Description
There's currently an example of this in the treemap module, since it's required to set methods like intersection in-order. An external iterator is basically a state machine, so they're somewhat painful to write by hand but it's possible to generate them automatically, and that could be tackled at some point (probably post-1.0).
The only method the trait will require is fn next(&mut self) -> Option<T>. A default implementation a wrapper (see {map, set}_advance in treemap.rs) for integrating with for loops can be provided.
Unlike internal iteration that works with a closure, external iterators can be interleaved and keep their state when you break out of a loop. All iterators (forward, reverse, range within a container, counter, etc.) can implement the same interface, which means a generic library like Python's itertools can be written around them.
Blocking on #5598 for now, because this should be done with methods.