fix panic-safety in specialized Zip::next_back #86452
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This was unsound since a panic in a.next_back() would result in the length not being updated which would then lead to the same element being revisited in the side-effect preserving code.
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fix panic-safety in specialized Zip::next_back This was unsound since a panic in a.next_back() would result in the length not being updated which would then lead to the same element being revisited in the side-effect preserving code. fixes rust-lang#86443
This was referenced Jun 21, 2021
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Rollup of 8 pull requests Successful merges: - rust-lang#83739 (Account for bad placeholder errors on consts/statics with trait objects) - rust-lang#85637 (document PartialEq, PartialOrd, Ord requirements more explicitly) - rust-lang#86152 (Lazify is_really_default condition in the RustdocGUI bootstrap step) - rust-lang#86156 (Fix a bug in the linkchecker) - rust-lang#86427 (Updated release note) - rust-lang#86452 (fix panic-safety in specialized Zip::next_back) - rust-lang#86484 (Do not set depth to 0 in fully_expand_fragment) - rust-lang#86491 (expand: Move some more derive logic to rustc_builtin_macros) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup
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…fJung Add comments around code where ordering is important due for panic-safety Iterators contain arbitrary code which may panic. Unsafe code has to be careful to do its state updates at the right point between calls that may panic. As requested in rust-lang#86452 (comment) r? `@RalfJung`
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Rollup of 8 pull requests Successful merges: - rust-lang#83739 (Account for bad placeholder errors on consts/statics with trait objects) - rust-lang#85637 (document PartialEq, PartialOrd, Ord requirements more explicitly) - rust-lang#86152 (Lazify is_really_default condition in the RustdocGUI bootstrap step) - rust-lang#86156 (Fix a bug in the linkchecker) - rust-lang#86427 (Updated release note) - rust-lang#86452 (fix panic-safety in specialized Zip::next_back) - rust-lang#86484 (Do not set depth to 0 in fully_expand_fragment) - rust-lang#86491 (expand: Move some more derive logic to rustc_builtin_macros) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup
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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. rust-lang#82292 tried to fix unsoundness (rust-lang#82291) in that side-effect-preservation code, but this introduced some panic-safety unsoundness (rust-lang#86443), but the fix rust-lang#86452 didn't fix it for nested Zip iterators (rust-lang#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 rust-lang#83791 but we haven't made use of that so far.
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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. rust-lang#82292 tried to fix unsoundness (rust-lang#82291) in that side-effect-preservation code, but this introduced some panic-safety unsoundness (rust-lang#86443), but the fix rust-lang#86452 didn't fix it for nested Zip iterators (rust-lang#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 rust-lang#83791 but we haven't made use of that so far.
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This was unsound since a panic in a.next_back() would result in the
length not being updated which would then lead to the same element
being revisited in the side-effect preserving code.
fixes #86443