Implement FusedIterator for core::iter::StepBy

philomathic_life:

Should the correctness of the impl be dependent on the correctness of the underlying Iterator's impl?

I don't understand why you'd answer that question with "no". Did you accidentally write the opposite of what you meant (dependent vs independent)?

The naive implementation of StepBy is only fused if the underlying iterator is fused. If the underlying iterator is incorrectly claiming to be fused, StepBy's claim to be fused would be just as incorrect. Which is completely fine.

Attempting to work around by fusing in StepBy would hurt performance, change existing behaviour, and require specialization. So it's clearly not a sane option.