Clarify modification time comparisons for .hi and .hie files

Addresses #591

Author: pepeiborra

src/Development/IDE/Core/Rules.hs

@@ -142,9 +142,12 @@ getHomeHieFile f = do
   mbHieTimestamp <- use GetModificationTime normal_hie_f
   srcTimestamp   <- use_ GetModificationTime f
 
-  let isUpToDate
-        | Just d <- mbHieTimestamp = comparing modificationTime d srcTimestamp == GT
-        | otherwise = False
+  let isUpToDate = case (modificationTime <$> mbHieTimestamp, modificationTime srcTimestamp) of
+          -- If the src file is in VFS, then we know it must have been typechecked
+          -- by now (due to kick) and therefore its .hie file is up to date
+          (Just _, Nothing) -> True
+          (Just (Just a), Just b) -> a >= b
+          _ -> False
 
   unless isUpToDate $
        void $ use_ TypeCheck f
@@ -555,8 +558,8 @@ getHiFileRule = defineEarlyCutoff $ \GetHiFile f -> do
     Just deps -> do
         mbHiVersion <- use  GetModificationTime hiFile
         modVersion  <- use_ GetModificationTime f
-        case mbHiVersion of
-            Just hiVersion
+        case (mbHiVersion, modVersion) of
+            (Just hiVersion, ModificationTime{})
               | modificationTime hiVersion >= modificationTime modVersion -> do
                 session <- hscEnv <$> use_ GhcSession f
                 r <- liftIO $ loadInterface session ms deps
@@ -567,6 +570,8 @@ getHiFileRule = defineEarlyCutoff $ \GetHiFile f -> do
                   Left err -> do
                     let diag = ideErrorWithSource (Just "interface file loading") (Just DsError) f . T.pack $ err
                     return (Nothing, (pure diag, Nothing))
+            (_, VFSVersion{}) ->
+                error "internal error - GetHiFile of file of interest"
             _ ->
               pure (Nothing, ([], Nothing))