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Add a UB-check to Vec::set_len #143813

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#143877
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Add a UB-check to Vec::set_len#143813
#143877
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xizheyin
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A-collectionsArea: `std::collections`C-enhancementCategory: An issue proposing an enhancement or a PR with one.E-easyCall for participation: Easy difficulty. Experience needed to fix: Not much. Good first issue.T-libsRelevant to the library team, which will review and decide on the PR/issue.
@scottmcm

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@scottmcm
scottmcm
opened on Jul 11, 2025

Today, set_len has a debug_assert! on the length being legal:

rust/library/alloc/src/vec/mod.rs

Lines 1950 to 1956 in 855e0fe

#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub unsafe fn set_len(&mut self, new_len: usize) {
debug_assert!(new_len <= self.capacity());
self.len = new_len;
}

It would be nice, assuming the compilation time hit is tolerable, to upgrade that to a UB-check in order to better catch people making over-long vectors.

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A-collectionsArea: `std::collections`C-enhancementCategory: An issue proposing an enhancement or a PR with one.E-easyCall for participation: Easy difficulty. Experience needed to fix: Not much. Good first issue.T-libsRelevant to the library team, which will review and decide on the PR/issue.

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