Warn GHC 9 Compatibility to LSP Client

README.md

@@ -140,7 +140,7 @@ ghcup install hls
 If you are using [`chocolatey`](https://chocolatey.org/) to manage your installations in windows, [you can install `haskell-language-server`](https://community.chocolatey.org/packages/haskell-language-server) with
 ```
 choco install haskell-language-server
-```` 
+````
 
 ### Visual Studio Code
 
@@ -703,6 +703,11 @@ args = ["--lsp"]
 
 ## Known limitations
 
+### Limited GHC 9 Support
+Currently, GHC 9 support in HLS is in beta stage; some features are unavailable with GHC >= 9.0.1 for the time being.
+This situation is expected to be resolved in the near future after all the dependent packages support GHC 9.
+See issue [#297](https://github.com/haskell/haskell-language-server/issues/297) for the supported features in GHC 9 and the status of migration progress.
+
 ### Preprocessor
 HLS is not yet able to find project preprocessors, which may result in `could not execute: <preprocessor>` errors. This problem is
 tracked in https://github.com/haskell/haskell-language-server/issues/176 and originally comes from https://github.com/mpickering/hie-bios/issues/125

ghcide/src/Development/IDE/Main.hs

@@ -25,7 +25,8 @@ import qualified Data.Text                             as T
 import qualified Data.Text.IO                          as T
 import           Data.Text.Lazy.Encoding               (decodeUtf8)
 import qualified Data.Text.Lazy.IO                     as LT
-import           Development.IDE                       (Action, Rules,
+import           Development.IDE                       (Action, GhcVersion (..),
+                                                        Rules, ghcVersion,
                                                         hDuplicateTo')
 import           Development.IDE.Core.Debouncer        (Debouncer,
                                                         newAsyncDebouncer)
@@ -256,6 +257,11 @@ defaultMain Arguments{..} = do
                             , optRunSubset = runSubset
                             }
                     caps = LSP.resClientCapabilities env
+                -- FIXME: Remove this after GHC 9 gets fully supported
+                when (ghcVersion == GHC90) $
+                    hPutStrLn stderr $
+                        "Currently, HLS supports GHC 9 only partially. "
+                        <> "See [issue #297](https://github.com/haskell/haskell-language-server/issues/297) for more detail."
                 initialise
                     argsDefaultHlsConfig
                     rules