Fix AST diffing for empty Haddock comments in data decls

CHANGELOG.md

@@ -9,6 +9,9 @@
 * Format parenthesized operators starting with a `#` correctly in the presence
   of `UnboxedSums`. [Issue 1062](https://github.com/tweag/ormolu/issues/1062).
 
+* Fix false positives in AST diffing related to empty Haddock comments in data
+  declarations. [Issue 1065](https://github.com/tweag/ormolu/issues/1065).
+
 ## Ormolu 0.7.1.0
 
 * Include `base` fixity information when formatting a Haskell file that's

data/examples/other/empty-haddock-out.hs

@@ -5,3 +5,5 @@ where
 
 test ::
   test
+
+data T = T

data/examples/other/empty-haddock.hs

@@ -9,3 +9,5 @@ where
 test ::
   -- |
   test
+
+data T = T {- ^ -}

src/Ormolu/Parser.hs

@@ -177,7 +177,7 @@ parseModuleSnippet Config {..} modFixityMap dynFlags path rawInput = liftIO $ do
 normalizeModule :: HsModule GhcPs -> HsModule GhcPs
 normalizeModule hsmod =
   everywhere
-    (extT (mkT dropBlankTypeHaddocks) patchContext)
+    (mkT dropBlankTypeHaddocks `extT` dropBlankDataDeclHaddocks `extT` patchContext)
     hsmod
       { hsmodImports =
           normalizeImports (hsmodImports hsmod),
@@ -204,6 +204,13 @@ normalizeModule hsmod =
       L _ (HsDocTy _ ty s) :: LHsType GhcPs
         | isBlankDocString s -> ty
       a -> a
+    dropBlankDataDeclHaddocks = \case
+      ConDeclGADT {con_doc = Just s, ..} :: ConDecl GhcPs
+        | isBlankDocString s -> ConDeclGADT {con_doc = Nothing, ..}
+      ConDeclH98 {con_doc = Just s, ..} :: ConDecl GhcPs
+        | isBlankDocString s -> ConDeclH98 {con_doc = Nothing, ..}
+      a -> a
+
     patchContext :: LHsContext GhcPs -> LHsContext GhcPs
     patchContext = fmap $ \case
       [x@(L _ (HsParTy _ _))] -> [x]