summ1:_)
-  = throwOneError $
-      mkPlainErrorMsgEnvelope noSrcSpan $
-      GhcDriverMessage $ DriverUnknownMessage $ mkPlainError $
-        text "module" <+> quotes (ppr mod) <+>
-        text "is defined in multiple files:" <+>
-        sep (map text files)
+  = throwOneError $ fmap GhcDriverMessage $
+    mkPlainErrorMsgEnvelope noSrcSpan $ DriverDuplicatedModuleDeclaration mod files
   where
     mod = ms_mod summ1
     files = map (expectJust "checkDup" . ml_hs_file . ms_location) summs
diff --git a/compiler/GHC/Iface/Errors.hs b/compiler/GHC/Iface/Errors.hs
index 9e0df9a9d8..83f9b03920 100644
--- a/compiler/GHC/Iface/Errors.hs
+++ b/compiler/GHC/Iface/Errors.hs
@@ -18,6 +18,7 @@ import GHC.Platform.Ways
 import GHC.Utils.Panic.Plain
 import GHC.Driver.Session
 import GHC.Driver.Env.Types
+import GHC.Driver.Errors.Types
 import GHC.Data.Maybe
 import GHC.Prelude
 import GHC.Unit
@@ -151,7 +152,7 @@ cannotFindModule hsc_env = cannotFindModule'
 
 cannotFindModule' :: DynFlags -> UnitEnv -> Profile -> ModuleName -> FindResult -> SDoc
 cannotFindModule' dflags unit_env profile mod res = pprWithUnitState (ue_units unit_env) $
-  cantFindErr (gopt Opt_BuildingCabalPackage dflags)
+  cantFindErr (checkBuildingCabalPackage dflags)
               cannotFindMsg
               (text "Ambiguous module name")
               unit_env
@@ -170,7 +171,7 @@ cannotFindModule' dflags unit_env profile mod res = pprWithUnitState (ue_units u
         _ -> text "Could not find module"
 
 cantFindErr
-    :: Bool -- ^ Using Cabal?
+    :: BuildingCabalPackage -- ^ Using Cabal?
     -> SDoc
     -> SDoc
     -> UnitEnv
@@ -273,7 +274,7 @@ cantFindErr using_cabal cannot_find _ unit_env profile tried_these mod_name find
         <> dot $$ pkg_hidden_hint uid
 
     pkg_hidden_hint uid
-     | using_cabal
+     | using_cabal == YesBuildingCabalPackage
         = let pkg = expectJust "pkg_hidden" (lookupUnit (ue_units unit_env) uid)
            in text "Perhaps you need to add" <+>
               quotes (ppr (unitPackageName pkg)) <+>
diff --git a/compiler/GHC/Utils/Error.hs b/compiler/GHC/Utils/Error.hs
index 7e614588f6..2380c95032 100644
--- a/compiler/GHC/Utils/Error.hs
+++ b/compiler/GHC/Utils/Error.hs
@@ -73,6 +73,7 @@ import GHC.Utils.Exception
 import GHC.Utils.Outputable as Outputable
 import GHC.Utils.Panic
 import GHC.Utils.Logger
+import GHC.Utils.Misc ( debugIsOn )
 import GHC.Types.Error
 import GHC.Types.SrcLoc as SrcLoc
 
@@ -151,7 +152,7 @@ mkErrorMsgEnvelope :: Diagnostic e
                    -> e
                    -> MsgEnvelope e
 mkErrorMsgEnvelope locn unqual msg =
-  mk_msg_envelope SevError locn unqual msg
+ ASSERT( diagnosticReason msg == ErrorWithoutFlag ) mk_msg_envelope SevError locn unqual msg
 
 -- | Variant that doesn't care about qualified/unqualified names.
 mkPlainMsgEnvelope :: Diagnostic e
diff --git a/testsuite/tests/warnings/should_compile/T10637/T10637.stderr b/testsuite/tests/warnings/should_compile/T10637/T10637.stderr
index 7be59d5f2b..ece4316739 100644
--- a/testsuite/tests/warnings/should_compile/T10637/T10637.stderr
+++ b/testsuite/tests/warnings/should_compile/T10637/T10637.stderr
@@ -1,3 +1,3 @@
 
-T10637.hs:3:23: warning:
+T10637.hs:3:23: warning: [-Wunused-imports (in -Wextra)]
     {-# SOURCE #-} unnecessary in import of  ‘A’