Add custom nth_back for Zip

[mdnight]

Implementation of nth_back for Zip.
Part of #54054
r? @scottmcm

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[timvermeulen]

Since the underlying iterators may have side-effects, I don't think we can return early when min_len < n.

[koalatux]

@timvermeulen, returning early might be ok, especially if you are leveraging the underlaying nth_back. The forward next also does short-circuiting, its documentation sais: "If the first iterator returns None, zip will short-circuit and next will not be called on the second iterator." Maybe we can use a similar behaviour here.

[timvermeulen]

@koalatux That guarantee is specific to Zip, whereas custom implementations of nth and nth_back are required by the Iterator trait to have the same behavior in regards to side effects as the default implementations (i.e. repeated calls to next/next_back).

So short-circuiting Zip::nth_back by forwarding to nth_back on the inner iterators is indeed fine, if done carefully, but short-circuiting based on the simple check min_len < n is not.

[Dylan-DPC-zz]

ping from triage @scottmcm waiting for your review on this

[scottmcm]

The default implementation of nth_back calls Iterator::next_back in a loop, which seems like it'd be just as good as this one? It looks like ZipImpl::nth exists, so I'd have expected the change here to add ZipImpl::nth_back and specialize it the same way.

[jonas-schievink]

Ping from triage @mdnight, can you address the review comment?

[JohnCSimon]

Ping from triage @mdnight please address the review comment

[JohnCSimon]

@scottmcm Triage: Thanks for the PR but I'm going to close this due to lack of movement.