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Implement split_at_spare_mut without Deref to a slice so that the spare slice is valid #92097

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Merged
merged 2 commits into from
Jan 1, 2022
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Implement split_at_spare_mut without Deref to a slice so that the spare slice is valid #92097

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4f80816
Implement split_at_spare_mut directly
saethlin Dec 19, 2021
777c853
Clarify safety comment
saethlin Dec 31, 2021
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9 changes: 7 additions & 2 deletions library/alloc/src/vec/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2141,12 +2141,17 @@ impl<T, A: Allocator> Vec<T, A> {
unsafe fn split_at_spare_mut_with_len(
&mut self,
) -> (&mut [T], &mut [MaybeUninit<T>], &mut usize) {
let Range { start: ptr, end: spare_ptr } = self.as_mut_ptr_range();
let ptr = self.as_mut_ptr();
// SAFETY:
// - `ptr` is guaranteed to be valid for `self.len` elements
// - but the allocation extends out to `self.buf.capacity()` elements, possibly
// uninitialized
let spare_ptr = unsafe { ptr.add(self.len) };
let spare_ptr = spare_ptr.cast::<MaybeUninit<T>>();
let spare_len = self.buf.capacity() - self.len;

// SAFETY:
// - `ptr` is guaranteed to be valid for `len` elements
// - `ptr` is guaranteed to be valid for `self.len` elements
// - `spare_ptr` is pointing one element past the buffer, so it doesn't overlap with `initialized`
unsafe {
let initialized = slice::from_raw_parts_mut(ptr, self.len);
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