Fix for ticket #10078: ensure that tcPluginStop is called even in case of type errors

Summary:
Remove unused variable that appeared through the fix for ticket #10078

Merge branch 'master' of git://git.haskell.org/ghc

Added comment with bug ID.

Reviewers: adamgundry, gridaphobe, austin

Reviewed By: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D667

GHC Trac Issues: #10078

(cherry picked from commit fd581a73)

compiler/typecheck/TcRnDriver.hs

@@ -2127,9 +2127,14 @@ withTcPlugins hsc_env m =
      case plugins of
        [] -> m  -- Common fast case
        _  -> do (solvers,stops) <- unzip `fmap` mapM startPlugin plugins
-                res <- updGblEnv (\e -> e { tcg_tc_plugins = solvers }) m
+                -- This ensures that tcPluginStop is called even if a type
+                -- error occurs during compilation (Fix of #10078)
+                eitherRes <- tryM $ do
+                  updGblEnv (\e -> e { tcg_tc_plugins = solvers }) m
                 mapM_ runTcPluginM stops
-                return res
+                case eitherRes of
+                  Left _ -> failM
+                  Right res -> return res
   where
   startPlugin (TcPlugin start solve stop) =
     do s <- runTcPluginM start