Bug in formatter with concatenated heredocs when assigned to attributes

[eksperimental]

Environment

• Elixir & Erlang/OTP versions (elixir --version): Elixir 1.13.0-dev (37a09fe)

Current behavior

given the code

defmodule Debug.ConcatenateHeredocs do
  @foo "Two"

  @moduledoc ~S"""
    One
    """ <>
      ~s"""
      #{@foo}
      """ <>
      ~S"""
      Three
      """

  def foo() do
    moduledoc =
      ~S"""
      One
      """ <>
        ~s"""
        #{@foo}
        """ <>
        ~S"""
        Three
        """
  end
end

mix format will wrongly indent the concatenated heredocs that are assigned to a the @moduledoc attribute

defmodule Debug.ConcatenateHeredocs do
  @foo "Two"

  @moduledoc ~S"""
             One
             """ <>
               ~s"""
               #{@foo}
               """ <>
               ~S"""
               Three
               """

  def foo() do
    moduledoc =
      ~S"""
      One
      """ <>
        ~s"""
        #{@foo}
        """ <>
        ~S"""
        Three
        """
  end
end

Expected behavior

Indent the attribute value as it does with the variable.

  @moduledoc ~S"""
    One
    """ <>
      ~s"""
      #{@foo}
      """ <>
      ~S"""
      Three
      """

[josevalim]

The current behavior is correct. The nice wrapping only happens in few circumstances (heredoxs, fn...). An operator is not one of them.