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Auto merge of #92138 - AngelicosPhosphoros:try_smarter_vec_from_iter_48994_2, r=Mark-Simulacrum

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Improve capacity estimation in Vec::from_iter

Iterates on the attempt made in #53086.

Closes #48994
This commit is contained in:
bors 2022-01-20 06:50:14 +00:00
parent 237949b6c8 ea570c689c
commit 74fbbefea8
2 changed files with 9 additions and 3 deletions
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2
library/alloc/src/raw_vec.rs
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@ -108,7 +108,7 @@ impl<T, A: Allocator> RawVec<T, A> {
// to round up a request of less than 8 bytes to at least 8 bytes.
// - 4 if elements are moderate-sized (<= 1 KiB).
// - 1 otherwise, to avoid wasting too much space for very short Vecs.
const MIN_NON_ZERO_CAP: usize = if mem::size_of::<T>() == 1 {
pub(crate) const MIN_NON_ZERO_CAP: usize = if mem::size_of::<T>() == 1 {
8
} else if mem::size_of::<T>() <= 1024 {
4

10
library/alloc/src/vec/spec_from_iter_nested.rs
View file

@ -1,5 +1,8 @@
use core::cmp;
use core::iter::TrustedLen;
use core::ptr::{self};
use core::ptr;
use crate::raw_vec::RawVec;
use super::{SpecExtend, Vec};
@ -24,8 +27,11 @@ where
None => return Vec::new(),
Some(element) => {
let (lower, _) = iterator.size_hint();
let mut vector = Vec::with_capacity(lower.saturating_add(1));
let initial_capacity =
cmp::max(RawVec::<T>::MIN_NON_ZERO_CAP, lower.saturating_add(1));
let mut vector = Vec::with_capacity(initial_capacity);
unsafe {
// SAFETY: We requested capacity at least 1
ptr::write(vector.as_mut_ptr(), element);
vector.set_len(1);
}