Edit (since I cannot edit my comment after 2 hours): for the code to properly verify the lifetimes one needs to write where 'b: 'a meaning 'b is at least

Edit (since I cannot edit my comment after 2 hours): for the code to properly verify the lifetimes one needs to write where 'b: 'a meaning 'b is at least as useful as 'a. So 'b can live the same time, or longer, but not less.