Always preserve expanded attribute parameters

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

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

An example of this is package recipes, which usually start out with just a
single attribute argument, but usually get more later. This would've been
contracted onto a single line before this change:

    {
      stdenv,
    }:

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

@@ -315,7 +315,9 @@ instance Pretty Parameter where
   pretty (IDParameter i) = pretty i
   -- {}:
   pretty (SetParameter bopen [] bclose) =
-    group $ pretty (moveTrailingCommentUp bopen) <> hardspace <> pretty bclose
+    group $ pretty (moveTrailingCommentUp bopen) <> sep <> pretty bclose
+    where
+      sep = if sourceLine bopen /= sourceLine bclose then hardline else hardspace
   -- { stuff }:
   pretty (SetParameter bopen attrs bclose) =
     group $
@@ -332,14 +334,17 @@ instance Pretty Parameter where
         [pretty (ParamAttr name maybeDefault Nothing) <> trailing ","]
       handleTrailingComma (x : xs) = pretty x : handleTrailingComma xs
 
-      sep = case attrs of
-        [ParamEllipsis _] -> line
-        -- Attributes must be without default
-        [ParamAttr _ Nothing _] -> line
-        [ParamAttr _ Nothing _, ParamEllipsis _] -> line
-        [ParamAttr _ Nothing _, ParamAttr _ Nothing _] -> line
-        [ParamAttr _ Nothing _, ParamAttr _ Nothing _, ParamEllipsis _] -> line
-        _ -> hardline
+      sep =
+        if sourceLine bopen /= sourceLine bclose
+          then hardline
+          else case attrs of
+            [ParamEllipsis _] -> line
+            -- Attributes must be without default
+            [ParamAttr _ Nothing _] -> line
+            [ParamAttr _ Nothing _, ParamEllipsis _] -> line
+            [ParamAttr _ Nothing _, ParamAttr _ Nothing _] -> line
+            [ParamAttr _ Nothing _, ParamAttr _ Nothing _, ParamEllipsis _] -> line
+            _ -> hardline
   pretty (ContextParameter param1 at param2) =
     pretty param1 <> pretty at <> pretty param2
 

test/diff/idioms_lib_3/out.nix

@@ -12,7 +12,9 @@
 #
 # Tests can be found in ./tests/misc.nix
 # Documentation in the manual, #sec-generators
-{ lib }:
+{
+  lib,
+}:
 with (lib).trivial;
 let
   libStr = lib.strings;
@@ -29,7 +31,8 @@ rec {
   # The builtin `toString` function has some strange defaults,
   # suitable for bash scripts but not much else.
   mkValueStringDefault =
-    { }:
+    {
+    }:
     v:
     with builtins;
     let
@@ -203,7 +206,10 @@ rec {
       # allow lists as values for duplicate keys
       listsAsDuplicateKeys ? false,
     }:
-    { globalSection, sections }:
+    {
+      globalSection,
+      sections,
+    }:
     (
       if globalSection == { } then
         ""
@@ -279,7 +285,10 @@ rec {
 
   # Generates JSON from an arbitrary (non-function) value.
   # For more information see the documentation of the builtin.
-  toJSON = { }: builtins.toJSON;
+  toJSON =
+    {
+    }:
+    builtins.toJSON;
 
   # YAML has been a strict superset of JSON since 1.2, so we
   # use toJSON. Before it only had a few differences referring
@@ -465,7 +474,8 @@ rec {
 
   # PLIST handling
   toPlist =
-    { }:
+    {
+    }:
     v:
     let
       isFloat = builtins.isFloat or (x: false);
@@ -546,7 +556,8 @@ rec {
   # Note that integers are translated to Integer and never
   # the Natural type.
   toDhall =
-    { }@args:
+    {
+    }@args:
     v:
     with builtins;
     let

test/diff/key_value/out.nix

@@ -56,7 +56,12 @@ rec {
       c = 2;
     };
   n = pkgs: { };
-  o = { pkgs, ... }: { };
+  o =
+    {
+      pkgs,
+      ...
+    }:
+    { };
 
   a
     # b

test/diff/lambda/in.nix

@@ -2,6 +2,13 @@ let
   inherit lib;
 in
 [
+  ({}: null)
+  ({
+   }: null)
+  ({
+
+   }: null)
+
   (
     { lib, }:
     let

test/diff/lambda/out.nix

@@ -2,6 +2,19 @@ let
   inherit lib;
 in
 [
+  ({ }: null)
+  (
+    {
+    }:
+    null
+  )
+  (
+    {
+
+    }:
+    null
+  )
+
   (
     { lib }:
     let
@@ -81,15 +94,22 @@ in
 
   (
     a:
-    { b, ... }:
+    {
+      b,
+      ...
+    }:
     c: {
       # Stuff
     }
   )
 
   (
     a:
-    { b, c, ... }:
+    {
+      b,
+      c,
+      ...
+    }:
     d: {
       # Stuff
     }