alloc/vec/
drain.rs

1use core::fmt;
2use core::iter::{FusedIterator, TrustedLen};
3use core::mem::{self, ManuallyDrop, SizedTypeProperties};
4use core::ptr::{self, NonNull};
5use core::slice::{self};
6
7use super::Vec;
8use crate::alloc::{Allocator, Global};
9
10/// A draining iterator for `Vec<T>`.
11///
12/// This `struct` is created by [`Vec::drain`].
13/// See its documentation for more.
14///
15/// # Example
16///
17/// ```
18/// let mut v = vec![0, 1, 2];
19/// let iter: std::vec::Drain<'_, _> = v.drain(..);
20/// ```
21#[stable(feature = "drain", since = "1.6.0")]
22pub struct Drain<
23    'a,
24    T: 'a,
25    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator + 'a = Global,
26> {
27    /// Index of tail to preserve
28    pub(super) tail_start: usize,
29    /// Length of tail
30    pub(super) tail_len: usize,
31    /// Current remaining range to remove
32    pub(super) iter: slice::Iter<'a, T>,
33    pub(super) vec: NonNull<Vec<T, A>>,
34}
35
36#[stable(feature = "collection_debug", since = "1.17.0")]
37impl<T: fmt::Debug, A: Allocator> fmt::Debug for Drain<'_, T, A> {
38    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
39        f.debug_tuple("Drain").field(&self.iter.as_slice()).finish()
40    }
41}
42
43impl<'a, T, A: Allocator> Drain<'a, T, A> {
44    /// Returns the remaining items of this iterator as a slice.
45    ///
46    /// # Examples
47    ///
48    /// ```
49    /// let mut vec = vec!['a', 'b', 'c'];
50    /// let mut drain = vec.drain(..);
51    /// assert_eq!(drain.as_slice(), &['a', 'b', 'c']);
52    /// let _ = drain.next().unwrap();
53    /// assert_eq!(drain.as_slice(), &['b', 'c']);
54    /// ```
55    #[must_use]
56    #[stable(feature = "vec_drain_as_slice", since = "1.46.0")]
57    pub fn as_slice(&self) -> &[T] {
58        self.iter.as_slice()
59    }
60
61    /// Returns a reference to the underlying allocator.
62    #[unstable(feature = "allocator_api", issue = "32838")]
63    #[must_use]
64    #[inline]
65    pub fn allocator(&self) -> &A {
66        unsafe { self.vec.as_ref().allocator() }
67    }
68
69    /// Keep unyielded elements in the source `Vec`.
70    ///
71    /// # Examples
72    ///
73    /// ```
74    /// #![feature(drain_keep_rest)]
75    ///
76    /// let mut vec = vec!['a', 'b', 'c'];
77    /// let mut drain = vec.drain(..);
78    ///
79    /// assert_eq!(drain.next().unwrap(), 'a');
80    ///
81    /// // This call keeps 'b' and 'c' in the vec.
82    /// drain.keep_rest();
83    ///
84    /// // If we wouldn't call `keep_rest()`,
85    /// // `vec` would be empty.
86    /// assert_eq!(vec, ['b', 'c']);
87    /// ```
88    #[unstable(feature = "drain_keep_rest", issue = "101122")]
89    pub fn keep_rest(self) {
90        // At this moment layout looks like this:
91        //
92        // [head] [yielded by next] [unyielded] [yielded by next_back] [tail]
93        //        ^-- start         \_________/-- unyielded_len        \____/-- self.tail_len
94        //                          ^-- unyielded_ptr                  ^-- tail
95        //
96        // Normally `Drop` impl would drop [unyielded] and then move [tail] to the `start`.
97        // Here we want to
98        // 1. Move [unyielded] to `start`
99        // 2. Move [tail] to a new start at `start + len(unyielded)`
100        // 3. Update length of the original vec to `len(head) + len(unyielded) + len(tail)`
101        //    a. In case of ZST, this is the only thing we want to do
102        // 4. Do *not* drop self, as everything is put in a consistent state already, there is nothing to do
103        let mut this = ManuallyDrop::new(self);
104
105        unsafe {
106            let source_vec = this.vec.as_mut();
107
108            let start = source_vec.len();
109            let tail = this.tail_start;
110
111            let unyielded_len = this.iter.len();
112            let unyielded_ptr = this.iter.as_slice().as_ptr();
113
114            // ZSTs have no identity, so we don't need to move them around.
115            if !T::IS_ZST {
116                let start_ptr = source_vec.as_mut_ptr().add(start);
117
118                // memmove back unyielded elements
119                if unyielded_ptr != start_ptr {
120                    let src = unyielded_ptr;
121                    let dst = start_ptr;
122
123                    ptr::copy(src, dst, unyielded_len);
124                }
125
126                // memmove back untouched tail
127                if tail != (start + unyielded_len) {
128                    let src = source_vec.as_ptr().add(tail);
129                    let dst = start_ptr.add(unyielded_len);
130                    ptr::copy(src, dst, this.tail_len);
131                }
132            }
133
134            source_vec.set_len(start + unyielded_len + this.tail_len);
135        }
136    }
137}
138
139#[stable(feature = "vec_drain_as_slice", since = "1.46.0")]
140impl<'a, T, A: Allocator> AsRef<[T]> for Drain<'a, T, A> {
141    fn as_ref(&self) -> &[T] {
142        self.as_slice()
143    }
144}
145
146#[stable(feature = "drain", since = "1.6.0")]
147unsafe impl<T: Sync, A: Sync + Allocator> Sync for Drain<'_, T, A> {}
148#[stable(feature = "drain", since = "1.6.0")]
149unsafe impl<T: Send, A: Send + Allocator> Send for Drain<'_, T, A> {}
150
151#[stable(feature = "drain", since = "1.6.0")]
152impl<T, A: Allocator> Iterator for Drain<'_, T, A> {
153    type Item = T;
154
155    #[inline]
156    fn next(&mut self) -> Option<T> {
157        self.iter.next().map(|elt| unsafe { ptr::read(elt as *const _) })
158    }
159
160    fn size_hint(&self) -> (usize, Option<usize>) {
161        self.iter.size_hint()
162    }
163}
164
165#[stable(feature = "drain", since = "1.6.0")]
166impl<T, A: Allocator> DoubleEndedIterator for Drain<'_, T, A> {
167    #[inline]
168    fn next_back(&mut self) -> Option<T> {
169        self.iter.next_back().map(|elt| unsafe { ptr::read(elt as *const _) })
170    }
171}
172
173#[stable(feature = "drain", since = "1.6.0")]
174impl<T, A: Allocator> Drop for Drain<'_, T, A> {
175    fn drop(&mut self) {
176        /// Moves back the un-`Drain`ed elements to restore the original `Vec`.
177        struct DropGuard<'r, 'a, T, A: Allocator>(&'r mut Drain<'a, T, A>);
178
179        impl<'r, 'a, T, A: Allocator> Drop for DropGuard<'r, 'a, T, A> {
180            fn drop(&mut self) {
181                if self.0.tail_len > 0 {
182                    unsafe {
183                        let source_vec = self.0.vec.as_mut();
184                        // memmove back untouched tail, update to new length
185                        let start = source_vec.len();
186                        let tail = self.0.tail_start;
187                        if tail != start {
188                            let src = source_vec.as_ptr().add(tail);
189                            let dst = source_vec.as_mut_ptr().add(start);
190                            ptr::copy(src, dst, self.0.tail_len);
191                        }
192                        source_vec.set_len(start + self.0.tail_len);
193                    }
194                }
195            }
196        }
197
198        let iter = mem::take(&mut self.iter);
199        let drop_len = iter.len();
200
201        let mut vec = self.vec;
202
203        if T::IS_ZST {
204            // ZSTs have no identity, so we don't need to move them around, we only need to drop the correct amount.
205            // this can be achieved by manipulating the Vec length instead of moving values out from `iter`.
206            unsafe {
207                let vec = vec.as_mut();
208                let old_len = vec.len();
209                vec.set_len(old_len + drop_len + self.tail_len);
210                vec.truncate(old_len + self.tail_len);
211            }
212
213            return;
214        }
215
216        // ensure elements are moved back into their appropriate places, even when drop_in_place panics
217        let _guard = DropGuard(self);
218
219        if drop_len == 0 {
220            return;
221        }
222
223        // as_slice() must only be called when iter.len() is > 0 because
224        // it also gets touched by vec::Splice which may turn it into a dangling pointer
225        // which would make it and the vec pointer point to different allocations which would
226        // lead to invalid pointer arithmetic below.
227        let drop_ptr = iter.as_slice().as_ptr();
228
229        unsafe {
230            // drop_ptr comes from a slice::Iter which only gives us a &[T] but for drop_in_place
231            // a pointer with mutable provenance is necessary. Therefore we must reconstruct
232            // it from the original vec but also avoid creating a &mut to the front since that could
233            // invalidate raw pointers to it which some unsafe code might rely on.
234            let vec_ptr = vec.as_mut().as_mut_ptr();
235            let drop_offset = drop_ptr.offset_from_unsigned(vec_ptr);
236            let to_drop = ptr::slice_from_raw_parts_mut(vec_ptr.add(drop_offset), drop_len);
237            ptr::drop_in_place(to_drop);
238        }
239    }
240}
241
242#[stable(feature = "drain", since = "1.6.0")]
243impl<T, A: Allocator> ExactSizeIterator for Drain<'_, T, A> {
244    fn is_empty(&self) -> bool {
245        self.iter.is_empty()
246    }
247}
248
249#[unstable(feature = "trusted_len", issue = "37572")]
250unsafe impl<T, A: Allocator> TrustedLen for Drain<'_, T, A> {}
251
252#[stable(feature = "fused", since = "1.26.0")]
253impl<T, A: Allocator> FusedIterator for Drain<'_, T, A> {}
alloc/vec/drain.rs

alloc/vec/
drain.rs
