Always preserve expanded attribute sets

This changes the formatting to take into account whether attribute sets
are expanded onto multiple lines already, and if so, preserves that.

This is desirable because if a user writes an attribute set over
multiple lines, there's most likely an intention to add more attributes,
which would become harder when formatted.

An example of this is module system options, which usually start out
with just a type (or even no attributes), but usually get more
attributes later. This would've been contracted onto a single line
before this change:

    options.foo = lib.mkOption {
      type = types.str;
    };

This change conforms to the standard due to this line:

> The formatter may take the input formatting into account in some cases in order to preserve multi-line syntax elements (which would otherwise have been contracted by the rules).

src/Nixfmt/Pretty.hs

@@ -159,7 +159,10 @@ instance Pretty Binder where
 prettySet :: Bool -> (Maybe Leaf, Leaf, Items Binder, Leaf) -> Doc
 -- Empty attribute set
 prettySet _ (krec, paropen@(LoneAnn _), Items [], parclose@Ann{preTrivia = []}) =
-  pretty (fmap (,hardspace) krec) <> pretty paropen <> hardspace <> pretty parclose
+  pretty (fmap (,hardspace) krec) <> pretty paropen <> sep <> pretty parclose
+  where
+    -- If the braces are on different lines, keep them like that
+    sep = if sourceLine paropen /= sourceLine parclose then hardline else hardspace
 -- Singleton sets are allowed to fit onto one line,
 -- but apart from that always expand.
 prettySet wide (krec, paropen@Ann{trailComment = post}, binders, parclose) =
@@ -168,7 +171,12 @@ prettySet wide (krec, paropen@Ann{trailComment = post}, binders, parclose) =
     <> surroundWith sep (nest $ pretty post <> prettyItems binders)
     <> pretty parclose
   where
-    sep = if wide && not (null (unItems binders)) then hardline else line
+    sep =
+      if wide && not (null (unItems binders))
+        -- If the braces are on different lines, keep them like that
+        || sourceLine paropen /= sourceLine parclose
+        then hardline
+        else line
 
 prettyTermWide :: Term -> Doc
 prettyTermWide (Set krec paropen items parclose) = prettySet True (krec, paropen, items, parclose)

test/diff/apply/out.nix

@@ -113,17 +113,25 @@
     name3 = function arg { asdf = 1; } { qwer = 12345; } argument;
   }
   {
-    name1 = function arg { asdf = 1; };
+    name1 = function arg {
+      asdf = 1;
+    };
 
     name2 = function arg {
       asdf = 1;
       # multiline
     } argument;
 
-    name3 = function arg {
-      asdf = 1;
-      # multiline
-    } { qwer = 12345; } argument;
+    name3 =
+      function arg
+        {
+          asdf = 1;
+          # multiline
+        }
+        {
+          qwer = 12345;
+        }
+        argument;
   }
   {
     name4 = function arg { asdf = 1; } {

test/diff/attr_set/in.nix

@@ -5,6 +5,8 @@
   {a=1;
   }
 
+  {
+  }
   {
 
   }

test/diff/attr_set/out-pure.nix

@@ -6,6 +6,7 @@
   { a = 1; }
   { a = 1; }
 
+  { }
   {
 
   }

test/diff/attr_set/out.nix

@@ -4,8 +4,12 @@
     # a
   }
   { a = 1; }
-  { a = 1; }
+  {
+    a = 1;
+  }
 
+  {
+  }
   {
 
   }

test/diff/idioms_lib_3/out.nix

@@ -263,9 +263,13 @@ rec {
             if isAttrs value && !lib.isDerivation value then
               lib.mapAttrsToList (name: value: recurse ([ name ] ++ path) value) value
             else if length path > 1 then
-              { ${concatStringsSep "." (lib.reverseList (tail path))}.${head path} = value; }
+              {
+                ${concatStringsSep "." (lib.reverseList (tail path))}.${head path} = value;
+              }
             else
-              { ${head path} = value; };
+              {
+                ${head path} = value;
+              };
         in
         attrs: lib.foldl lib.recursiveUpdate { } (lib.flatten (recurse [ ] attrs));
 

test/diff/idioms_nixos_2/out.nix

@@ -51,7 +51,10 @@ let
       post_max_size = cfg.maxUploadSize;
       memory_limit = cfg.maxUploadSize;
     }
-    // cfg.phpOptions // optionalAttrs cfg.caching.apcu { "apc.enable_cli" = "1"; };
+    // cfg.phpOptions
+    // optionalAttrs cfg.caching.apcu {
+      "apc.enable_cli" = "1";
+    };
 
   occ = pkgs.writeScriptBin "nextcloud-occ" ''
     #! ${pkgs.runtimeShell}

test/diff/idioms_pkgs_1/out.nix

@@ -8,7 +8,9 @@
 stdenv.mkDerivation rec {
   pname = "test";
   version = "0.0";
-  src = fetchFrom { url = "example/${version}"; };
+  src = fetchFrom {
+    url = "example/${version}";
+  };
   meta = with lib; {
     maintainers = with maintainers; [ someone ];
     description = "something";

test/diff/idioms_pkgs_4/out.nix

@@ -197,9 +197,13 @@ in
   # First build a stdenv based only on tools outside the store.
   (prevStage: {
     inherit config overlays;
-    stdenv = makeStdenv { inherit (prevStage) cc fetchurl; } // {
-      inherit (prevStage) fetchurl;
-    };
+    stdenv =
+      makeStdenv {
+        inherit (prevStage) cc fetchurl;
+      }
+      // {
+        inherit (prevStage) fetchurl;
+      };
   })
 
   # Using that, build a stdenv that adds the ‘xz’ command (which most systems

test/diff/idioms_pkgs_5/out.nix

@@ -628,9 +628,14 @@ let
             enableParallelChecking = attrs.enableParallelChecking or true;
             enableParallelInstalling = attrs.enableParallelInstalling or true;
           }
-          // optionalAttrs (
-            hardeningDisable != [ ] || hardeningEnable != [ ] || stdenv.hostPlatform.isMusl
-          ) { NIX_HARDENING_ENABLE = enabledHardeningOptions; }
+          //
+            optionalAttrs
+              (
+                hardeningDisable != [ ] || hardeningEnable != [ ] || stdenv.hostPlatform.isMusl
+              )
+              {
+                NIX_HARDENING_ENABLE = enabledHardeningOptions;
+              }
           //
             optionalAttrs (stdenv.hostPlatform.isx86_64 && stdenv.hostPlatform ? gcc.arch)
               {

test/diff/if_else/out.nix

@@ -1,5 +1,14 @@
 [
-  (if true then { version = "1.2.3"; } else { version = "3.2.1"; })
+  (
+    if true then
+      {
+        version = "1.2.3";
+      }
+    else
+      {
+        version = "3.2.1";
+      }
+  )
   (
     if true then
       ''

test/diff/inherit/out.nix

@@ -18,7 +18,9 @@
       j
       ;
   }
-  { inherit aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa; }
+  {
+    inherit aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa;
+  }
   { inherit b d; }
   {
     inherit

test/diff/with/out.nix

@@ -89,7 +89,9 @@
       b = 2;
     }
   )
-  { a = with b; with b; with b; 1; }
+  {
+    a = with b; with b; with b; 1;
+  }
   {
     binPath =
       with pkgs;