Fix: Show actual parse errors for external Fourmolu/Ormolu

haskell-language-server.cabal

@@ -1481,11 +1481,15 @@ test-suite hls-fourmolu-plugin-tests
     fourmolu:fourmolu
   build-depends:
     , aeson
+    , containers
     , filepath
     , haskell-language-server:hls-fourmolu-plugin
     , hls-plugin-api
     , hls-test-utils       == 2.13.0.0
+    , lens
     , lsp-test
+    , lsp-types
+    , text
 
 -----------------------------
 -- ormolu plugin
@@ -1538,12 +1542,15 @@ test-suite hls-ormolu-plugin-tests
     ormolu:ormolu
   build-depends:
     , aeson
+    , containers
     , filepath
     , haskell-language-server:hls-ormolu-plugin
     , hls-plugin-api
     , hls-test-utils     == 2.13.0.0
+    , lens
     , lsp-types
     , ormolu
+    , text
 
 -----------------------------
 -- stylish-haskell plugin

plugins/hls-fourmolu-plugin/src/Ide/Plugin/Fourmolu.hs

@@ -145,7 +145,12 @@ provider recorder plId ideState token typ contents fp fo = ExceptT $ pluginWithI
                 pure $ InL $ makeDiffTextEdit contents out
             ExitFailure n -> do
                 logWith recorder Info $ StdErr err
-                throwError $ PluginInternalError $ "Fourmolu failed with exit code " <> T.pack (show n)
+                let cleanErr = T.stripEnd err
+                let cliError = if T.null cleanErr
+                               then ""
+                               else "\n" <> cleanErr
+                throwError $ PluginInternalError $
+                    "Fourmolu failed with exit code " <> T.pack (show n) <> cliError
 
 loadConfig ::
     Recorder (WithPriority LogEvent) ->

plugins/hls-fourmolu-plugin/test/Main.hs

@@ -3,11 +3,15 @@ module Main
   ( main
   ) where
 
+import           Control.Lens                ((^.))
 import           Data.Aeson
 import qualified Data.Aeson.KeyMap           as KM
 import           Data.Functor
+import qualified Data.Map                    as M
+import qualified Data.Text                   as T
 import           Ide.Plugin.Config
 import qualified Ide.Plugin.Fourmolu         as Fourmolu
+import qualified Language.LSP.Protocol.Lens  as L
 import           Language.LSP.Protocol.Types
 import           Language.LSP.Test
 import           System.FilePath
@@ -31,6 +35,28 @@ tests =
             formatDoc doc (FormattingOptions 4 True Nothing Nothing Nothing)
         , goldenWithFourmolu cli "uses correct operator fixities" "Fourmolu3" "formatted" $ \doc -> do
             formatDoc doc (FormattingOptions 4 True Nothing Nothing Nothing)
+        , testCase "error message contains stderr output" $ do
+            let cliConfig = def {
+                    formattingProvider = "fourmolu",
+                    plugins = M.fromList [("fourmolu", def { plcConfig = KM.fromList ["external" .= True] })]
+                }
+            runSessionWithServer cliConfig fourmoluPlugin testDataDir $ do
+                doc <- openDoc "FormatError.hs" "haskell"
+                void waitForBuildQueue
+                resp <- request SMethod_TextDocumentFormatting $
+                    DocumentFormattingParams Nothing doc (FormattingOptions 4 True Nothing Nothing Nothing)
+                liftIO $ case resp ^. L.result of
+                    Left err -> do
+                        let msg = err ^. L.message
+                        -- Verify the error message structure:
+                        -- 1. Contains the exit code prefix (base message intact)
+                        assertBool ("Expected exit code prefix, got: " <> T.unpack msg)
+                            ("failed with exit code" `T.isInfixOf` msg)
+                        -- 2. Contains a stable parse-error phrase from formatter stderr
+                        assertBool ("Expected parse error details from stderr, got: " <> T.unpack msg)
+                            ("parse error on input" `T.isInfixOf` msg)
+                    Right _ ->
+                        assertFailure "Expected formatting to fail on unparsable file"
         ]
 
 goldenWithFourmolu :: Bool -> TestName -> FilePath -> FilePath -> (TextDocumentIdentifier -> Session ()) -> TestTree

plugins/hls-fourmolu-plugin/test/testdata/FormatError.hs

@@ -0,0 +1,3 @@
+module FormatError where
+
+foo = let x = 1 in in x

plugins/hls-ormolu-plugin/src/Ide/Plugin/Ormolu.hs

@@ -170,7 +170,12 @@ provider recorder plId ideState token typ contents fp _ = ExceptT $ pluginWithIn
                pure $ InL $ makeDiffTextEdit contents out
            ExitFailure n -> do
                logWith recorder Info $ StdErr err
-               throwError $ PluginInternalError $ "Ormolu failed with exit code " <> T.pack (show n)
+               let cleanErr = T.stripEnd err
+               let cliError = if T.null cleanErr
+                              then ""
+                              else "\n" <> cleanErr
+               throwError $ PluginInternalError $
+                   "Ormolu failed with exit code " <> T.pack (show n) <> cliError
 
 newtype CLIVersionInfo =   CLIVersionInfo
     { noCabal :: Bool

plugins/hls-ormolu-plugin/test/Main.hs

@@ -4,11 +4,15 @@ module Main
   ( main
   ) where
 
+import           Control.Lens                ((^.))
 import           Data.Aeson
 import qualified Data.Aeson.KeyMap           as KM
 import           Data.Functor
+import qualified Data.Map                    as M
+import qualified Data.Text                   as T
 import           Ide.Plugin.Config
 import qualified Ide.Plugin.Ormolu           as Ormolu
+import qualified Language.LSP.Protocol.Lens  as L
 import           Language.LSP.Protocol.Types
 import           System.FilePath
 import           Test.Hls
@@ -31,6 +35,28 @@ tests = testGroup "ormolu" $
       , goldenWithOrmolu cli "formats operators correctly" "Ormolu3" "formatted" $ \doc -> do
           formatDoc doc (FormattingOptions 4 True Nothing Nothing Nothing)
 #endif
+      , testCase "error message contains stderr output" $ do
+          let cliConfig = def {
+                  formattingProvider = "ormolu",
+                  plugins = M.fromList [("ormolu", def { plcConfig = KM.fromList ["external" .= True] })]
+              }
+          runSessionWithServer cliConfig ormoluPlugin testDataDir $ do
+              doc <- openDoc "FormatError.hs" "haskell"
+              void waitForBuildQueue
+              resp <- request SMethod_TextDocumentFormatting $
+                  DocumentFormattingParams Nothing doc (FormattingOptions 4 True Nothing Nothing Nothing)
+              liftIO $ case resp ^. L.result of
+                  Left err -> do
+                      let msg = err ^. L.message
+                      -- Verify the error message structure:
+                      -- 1. Contains the exit code prefix (base message intact)
+                      assertBool ("Expected exit code prefix, got: " <> T.unpack msg)
+                          ("failed with exit code" `T.isInfixOf` msg)
+                      -- 2. Contains a stable parse-error phrase from formatter stderr
+                      assertBool ("Expected parse error details from stderr, got: " <> T.unpack msg)
+                          ("parse error on input" `T.isInfixOf` msg)
+                  Right _ ->
+                      assertFailure "Expected formatting to fail on unparsable file"
       ]
 
 goldenWithOrmolu :: Bool -> TestName -> FilePath -> FilePath -> (TextDocumentIdentifier -> Session ()) -> TestTree

plugins/hls-ormolu-plugin/test/testdata/FormatError.hs

@@ -0,0 +1,3 @@
+module FormatError where
+
+foo = let x = 1 in in x