As far as I can tell, this can't actually work, because the return type of `fn next(&'a mut self)` should actually live for some lifetime `'b` that is shorter t

As far as I can tell, this can't actually work, because the return type of `fn next(&'a mut self)` should actually live for some lifetime `'b` that is shorter than `'a`, instead of at least as long as `'a`; mostly because `next` borrows `self` mutably.