def MessageSetItemSizer(field_number):
  """Returns a sizer for extensions of MessageSet.

  The message set message looks like this:
    message MessageSet {
      repeated group Item = 1 {
        required int32 type_id = 2;
        required string message = 3;
      }
    }
  """
  static_size = (_TagSize(1) * 2 + _TagSize(2) + _VarintSize(field_number) +
                 _TagSize(3))
  local_VarintSize = _VarintSize

  def FieldSize(value):
    l = value.ByteSize()
    return static_size + local_VarintSize(l) + l

  return FieldSize
Returns a sizer for extensions of MessageSet.

  The message set message looks like this:
    message MessageSet {
      repeated group Item = 1 {
        required int32 type_id = 2;
        required string message = 3;
      }
    }