List, Attrset: Preserve empty line when there are no items

src/Nixfmt/Pretty.hs

@@ -64,7 +64,10 @@ instance Pretty a => Pretty (Item a) where
 
 -- For lists, attribute sets and let bindings
 prettyItems :: Pretty a => Doc -> Items a -> Doc
-prettyItems sep = prettyItems' . unItems
+-- Special case: Preserve an empty line with no items
+-- usually, trailing newlines after the last element are not preserved
+prettyItems _ (Items [DetachedComments []]) = emptyline
+prettyItems sep items = prettyItems' $ unItems items
   where
     prettyItems' :: Pretty a => [Item a] -> Doc
     prettyItems' [] = mempty
@@ -124,9 +127,9 @@ instance Pretty Binder where
 -- in some situations even that is not sufficient. The wide parameter will
 -- be even more eager at expanding, except for empty sets and inherit statements.
 prettySet :: Bool -> (Maybe Leaf, Leaf, Items Binder, Leaf) -> Doc
--- Empty, non-recursive attribute set
-prettySet _ (Nothing, Ann [] paropen Nothing, Items [], parclose@(Ann [] _ _))
-    = pretty paropen <> hardspace <> pretty parclose
+-- Empty attribute set
+prettySet _ (krec, Ann [] paropen Nothing, Items [], parclose@(Ann [] _ _))
+    = pretty (fmap (, hardspace) krec) <> pretty paropen <> hardspace <> pretty parclose
 -- Singleton sets are allowed to fit onto one line,
 -- but apart from that always expand.
 prettySet wide (krec, Ann pre paropen post, binders, parclose)

test/diff/attr_set/in.nix

@@ -5,6 +5,14 @@
   {a=1;
   }
 
+  {
+
+  }
+
+  { a = {
+
+  };}
+
   {       b=1;       }
   {       b=1; /*c*/ }
   { /*a*/ b=1;       }

test/diff/attr_set/out.nix

@@ -6,6 +6,16 @@
   { a = 1; }
   { a = 1; }
 
+  {
+
+  }
+
+  {
+    a = {
+
+    };
+  }
+
   { b = 1; }
   {
     b = 1; # c

test/diff/lists/in.nix

@@ -1,4 +1,12 @@
 [
+  [
+
+  ]
+
+  [ [
+
+  ] ]
+
   [ {
     # multiline
     foo = "bar";

test/diff/lists/out.nix

@@ -1,4 +1,14 @@
 [
+  [
+
+  ]
+
+  [
+    [
+
+    ]
+  ]
+
   [
     {
       # multiline