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unchecked layout calculations when shrinking during in-place collect

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Reduces the amount of emitted IR. RawVec has similar optimizations
This commit is contained in:
The 8472 2023-05-29 13:04:26 +02:00
parent c98070d522
commit 072b51cbb5
1 changed files with 12 additions and 2 deletions
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14
library/alloc/src/vec/in_place_collect.rs
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@ -267,10 +267,20 @@ where
{
let alloc = Global;
unsafe {
let new_layout = Layout::array::<T>(dst_cap).unwrap();
// The old allocation exists, therefore it must have a valid layout.
let src_align = mem::align_of::<I::Src>();
let src_size = mem::size_of::<I::Src>().unchecked_mul(src_cap);
let old_layout = Layout::from_size_align_unchecked(src_size, src_align);
// The must be equal or smaller for in-place iteration to be possible
// therefore the new layout must be ≤ the old one and therefore valid.
let dst_align = mem::align_of::<T>();
let dst_size = mem::size_of::<T>().unchecked_mul(dst_cap);
let new_layout = Layout::from_size_align_unchecked(dst_size, dst_align);
let result = alloc.shrink(
NonNull::new_unchecked(dst_buf as *mut u8),
Layout::array::<I::Src>(src_cap).unwrap(),
old_layout,
new_layout,
);
let Ok(reallocated) = result else { handle_alloc_error(new_layout) };