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fff53f1dbb
fix Zip unsoundness (again) Some history: The Zip TrustedRandomAccess specialization has tried to emulate the side-effects of the naive implementation for a long time, including backwards iteration. #82292 tried to fix unsoundness (#82291) in that side-effect-preservation code, but this introduced some panic-safety unsoundness (#86443), but the fix #86452 didn't fix it for nested Zip iterators (#137255). Rather than piling yet another fix ontop of this heap of fixes this PR reduces the number of cases in which side-effects will be preserved; the necessary API guarantee change was approved in #83791 but we haven't made use of that so far. fixes #137255