Allow iodata in sigil formatting functions

Author: josevalim

lib/elixir/lib/code/formatter.ex

@@ -1330,12 +1330,12 @@ defmodule Code.Formatter do
             ]
 
             case callback.(hd(entries), metadata) do
-              binary when is_binary(binary) ->
-                [binary]
+              iodata when is_binary(iodata) or is_list(iodata) ->
+                [IO.iodata_to_binary(iodata)]
 
               other ->
                 raise ArgumentError,
-                      "expected sigil callback to return a binary, got: #{inspect(other)}"
+                      "expected sigil callback to return iodata, got: #{inspect(other)}"
             end
 
           %{} ->

lib/elixir/test/elixir/code_formatter/general_test.exs

@@ -142,7 +142,7 @@ defmodule Code.Formatter.GeneralTest do
 
       formatter = fn content, opts ->
         assert opts == [file: nil, line: 1, sigil: :W, modifiers: 'abc']
-        content |> String.split(~r/ +/) |> Enum.join(" ")
+        content |> String.split(~r/ +/) |> Enum.intersperse(" ")
       end
 
       assert_format bad, good, sigils: [W: formatter]