alloc/collections/vec_deque/
spec_extend.rs

1use core::iter::TrustedLen;
2use core::slice;
3
4use super::VecDeque;
5use crate::alloc::Allocator;
6#[cfg(not(test))]
7use crate::vec;
8
9// Specialization trait used for VecDeque::extend
10pub(super) trait SpecExtend<T, I> {
11    fn spec_extend(&mut self, iter: I);
12}
13
14impl<T, I, A: Allocator> SpecExtend<T, I> for VecDeque<T, A>
15where
16    I: Iterator<Item = T>,
17{
18    default fn spec_extend(&mut self, mut iter: I) {
19        // This function should be the moral equivalent of:
20        //
21        // for item in iter {
22        //     self.push_back(item);
23        // }
24
25        while let Some(element) = iter.next() {
26            let (lower, _) = iter.size_hint();
27            self.reserve(lower.saturating_add(1));
28
29            // SAFETY: We just reserved space for at least one element.
30            unsafe { self.push_unchecked(element) };
31
32            // Inner loop to avoid repeatedly calling `reserve`.
33            while self.len < self.capacity() {
34                let Some(element) = iter.next() else {
35                    return;
36                };
37                // SAFETY: The loop condition guarantees that `self.len() < self.capacity()`.
38                unsafe { self.push_unchecked(element) };
39            }
40        }
41    }
42}
43
44impl<T, I, A: Allocator> SpecExtend<T, I> for VecDeque<T, A>
45where
46    I: TrustedLen<Item = T>,
47{
48    default fn spec_extend(&mut self, iter: I) {
49        // This is the case for a TrustedLen iterator.
50        let (low, high) = iter.size_hint();
51        if let Some(additional) = high {
52            debug_assert_eq!(
53                low,
54                additional,
55                "TrustedLen iterator's size hint is not exact: {:?}",
56                (low, high)
57            );
58            self.reserve(additional);
59
60            let written = unsafe {
61                self.write_iter_wrapping(self.to_physical_idx(self.len), iter, additional)
62            };
63
64            debug_assert_eq!(
65                additional, written,
66                "The number of items written to VecDeque doesn't match the TrustedLen size hint"
67            );
68        } else {
69            // Per TrustedLen contract a `None` upper bound means that the iterator length
70            // truly exceeds usize::MAX, which would eventually lead to a capacity overflow anyway.
71            // Since the other branch already panics eagerly (via `reserve()`) we do the same here.
72            // This avoids additional codegen for a fallback code path which would eventually
73            // panic anyway.
74            panic!("capacity overflow");
75        }
76    }
77}
78
79#[cfg(not(test))]
80impl<T, A: Allocator> SpecExtend<T, vec::IntoIter<T>> for VecDeque<T, A> {
81    fn spec_extend(&mut self, mut iterator: vec::IntoIter<T>) {
82        let slice = iterator.as_slice();
83        self.reserve(slice.len());
84
85        unsafe {
86            self.copy_slice(self.to_physical_idx(self.len), slice);
87            self.len += slice.len();
88        }
89        iterator.forget_remaining_elements();
90    }
91}
92
93impl<'a, T: 'a, I, A: Allocator> SpecExtend<&'a T, I> for VecDeque<T, A>
94where
95    I: Iterator<Item = &'a T>,
96    T: Copy,
97{
98    default fn spec_extend(&mut self, iterator: I) {
99        self.spec_extend(iterator.copied())
100    }
101}
102
103impl<'a, T: 'a, A: Allocator> SpecExtend<&'a T, slice::Iter<'a, T>> for VecDeque<T, A>
104where
105    T: Copy,
106{
107    fn spec_extend(&mut self, iterator: slice::Iter<'a, T>) {
108        let slice = iterator.as_slice();
109        self.reserve(slice.len());
110
111        unsafe {
112            self.copy_slice(self.to_physical_idx(self.len), slice);
113            self.len += slice.len();
114        }
115    }
116}
alloc/collections/vec_deque/spec_extend.rs

alloc/collections/vec_deque/
spec_extend.rs
