A Generator is currently defined as any closure that uses the new yield keyword. When it is executed with .resume() [11], it will run until it hits a yield. If run again, the generator will continue where it left off until it reaches another yield or encounters a return. If there are no more yields left in the generator, it will simply return an empty tuple, behaving as if encountering return ();.

Because there are two scenarios of what a generator does (yield vs return), you have to check the result of .resume() every time you use it, as it could be GeneratorState::Yielded or GeneratorState::Complete.

At the time of writing, you can return a different type than you yield. The situation around this is somewhat unclear, as the reason for this quirk is the aforementioned convention to return (); when running out of yields. Maybe the final version of generators in Rust will not rely on this behavior, and only allow returning the same type as yielding. You can find the discussion...