Avoid lookupName on ghc-lib (#327)

This fixes an issue that we encountered in DAML. I’ll add a test for
this in the DAML repo since we cannot test the ghc-lib codepath
here (since we don’t setup an environment that works).

Author: cocreature

.hlint.yaml

@@ -76,6 +76,7 @@
   - {name: ImplicitParams, within: []}
   - name: CPP
     within:
+    - Development.IDE.Core.Completions
     - Development.IDE.Core.FileStore
     - Development.IDE.Core.Compile
     - Development.IDE.GHC.Compat

src/Development/IDE/Core/Completions.hs

@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 -- Mostly taken from "haskell-ide-engine"
 module Development.IDE.Core.Completions (
   CachedCompletions
@@ -32,22 +33,26 @@ import Language.Haskell.LSP.Types.Capabilities
 import qualified Language.Haskell.LSP.VFS as VFS
 import Development.IDE.Core.CompletionsTypes
 import Development.IDE.Spans.Documentation
-import Development.IDE.GHC.Util
 import Development.IDE.GHC.Error
 import Development.IDE.Types.Options
 
--- From haskell-ide-engine/src/Haskell/Ide/Engine/Support/HieExtras.hs
+#ifndef GHC_LIB
+import Development.IDE.GHC.Util
 
-safeTyThingId :: TyThing -> Maybe Id
-safeTyThingId (AnId i)                    = Just i
-safeTyThingId (AConLike (RealDataCon dc)) = Just $ dataConWrapId dc
-safeTyThingId _                           = Nothing
 
 safeTyThingType :: TyThing -> Maybe Type
 safeTyThingType thing
   | Just i <- safeTyThingId thing = Just (varType i)
 safeTyThingType (ATyCon tycon)    = Just (tyConKind tycon)
 safeTyThingType _                 = Nothing
+#endif
+
+-- From haskell-ide-engine/src/Haskell/Ide/Engine/Support/HieExtras.hs
+
+safeTyThingId :: TyThing -> Maybe Id
+safeTyThingId (AnId i)                    = Just i
+safeTyThingId (AConLike (RealDataCon dc)) = Just $ dataConWrapId dc
+safeTyThingId _                           = Nothing
 
 -- From haskell-ide-engine/hie-plugin-api/Haskell/Ide/Engine/Context.hs
 
@@ -276,9 +281,15 @@ cacheDataProducer packageState dflags tm tcs = do
       toCompItem :: ModuleName -> Name -> IO CompItem
       toCompItem mn n = do
         docs <- getDocumentationTryGhc packageState (tm:tcs) n
+-- lookupName uses runInteractiveHsc, i.e., GHCi stuff which does not work with GHCi
+-- and leads to fun errors like "Cannot continue after interface file error".
+#ifdef GHC_LIB
+        let ty = Right Nothing
+#else
         ty <- runGhcEnv packageState $ catchSrcErrors "completion" $ do
                 name' <- lookupName n
                 return $ name' >>= safeTyThingType
+#endif
         return $ CI n (showModName mn) (either (const Nothing) id ty) (T.pack $ showGhc n) Nothing docs
 
   (unquals,quals) <- getCompls rdrElts
@@ -515,4 +526,4 @@ prefixes =
   , "$t"
   , "$c"
   , "$m"
-  ]
+  ]