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Use spare_capacity_mut instead of invalid unchecked indexing when joining str #91680

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Merged
merged 1 commit into from
Dec 14, 2021
Merged

Use spare_capacity_mut instead of invalid unchecked indexing when joining str #91680

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12 changes: 10 additions & 2 deletions library/alloc/src/str.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -178,12 +178,20 @@ where

unsafe {
let pos = result.len();
let target = result.get_unchecked_mut(pos..reserved_len);
let target = result.spare_capacity_mut().get_unchecked_mut(..reserved_len - pos);

// Convert the separator and slices to slices of MaybeUninit
// to simplify implementation in specialize_for_lengths
let sep_uninit = core::slice::from_raw_parts(sep.as_ptr().cast(), sep.len());
let iter_uninit = iter.map(|it| {
let it = it.borrow().as_ref();
core::slice::from_raw_parts(it.as_ptr().cast(), it.len())
});

// copy separator and slices over without bounds checks
// generate loops with hardcoded offsets for small separators
// massive improvements possible (~ x2)
let remain = specialize_for_lengths!(sep, target, iter; 0, 1, 2, 3, 4);
let remain = specialize_for_lengths!(sep_uninit, target, iter_uninit; 0, 1, 2, 3, 4);

// A weird borrow implementation may return different
// slices for the length calculation and the actual copy.
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