Support DatatypeContexts

CHANGELOG.md

@@ -5,6 +5,9 @@
   1002](https://github.com/tweag/ormolu/issues/1002) and [issue
   730](https://github.com/tweag/ormolu/issues/730).
 
+* Support the (deprecated) `DatatypeContexts` extension to avoid surprises.
+  [Issue 1012](https://github.com/tweag/ormolu/issues/1012).
+
 ## Ormolu 0.6.0.1
 
 * Fix false positives in AST diffing related to `UnicodeSyntax`. [PR

data/examples/declaration/data/datatype-contexts-out.hs

@@ -0,0 +1,11 @@
+data (IsString s) => T s = T
+
+data
+  (IsString s) =>
+  T s = T
+
+data
+  ( IsString s,
+    IsString s
+  ) =>
+  T s = T

data/examples/declaration/data/datatype-contexts.hs

@@ -0,0 +1,8 @@
+data IsString s => T s = T
+
+data IsString s =>
+  T s = T
+
+data
+  ( IsString s
+  , IsString s ) =>  T s = T

expected-failures/brittany.txt

@@ -23,36 +23,6 @@ data/Test394.hs
 
   Formatting is not idempotent.
   Please, consider reporting the bug.
-data/Test51.hs
-@@ -1,14 +1,11 @@
-  {-# LANGUAGE DatatypeContexts #-}
-- data
--   ( LooooooooooooooooooooongConstraint a
--   , LooooooooooooooooooooongConstraint b
--   ) =>
--   MyRecord a b
--   = MyConstructor
--     { foo1, foo2
--         :: loooooooooooooooooooooooooooooooong
--         -> loooooooooooooooooooooooooooooooong
--     , bar  :: a
--     , bazz :: b
--     }
-
-+ data MyRecord a b = MyConstructor
-+   { foo1,
-+     foo2 ::
-+       loooooooooooooooooooooooooooooooong ->
-+       loooooooooooooooooooooooooooooooong,
-+     bar :: a,
-+     bazz :: b
-+   }
-+
-
-  AST of input and AST of formatted code differ.
-    at data/Test51.hs:(2,1)-(13,5)
-  Please, consider reporting the bug.
-  To format anyway, use --unsafe.
 data/Test65.hs
 @@ -1,9 +1,9 @@
   data Foo = Bar

src/Ormolu/Printer/Meat/Declaration/Data.hs

@@ -59,10 +59,18 @@ p_dataDecl style name tpats fixity HsDataDefn {..} = do
       txt " #-}"
   let constructorSpans = getLocA name : fmap lhsTypeArgSrcSpan tpats
       sigSpans = maybeToList . fmap getLocA $ dd_kindSig
-      declHeaderSpans = constructorSpans ++ sigSpans
+      declHeaderSpans =
+        maybeToList (getLocA <$> dd_ctxt) ++ constructorSpans ++ sigSpans
   switchLayout declHeaderSpans $ do
     breakpoint
     inci $ do
+      case dd_ctxt of
+        Nothing -> pure ()
+        Just ctxt -> do
+          located ctxt p_hsContext
+          space
+          txt "=>"
+          breakpoint
       switchLayout constructorSpans $
         p_infixDefHelper
           (isInfix fixity)