Support plugins (haskell/ghcide#192)

* Support plugins

Call initializePlugins before running typecheck.

* call initializePlugins only for GHC >= 8.6

initializePlugins doesn't exist in older GHC versions.

* A separate function for initializing plugins

* Add a test for plugins

ghcide/ghcide.cabal

@@ -187,6 +187,7 @@ test-suite ghcide-tests
         ghc,
         --------------------------------------------------------------
         ghcide,
+        ghc-typelits-knownnat,
         haskell-lsp-types,
         lens,
         lsp-test >= 0.8,

ghcide/src/Development/IDE/Core/Compile.hs

@@ -28,6 +28,10 @@ import qualified GHC.LanguageExtensions.Type as GHC
 import Development.IDE.Types.Options
 import Development.IDE.Types.Location
 
+#if MIN_GHC_API_VERSION(8,6,0)
+import           DynamicLoading (initializePlugins)
+#endif
+
 import           GHC hiding (parseModule, typecheckModule)
 import qualified Parser
 import           Lexer
@@ -95,11 +99,23 @@ typecheckModule (IdeDefer defer) packageState deps pm =
     runGhcEnv packageState $
         catchSrcErrors "typecheck" $ do
             setupEnv deps
+            let modSummary = pm_mod_summary pm
+            modSummary' <- initPlugins modSummary
             (warnings, tcm) <- withWarnings "typecheck" $ \tweak ->
-                GHC.typecheckModule $ demoteIfDefer pm{pm_mod_summary = tweak $ pm_mod_summary pm}
+                GHC.typecheckModule $ demoteIfDefer pm{pm_mod_summary = tweak modSummary'}
             tcm2 <- mkTcModuleResult tcm
             return (map unDefer warnings, tcm2)
 
+initPlugins :: GhcMonad m => ModSummary -> m ModSummary
+initPlugins modSummary = do
+#if MIN_GHC_API_VERSION(8,6,0)
+    session <- getSession
+    dflags <- liftIO $ initializePlugins session (ms_hspp_opts modSummary)
+    return modSummary{ms_hspp_opts = dflags}
+#else
+    return modSummary
+#endif
+
 -- | Compile a single type-checked module to a 'CoreModule' value, or
 -- provide errors.
 compileModule

ghcide/test/exe/Main.hs

@@ -40,6 +40,7 @@ main = defaultMain $ testGroup "HIE"
   , diagnosticTests
   , codeActionTests
   , findDefinitionAndHoverTests
+  , pluginTests
   ]
 
 initializeResponseTests :: TestTree
@@ -810,6 +811,28 @@ findDefinitionAndHoverTests = let
         broken = Just . (`xfail` "known broken")
       --  no = const Nothing -- don't run this test at all
 
+pluginTests :: TestTree
+pluginTests = testSessionWait "plugins" $ do
+  let content =
+        T.unlines
+          [ "{-# OPTIONS_GHC -fplugin GHC.TypeLits.KnownNat.Solver #-}"
+          , "{-# LANGUAGE DataKinds, ScopedTypeVariables, TypeOperators #-}"
+          , "module Testing where"
+          , "import Data.Proxy"
+          , "import GHC.TypeLits"
+          -- This function fails without plugins being initialized.
+          , "f :: forall n. KnownNat n => Proxy n -> Integer"
+          , "f _ = natVal (Proxy :: Proxy n) + natVal (Proxy :: Proxy (n+2))"
+          , "foo :: Int -> Int -> Int"
+          , "foo a b = a + c"
+          ]
+  _ <- openDoc' "Testing.hs" "haskell" content
+  expectDiagnostics
+    [ ( "Testing.hs",
+        [(DsError, (8, 14), "Variable not in scope: c")]
+      )
+    ]
+
 xfail :: TestTree -> String -> TestTree
 xfail = flip expectFailBecause