alloc/vec/
splice.rs

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1use core::ptr::{self};
2use core::slice::{self};
3
4use super::{Drain, Vec};
5use crate::alloc::{Allocator, Global};
6
7/// A splicing iterator for `Vec`.
8///
9/// This struct is created by [`Vec::splice()`].
10/// See its documentation for more.
11///
12/// # Example
13///
14/// ```
15/// let mut v = vec![0, 1, 2];
16/// let new = [7, 8];
17/// let iter: std::vec::Splice<'_, _> = v.splice(1.., new);
18/// ```
19#[derive(Debug)]
20#[stable(feature = "vec_splice", since = "1.21.0")]
21pub struct Splice<
22    'a,
23    I: Iterator + 'a,
24    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator + 'a = Global,
25> {
26    pub(super) drain: Drain<'a, I::Item, A>,
27    pub(super) replace_with: I,
28}
29
30#[stable(feature = "vec_splice", since = "1.21.0")]
31impl<I: Iterator, A: Allocator> Iterator for Splice<'_, I, A> {
32    type Item = I::Item;
33
34    fn next(&mut self) -> Option<Self::Item> {
35        self.drain.next()
36    }
37
38    fn size_hint(&self) -> (usize, Option<usize>) {
39        self.drain.size_hint()
40    }
41}
42
43#[stable(feature = "vec_splice", since = "1.21.0")]
44impl<I: Iterator, A: Allocator> DoubleEndedIterator for Splice<'_, I, A> {
45    fn next_back(&mut self) -> Option<Self::Item> {
46        self.drain.next_back()
47    }
48}
49
50#[stable(feature = "vec_splice", since = "1.21.0")]
51impl<I: Iterator, A: Allocator> ExactSizeIterator for Splice<'_, I, A> {}
52
53#[stable(feature = "vec_splice", since = "1.21.0")]
54impl<I: Iterator, A: Allocator> Drop for Splice<'_, I, A> {
55    #[track_caller]
56    fn drop(&mut self) {
57        self.drain.by_ref().for_each(drop);
58        // At this point draining is done and the only remaining tasks are splicing
59        // and moving things into the final place.
60        // Which means we can replace the slice::Iter with pointers that won't point to deallocated
61        // memory, so that Drain::drop is still allowed to call iter.len(), otherwise it would break
62        // the ptr.sub_ptr contract.
63        self.drain.iter = (&[]).iter();
64
65        unsafe {
66            if self.drain.tail_len == 0 {
67                self.drain.vec.as_mut().extend(self.replace_with.by_ref());
68                return;
69            }
70
71            // First fill the range left by drain().
72            if !self.drain.fill(&mut self.replace_with) {
73                return;
74            }
75
76            // There may be more elements. Use the lower bound as an estimate.
77            // FIXME: Is the upper bound a better guess? Or something else?
78            let (lower_bound, _upper_bound) = self.replace_with.size_hint();
79            if lower_bound > 0 {
80                self.drain.move_tail(lower_bound);
81                if !self.drain.fill(&mut self.replace_with) {
82                    return;
83                }
84            }
85
86            // Collect any remaining elements.
87            // This is a zero-length vector which does not allocate if `lower_bound` was exact.
88            let mut collected = self.replace_with.by_ref().collect::<Vec<I::Item>>().into_iter();
89            // Now we have an exact count.
90            if collected.len() > 0 {
91                self.drain.move_tail(collected.len());
92                let filled = self.drain.fill(&mut collected);
93                debug_assert!(filled);
94                debug_assert_eq!(collected.len(), 0);
95            }
96        }
97        // Let `Drain::drop` move the tail back if necessary and restore `vec.len`.
98    }
99}
100
101/// Private helper methods for `Splice::drop`
102impl<T, A: Allocator> Drain<'_, T, A> {
103    /// The range from `self.vec.len` to `self.tail_start` contains elements
104    /// that have been moved out.
105    /// Fill that range as much as possible with new elements from the `replace_with` iterator.
106    /// Returns `true` if we filled the entire range. (`replace_with.next()` didn’t return `None`.)
107    unsafe fn fill<I: Iterator<Item = T>>(&mut self, replace_with: &mut I) -> bool {
108        let vec = unsafe { self.vec.as_mut() };
109        let range_start = vec.len;
110        let range_end = self.tail_start;
111        let range_slice = unsafe {
112            slice::from_raw_parts_mut(vec.as_mut_ptr().add(range_start), range_end - range_start)
113        };
114
115        for place in range_slice {
116            if let Some(new_item) = replace_with.next() {
117                unsafe { ptr::write(place, new_item) };
118                vec.len += 1;
119            } else {
120                return false;
121            }
122        }
123        true
124    }
125
126    /// Makes room for inserting more elements before the tail.
127    #[track_caller]
128    unsafe fn move_tail(&mut self, additional: usize) {
129        let vec = unsafe { self.vec.as_mut() };
130        let len = self.tail_start + self.tail_len;
131        vec.buf.reserve(len, additional);
132
133        let new_tail_start = self.tail_start + additional;
134        unsafe {
135            let src = vec.as_ptr().add(self.tail_start);
136            let dst = vec.as_mut_ptr().add(new_tail_start);
137            ptr::copy(src, dst, self.tail_len);
138        }
139        self.tail_start = new_tail_start;
140    }
141}
alloc/vec/splice.rs

alloc/vec/
splice.rs
