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- My initial thought is that the performance implication (e.g. escaping to the heap) of impure iterators is an implementation detail and seems like a good candidate for a future compiler optimization.

- impure iterators are strictly more powerful than pure iterators in that they can be resumed but do not have to be. If I wanted to pick one pattern for consistent behavior across all iterators it would be this one.

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Not all impure iterators can be resumed. But any pure iterator can be converted to a resumable iterator with a generic conversion function.

> any pure iterator can be converted to a resumable iterator with a generic conversion function.

Makes sense

> Not all impure iterators can be resumed

How do you mean? In my head resuming an iterator is just partially unrolling a loop.

An impure iterator can panic, so you can't resume?

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