1use core::iter::TrustedLen;
2use core::{cmp, ptr};
3
4use super::{SpecExtend, Vec};
5use crate::raw_vec::RawVec;
6
7/// Another specialization trait for Vec::from_iter
8/// necessary to manually prioritize overlapping specializations
9/// see [`SpecFromIter`](super::SpecFromIter) for details.
10pub(super) trait SpecFromIterNested<T, I> {
11 fn from_iter(iter: I) -> Self;
12}
13
14impl<T, I> SpecFromIterNested<T, I> for Vec<T>
15where
16 I: Iterator<Item = T>,
17{
18 #[track_caller]
19 default fn from_iter(mut iterator: I) -> Self {
20 // Unroll the first iteration, as the vector is going to be
21 // expanded on this iteration in every case when the iterable is not
22 // empty, but the loop in extend_desugared() is not going to see the
23 // vector being full in the few subsequent loop iterations.
24 // So we get better branch prediction.
25 let mut vector = match iterator.next() {
26 None => return Vec::new(),
27 Some(element) => {
28 let (lower, _) = iterator.size_hint();
29 let initial_capacity =
30 cmp::max(RawVec::<T>::MIN_NON_ZERO_CAP, lower.saturating_add(1));
31 let mut vector = Vec::with_capacity(initial_capacity);
32 unsafe {
33 // SAFETY: We requested capacity at least 1
34 ptr::write(vector.as_mut_ptr(), element);
35 vector.set_len(1);
36 }
37 vector
38 }
39 };
40 // must delegate to spec_extend() since extend() itself delegates
41 // to spec_from for empty Vecs
42 <Vec<T> as SpecExtend<T, I>>::spec_extend(&mut vector, iterator);
43 vector
44 }
45}
46
47impl<T, I> SpecFromIterNested<T, I> for Vec<T>
48where
49 I: TrustedLen<Item = T>,
50{
51 #[track_caller]
52 fn from_iter(iterator: I) -> Self {
53 let mut vector: Vec = match iterator.size_hint() {
54 (_, Some(upper: usize)) => Vec::with_capacity(upper),
55 // TrustedLen contract guarantees that `size_hint() == (_, None)` means that there
56 // are more than `usize::MAX` elements.
57 // Since the previous branch would eagerly panic if the capacity is too large
58 // (via `with_capacity`) we do the same here.
59 _ => panic!("capacity overflow"),
60 };
61 // reuse extend specialization for TrustedLen
62 vector.spec_extend(iter:iterator);
63 vector
64 }
65}
66