Always preserve expanded lists

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

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

An example of this is build inputs, which usually start out with just a
single element, but usually get more elements later. This would've been
contracted onto a single line before this change:

    buildInputs = [
      libfoo
    ];

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

@@ -205,13 +205,19 @@ prettyTerm (Selection term selectors rest) =
 
 -- Empty list
 prettyTerm (List paropen@Ann{trailComment = Nothing} (Items []) parclose@Ann{preTrivia = []}) =
-  pretty paropen <> hardspace <> pretty parclose
+  pretty paropen <> sep <> pretty parclose
+  where
+    -- If the brackets are on different lines, keep them like that
+    sep = if sourceLine paropen /= sourceLine parclose then hardline else hardspace
 -- General list
 -- Always expand if len > 1
 prettyTerm (List paropen@Ann{trailComment = post} items parclose) =
   pretty (paropen{trailComment = Nothing})
-    <> surroundWith line (nest $ pretty post <> prettyItems items)
+    <> surroundWith sur (nest $ pretty post <> prettyItems items)
     <> pretty parclose
+  where
+    -- If the brackets are on different lines, keep them like that
+    sur = if sourceLine paropen /= sourceLine parclose then hardline else line
 prettyTerm (Set krec paropen items parclose) = prettySet False (krec, paropen, items, parclose)
 -- Parentheses
 prettyTerm (Parenthesized paropen expr parclose@Ann{preTrivia = closePre}) =

test/diff/idioms_pkgs_3/out.nix

@@ -237,7 +237,9 @@ buildStdenv.mkDerivation ({
 
   inherit src unpackPhase meta;
 
-  outputs = [ "out" ] ++ lib.optionals crashreporterSupport [ "symbols" ];
+  outputs = [
+    "out"
+  ] ++ lib.optionals crashreporterSupport [ "symbols" ];
 
   # Add another configure-build-profiling run before the final configure phase if we build with pgo
   preConfigurePhases = lib.optionals pgoSupport [

test/diff/idioms_pkgs_4/out.nix

@@ -84,9 +84,13 @@ let
     ]
     ++ (
       if system == "i686-cygwin" then
-        [ ../cygwin/rebase-i686.sh ]
+        [
+          ../cygwin/rebase-i686.sh
+        ]
       else if system == "x86_64-cygwin" then
-        [ ../cygwin/rebase-x86_64.sh ]
+        [
+          ../cygwin/rebase-x86_64.sh
+        ]
       else
         [ ]
     );

test/diff/lists/in.nix

@@ -1,4 +1,7 @@
 [
+  []
+  [
+  ]
   [
 
   ]

test/diff/lists/out-pure.nix

@@ -1,4 +1,6 @@
 [
+  [ ]
+  [ ]
   [
 
   ]

test/diff/lists/out.nix

@@ -1,4 +1,7 @@
 [
+  [ ]
+  [
+  ]
   [
 
   ]
@@ -46,7 +49,9 @@
   ]
   [ 1 ]
 
-  [ 1 ]
+  [
+    1
+  ]
 
   [
     b

test/diff/operation/out.nix

@@ -230,7 +230,9 @@
       zip
       zlib
     ]
-    ++ [ (if (lib.versionAtLeast version "103") then nss_latest else nss_esr) ]
+    ++ [
+      (if (lib.versionAtLeast version "103") then nss_latest else nss_esr)
+    ]
   )
 
   # Indentation with parenthesized multiline function call