alloc/vec/
spec_from_iter_nested.rs

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    default fn from_iter(mut iterator: I) -> Self {
19        // Unroll the first iteration, as the vector is going to be
20        // expanded on this iteration in every case when the iterable is not
21        // empty, but the loop in extend_desugared() is not going to see the
22        // vector being full in the few subsequent loop iterations.
23        // So we get better branch prediction.
24        let mut vector = match iterator.next() {
25            None => return Vec::new(),
26            Some(element) => {
27                let (lower, _) = iterator.size_hint();
28                let initial_capacity =
29                    cmp::max(RawVec::<T>::MIN_NON_ZERO_CAP, lower.saturating_add(1));
30                let mut vector = Vec::with_capacity(initial_capacity);
31                unsafe {
32                    // SAFETY: We requested capacity at least 1
33                    ptr::write(vector.as_mut_ptr(), element);
34                    vector.set_len(1);
35                }
36                vector
37            }
38        };
39        // must delegate to spec_extend() since extend() itself delegates
40        // to spec_from for empty Vecs
41        <Vec<T> as SpecExtend<T, I>>::spec_extend(&mut vector, iterator);
42        vector
43    }
44}
45
46impl<T, I> SpecFromIterNested<T, I> for Vec<T>
47where
48    I: TrustedLen<Item = T>,
49{
50    fn from_iter(iterator: I) -> Self {
51        let mut vector = match iterator.size_hint() {
52            (_, Some(upper)) => Vec::with_capacity(upper),
53            // TrustedLen contract guarantees that `size_hint() == (_, None)` means that there
54            // are more than `usize::MAX` elements.
55            // Since the previous branch would eagerly panic if the capacity is too large
56            // (via `with_capacity`) we do the same here.
57            _ => panic!("capacity overflow"),
58        };
59        // reuse extend specialization for TrustedLen
60        vector.spec_extend(iterator);
61        vector
62    }
63}
alloc/vec/spec_from_iter_nested.rs

alloc/vec/
spec_from_iter_nested.rs
