From ea570c689cf5dfb07f39853cc26701f382dff9a6 Mon Sep 17 00:00:00 2001 From: AngelicosPhosphoros Date: Mon, 20 Dec 2021 23:58:45 +0300 Subject: [PATCH] Improve estimation of capacity in Vec::from_iter Closes #48994 --- library/alloc/src/raw_vec.rs | 2 +- library/alloc/src/vec/spec_from_iter_nested.rs | 10 ++++++++-- 2 files changed, 9 insertions(+), 3 deletions(-) diff --git a/library/alloc/src/raw_vec.rs b/library/alloc/src/raw_vec.rs index 3806bc546ee7b..8fa0242ca9a9f 100644 --- a/library/alloc/src/raw_vec.rs +++ b/library/alloc/src/raw_vec.rs @@ -108,7 +108,7 @@ impl RawVec { // 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::() == 1 { + pub(crate) const MIN_NON_ZERO_CAP: usize = if mem::size_of::() == 1 { 8 } else if mem::size_of::() <= 1024 { 4 diff --git a/library/alloc/src/vec/spec_from_iter_nested.rs b/library/alloc/src/vec/spec_from_iter_nested.rs index 948cf044197c2..f915ebb86e5a5 100644 --- a/library/alloc/src/vec/spec_from_iter_nested.rs +++ b/library/alloc/src/vec/spec_from_iter_nested.rs @@ -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::::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); }