fix Zip unsoundness (again) #141076
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fix Zip unsoundness (again) #141076
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rustbot has assigned @workingjubilee. Use |
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exciting! |
workingjubilee
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Jun 3, 2025
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Could you either commit these comments or, if they're wrong (and that seems reasonably likely), correct them? The description of the issue so far is... a bit in-the-weeds. Tests are a valuable opportunity to spend time describing the actual issues we are trying to avoid.
| }); | ||
| let b = [1, 2, 3, 4].as_slice().iter().copied(); | ||
| let c = [5, 6, 7].as_slice().iter().copied(); | ||
| let ab = zip(a, b); |
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Suggested change
| let ab = zip(a, b); | |
| // The actual case we are testing: this Zip gets nested in the next, and | |
| // `a` forces a premature unwind from Zip's iterator-driving code. | |
| let ab = zip(a, b); |
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Some history: The Zip TrustedRandomAccess specialization has tried to emulate the side-effects of the naive implementation for a long time, including backwards iteration. 82292¹ tried to fix unsoundness (82291¹) in that side-effect-preservation code, but this introduced some panic-safety unsoundness (86443¹), but the fix 86452¹ didn't fix it for nested Zip iterators (137255¹). 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 83791¹ but we haven't made use of that so far. ¹ see merge commit for linkfied issues.
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@rustbot ready The issue is complicated, so I don't think those few suggested comments would a good job, so I tried to add some more instead. |
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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