alloc/vec/
spec_from_iter_nested.rs

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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 = match iterator.size_hint() {
54            (_, Some(upper)) => 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(iterator);
63        vector
64    }
65}
alloc/vec/spec_from_iter_nested.rs

alloc/vec/
spec_from_iter_nested.rs
