Send warning to LSP client when used on GHC 9

Author: konn

ghcide/src/Development/IDE/Main.hs

@@ -1,4 +1,5 @@
 {-# OPTIONS_GHC -Wno-orphans #-}
+{-# LANGUAGE CPP #-}
 module Development.IDE.Main
 (Arguments(..)
 ,Command(..)
@@ -85,6 +86,9 @@ import           Ide.Types                             (IdeCommand (IdeCommand),
                                                         PluginId (PluginId),
                                                         ipMap)
 import qualified Language.LSP.Server                   as LSP
+#if (defined(MIN_VERSION_GLASGOW_HASKELL) && (MIN_VERSION_GLASGOW_HASKELL(9,0,1,0)))
+import qualified Language.LSP.Types                    as LSP
+#endif
 import           Options.Applicative                   hiding (action)
 import qualified System.Directory.Extra                as IO
 import           System.Exit                           (ExitCode (ExitFailure),
@@ -256,6 +260,14 @@ defaultMain Arguments{..} = do
                             , optRunSubset = runSubset
                             }
                     caps = LSP.resClientCapabilities env
+-- FIXME: Remove this after GHC 9 gets fully supported
+#if (defined(MIN_VERSION_GLASGOW_HASKELL) && (MIN_VERSION_GLASGOW_HASKELL(9,0,1,0)))
+                LSP.runLspT env $
+                    LSP.sendNotification LSP.SWindowShowMessage $
+                    LSP.ShowMessageParams LSP.MtWarning $
+                    "Currently, HLS supports GHC 9 only partially. "
+                    <> "See [issue #297](https://github.com/haskell/haskell-language-server/issues/297) for more detail."
+#endif
                 initialise
                     argsDefaultHlsConfig
                     rules