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Co-authored-by: Josh Stone <cuviper@gmail.com>
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the8472 and cuviper committed Jan 30, 2024
1 parent c780fe6 commit 39dc315
Showing 1 changed file with 11 additions and 12 deletions.
23 changes: 11 additions & 12 deletions library/alloc/src/vec/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -2788,7 +2788,7 @@ impl<T, I: SliceIndex<[T]>, A: Allocator> IndexMut<I> for Vec<T, A> {
///
/// # Allocation behavior
///
/// In general `Vec` does not guarantee any particular grow/allocation stategy.
/// In general `Vec` does not guarantee any particular growth or allocation strategy.
/// That also applies to this trait impl.
///
/// **Note:** This section covers implementation details and is therefore exempt from
Expand All @@ -2798,29 +2798,28 @@ impl<T, I: SliceIndex<[T]>, A: Allocator> IndexMut<I> for Vec<T, A> {
/// depending on the supplied iterator:
///
/// * preallocate based on [`Iterator::size_hint()`]
/// * and panic if the number of items is not outside the provided lower/upper bounds
/// * and panic if the number of items is outside the provided lower/upper bounds
/// * use an amortized growth strategy similar to `pushing` one item at a time
/// * perform the iteration in-place on the original allocation backing the iterator
///
/// The last case warrants some attention. It is an optimization that in many cases reduces peak memory
/// consumption and improves cache locality. But when a large number of big, short-lived
/// allocations are created, only a small fraction of their items gets collected, no further use
/// is made of the spare capacity and the resulting `Vec` is moved into a longer-lived structure
/// this can lead to the large allocations having their lifetimes unnecessarily extended which
/// can result in increased memory footprint.
/// consumption and improves cache locality. But when big, short-lived allocations are created,
/// only a small fraction of their items get collected, no further use is made of the spare capacity
/// and the resulting `Vec` is moved into a longer-lived structure, then this can lead to the large
/// allocations having their lifetimes unnecessarily extended which can result in increased memory
/// footprint.
///
/// In cases where this is an issue the excess capacity can be discard with [`Vec::shrink_to()`],
/// [`Vec::shrink_to_fit()`] or by collecting into [`Box<[T]>`][owned slice] instead which additionally reduces
/// the size of the longlived struct.
/// In cases where this is an issue, the excess capacity can be discarded with [`Vec::shrink_to()`],
/// [`Vec::shrink_to_fit()`] or by collecting into [`Box<[T]>`][owned slice] instead, which additionally reduces
/// the size of the long-lived struct.
///
/// [owned slice]: Box
///
/// ```rust
/// # use std::sync::Mutex;
/// static LONG_LIVED: Mutex<Vec<Vec<u16>>> = Mutex::new(Vec::new());
///
/// // many short-lived allocations
/// for i in 0..100 {
/// for i in 0..10 {
/// let big_temporary: Vec<u16> = (0..1024).collect();
/// // discard most items
/// let mut result: Vec<_> = big_temporary.into_iter().filter(|i| i % 100 == 0).collect();
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