I am wondering.
what is 'fuse'? why so need here?
It is documented.
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Doc:
A stream that yields None forever after the underlying stream yields None once.
Iterators aren't necessarily expected to exhibit correct behavior if next calls are made after already returning None once, so, if one is given an arbitrary iterator and wants to be able to call it after it's already returned None once, they can use fuse which will guarantee that it just keeps returning None and doesn't have other side effects
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For more of a why you'd use it answer:
If an iterator isn't fuse, to consume it correctly you have to make sure that you always stop when something says it hit the end.
So something like let _ = it.nth(10); … use it … is only correct if it is fused -- if it might not be fused, you need to check for an Option from nth saying that it's exhausted, and make sure not to use it further.
Thus if you're accepting something where you're not sure whether it's fused and don't want to be careful about stuff like that, then you call .fuse() on it to get a fused iterator and you can just call whatever since it'll stay exhausted once exhausted.
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"Correctly" if the iteratator wasn't designed to be reused anyway (which, granted, is most of them).
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But what example can there be where someone might call .next() after a None, and thus needs the iterator to be fused, just in case?
The simplest example is something like chain -- it needs to give you something from the second iterator when the first one is exhausted. If the first iterator is fused, that's easily done by just first.next().or_else(|| second.next()).
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I think it's beneficial for optimizations in some cases. At least, there are or have been specialized implementations in the standard library based on Fuse.
You may also run into non-ideally implemented iterators; that one occasionally gives unexpected results on release or panics on debug.
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But then doesn't nth have to check n-1th etc? Wouldn't it be better to store a reference of which iter is the current iter?
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