Only rearrange not in if explicitly opted-in

Author: José Valim

lib/elixir/lib/code/formatter.ex

@@ -506,8 +506,15 @@ defmodule Code.Formatter do
 
   # not(left in right)
   # left not in right
-  defp quoted_to_algebra({:not, meta, [{:in, _, [left, right]}]}, context, state) do
-    binary_op_to_algebra(:in, "not in", meta, left, right, context, state)
+  defp quoted_to_algebra({:not, meta, [{:in, _, [left, right]} = arg]}, context, state) do
+    %{rename_deprecated_at: since} = state
+
+    # TODO: Remove since check on Elixir v2.0 and the OP arrengement is removed.
+    if meta[:operator] == :"not in" || (since && Version.match?(since, "~> 1.5")) do
+      binary_op_to_algebra(:in, "not in", meta, left, right, context, state)
+    else
+      unary_op_to_algebra(:not, meta, arg, context, state)
+    end
   end
 
   defp quoted_to_algebra({:fn, meta, [_ | _] = clauses}, _context, state) do

lib/elixir/src/elixir_parser.yrl

@@ -688,7 +688,13 @@ build_op({_Kind, Location, 'in'}, {UOp, _, [Left]}, Right) when ?rearrange_uop(U
   {UOp, meta_from_location(Location), [{'in', meta_from_location(Location), [Left, Right]}]};
 
 build_op({_Kind, Location, 'not in'}, Left, Right) ->
-  {'not', meta_from_location(Location), [{'in', meta_from_location(Location), [Left, Right]}]};
+  InMeta = meta_from_location(Location),
+  NotMeta =
+    case ?formatter_metadata() of
+      true -> [{operator, 'not in'} | InMeta];
+      false -> InMeta
+    end,
+  {'not', NotMeta, [{'in', InMeta, [Left, Right]}]};
 build_op({_Kind, Location, Op}, Left, Right) ->
   {Op, eol_op(Location) ++ meta_from_location(Location), [Left, Right]}.
 

lib/elixir/test/elixir/code_formatter/operators_test.exs

@@ -121,7 +121,13 @@ defmodule Code.Formatter.OperatorsTest do
     end
 
     test "not in" do
-      assert_format "not(foo in bar)", "foo not in bar"
+      assert_format "not foo in bar", "not (foo in bar)"
+      assert_format "not foo in bar", "foo not in bar", rename_deprecated_at: "1.5.0"
+
+      assert_format "not(foo in bar)", "not (foo in bar)"
+      assert_format "not(foo in bar)", "foo not in bar", rename_deprecated_at: "1.5.0"
+
+      assert_same "foo not in bar"
       assert_same "(not foo) in bar"
       assert_same "(!foo) in bar"
     end