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Auto merge of #91680 - saethlin:spare_capacity_mut-in-join, r=dtolnay

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Use spare_capacity_mut instead of invalid unchecked indexing when joining str

This is a fix for https://github.com/rust-lang/rust/issues/91574

I think in general I'd prefer to see this code implemented with raw pointers or `MaybeUninit::write_slice`, but there's existing code in here based on copying from slice to slice, so converting everything from `&[T]` to `&[MaybeUninit<T>]` is less disruptive.
This commit is contained in:
bors 2021-12-14 04:29:54 +00:00
commit 7ca74ea0af
1 changed files with 10 additions and 2 deletions
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12
library/alloc/src/str.rs
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@ -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.