enable GhcSessionDeps reuse

Author: pepeiborra

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

@@ -697,9 +697,16 @@ loadGhcSession ghcSessionDepsConfig = do
                 Nothing -> LBS.toStrict $ B.encode (hash (snd val))
         return (Just cutoffHash, val)
 
-    defineNoDiagnostics $ \(GhcSessionDeps_ fullModSummary) file -> do
-        env <- use_ GhcSession file
-        ghcSessionDepsDefinition fullModSummary ghcSessionDepsConfig env file
+    defineEarlyCutoff $ RuleWithCustomNewnessCheck (<=) $
+    -- This rule uses a custom newness check that relies on the derived 'Ord' instance:
+    --   * <previous> -> <new>
+    --   * GhcSessionDeps True -> GhcSessionDeps False: the prev session can be reused
+    --   * GhcSessionDeps False -> GhcSessionDeps True: the prev session cannot be reused
+        \(GhcSessionDeps_ fullModSummary) file -> do
+            env <- use_ GhcSession file
+            res <- ghcSessionDepsDefinition fullModSummary ghcSessionDepsConfig env file
+            return (Nothing, res)
+
 
 newtype GhcSessionDepsConfig = GhcSessionDepsConfig
     { checkForImportCycles :: Bool