add comments and use custom newness check in ghcide too

Author: pepeiborra

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

@@ -49,6 +49,9 @@ data LinkableType = ObjectLinkable | BCOLinkable
 instance Hashable LinkableType
 instance NFData   LinkableType
 
+-- | Encode the linkable into an ordered bytestring.
+--   This is used to drive an ordered "newness" predicate in the
+--   'NeedsCompilation' build rule.
 encodeLinkableType :: Maybe LinkableType -> ByteString
 encodeLinkableType Nothing               = "0"
 encodeLinkableType (Just BCOLinkable)    = "1"

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

@@ -1080,7 +1080,13 @@ mainRule = do
     getClientSettingsRule
     getHieAstsRule
     getBindingsRule
-    defineEarlyCutoff $ RuleNoDiagnostics $ \NeedsCompilation file ->
+    -- This rule uses a custom newness check that relies on the encoding
+    --  produced by 'encodeLinkable'. This works as follows:
+    --   * <previous> -> <new>
+    --   * ObjectLinkable -> BCOLinkable : the prev linkable can be reused,  signal "no change"
+    --   * Object/BCO -> NoLinkable      : the prev linkable can be ignored, signal "no change"
+    --   * otherwise                     : the prev linkable cannot be reused, signal "value has changed"
+    defineEarlyCutoff $ RuleWithCustomNewnessCheck (<=) $ \NeedsCompilation file ->
         needsCompilationRule file
     generateCoreRule
     getImportMapRule