Only rearrange not in if explicitly opted-in

Closes #7227.

lib/elixir/src/elixir_parser.yrl

@@ -684,13 +684,19 @@ number_value({_, {_, _, Value}, _}) ->
 %% Operators
 
 build_op({_Kind, Location, 'in'}, {UOp, _, [Left]}, Right) when ?rearrange_uop(UOp) ->
-  %% TODO: Raise on "not left in right" rearrangement on 2.0
+  %% TODO: Remove "not left in right" rearrangement on 2.0
   elixir_errors:warn(line_from_location(Location), ?file(),
     "\"not expr1 in expr2\" is deprecated. Use \"expr1 not in expr2\" instead"),
   {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]}.