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1// SPDX-License-Identifier: Apache-2.0 OR MIT
2
3use crate::alloc::{Allocator, Global};
4use core::fmt;
5use core::iter::{FusedIterator, TrustedLen};
6use core::mem::{self, ManuallyDrop, SizedTypeProperties};
7use core::ptr::{self, NonNull};
8use core::slice::{self};
9
10use super::Vec;
11
12/// A draining iterator for `Vec<T>`.
13///
14/// This `struct` is created by [`Vec::drain`].
15/// See its documentation for more.
16///
17/// # Example
18///
19/// ```
20/// let mut v = vec![0, 1, 2];
21/// let iter: std::vec::Drain<'_, _> = v.drain(..);
22/// ```
23#[stable(feature = "drain", since = "1.6.0")]
24pub struct Drain<
25 'a,
26 T: 'a,
27 #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator + 'a = Global,
28> {
29 /// Index of tail to preserve
30 pub(super) tail_start: usize,
31 /// Length of tail
32 pub(super) tail_len: usize,
33 /// Current remaining range to remove
34 pub(super) iter: slice::Iter<'a, T>,
35 pub(super) vec: NonNull<Vec<T, A>>,
36}
37
38#[stable(feature = "collection_debug", since = "1.17.0")]
39impl<T: fmt::Debug, A: Allocator> fmt::Debug for Drain<'_, T, A> {
40 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
41 f.debug_tuple("Drain").field(&self.iter.as_slice()).finish()
42 }
43}
44
45impl<'a, T, A: Allocator> Drain<'a, T, A> {
46 /// Returns the remaining items of this iterator as a slice.
47 ///
48 /// # Examples
49 ///
50 /// ```
51 /// let mut vec = vec!['a', 'b', 'c'];
52 /// let mut drain = vec.drain(..);
53 /// assert_eq!(drain.as_slice(), &['a', 'b', 'c']);
54 /// let _ = drain.next().unwrap();
55 /// assert_eq!(drain.as_slice(), &['b', 'c']);
56 /// ```
57 #[must_use]
58 #[stable(feature = "vec_drain_as_slice", since = "1.46.0")]
59 pub fn as_slice(&self) -> &[T] {
60 self.iter.as_slice()
61 }
62
63 /// Returns a reference to the underlying allocator.
64 #[unstable(feature = "allocator_api", issue = "32838")]
65 #[must_use]
66 #[inline]
67 pub fn allocator(&self) -> &A {
68 unsafe { self.vec.as_ref().allocator() }
69 }
70
71 /// Keep unyielded elements in the source `Vec`.
72 ///
73 /// # Examples
74 ///
75 /// ```
76 /// #![feature(drain_keep_rest)]
77 ///
78 /// let mut vec = vec!['a', 'b', 'c'];
79 /// let mut drain = vec.drain(..);
80 ///
81 /// assert_eq!(drain.next().unwrap(), 'a');
82 ///
83 /// // This call keeps 'b' and 'c' in the vec.
84 /// drain.keep_rest();
85 ///
86 /// // If we wouldn't call `keep_rest()`,
87 /// // `vec` would be empty.
88 /// assert_eq!(vec, ['b', 'c']);
89 /// ```
90 #[unstable(feature = "drain_keep_rest", issue = "101122")]
91 pub fn keep_rest(self) {
92 // At this moment layout looks like this:
93 //
94 // [head] [yielded by next] [unyielded] [yielded by next_back] [tail]
95 // ^-- start \_________/-- unyielded_len \____/-- self.tail_len
96 // ^-- unyielded_ptr ^-- tail
97 //
98 // Normally `Drop` impl would drop [unyielded] and then move [tail] to the `start`.
99 // Here we want to
100 // 1. Move [unyielded] to `start`
101 // 2. Move [tail] to a new start at `start + len(unyielded)`
102 // 3. Update length of the original vec to `len(head) + len(unyielded) + len(tail)`
103 // a. In case of ZST, this is the only thing we want to do
104 // 4. Do *not* drop self, as everything is put in a consistent state already, there is nothing to do
105 let mut this = ManuallyDrop::new(self);
106
107 unsafe {
108 let source_vec = this.vec.as_mut();
109
110 let start = source_vec.len();
111 let tail = this.tail_start;
112
113 let unyielded_len = this.iter.len();
114 let unyielded_ptr = this.iter.as_slice().as_ptr();
115
116 // ZSTs have no identity, so we don't need to move them around.
117 if !T::IS_ZST {
118 let start_ptr = source_vec.as_mut_ptr().add(start);
119
120 // memmove back unyielded elements
121 if unyielded_ptr != start_ptr {
122 let src = unyielded_ptr;
123 let dst = start_ptr;
124
125 ptr::copy(src, dst, unyielded_len);
126 }
127
128 // memmove back untouched tail
129 if tail != (start + unyielded_len) {
130 let src = source_vec.as_ptr().add(tail);
131 let dst = start_ptr.add(unyielded_len);
132 ptr::copy(src, dst, this.tail_len);
133 }
134 }
135
136 source_vec.set_len(start + unyielded_len + this.tail_len);
137 }
138 }
139}
140
141#[stable(feature = "vec_drain_as_slice", since = "1.46.0")]
142impl<'a, T, A: Allocator> AsRef<[T]> for Drain<'a, T, A> {
143 fn as_ref(&self) -> &[T] {
144 self.as_slice()
145 }
146}
147
148#[stable(feature = "drain", since = "1.6.0")]
149unsafe impl<T: Sync, A: Sync + Allocator> Sync for Drain<'_, T, A> {}
150#[stable(feature = "drain", since = "1.6.0")]
151unsafe impl<T: Send, A: Send + Allocator> Send for Drain<'_, T, A> {}
152
153#[stable(feature = "drain", since = "1.6.0")]
154impl<T, A: Allocator> Iterator for Drain<'_, T, A> {
155 type Item = T;
156
157 #[inline]
158 fn next(&mut self) -> Option<T> {
159 self.iter.next().map(|elt| unsafe { ptr::read(elt as *const _) })
160 }
161
162 fn size_hint(&self) -> (usize, Option<usize>) {
163 self.iter.size_hint()
164 }
165}
166
167#[stable(feature = "drain", since = "1.6.0")]
168impl<T, A: Allocator> DoubleEndedIterator for Drain<'_, T, A> {
169 #[inline]
170 fn next_back(&mut self) -> Option<T> {
171 self.iter.next_back().map(|elt| unsafe { ptr::read(elt as *const _) })
172 }
173}
174
175#[stable(feature = "drain", since = "1.6.0")]
176impl<T, A: Allocator> Drop for Drain<'_, T, A> {
177 fn drop(&mut self) {
178 /// Moves back the un-`Drain`ed elements to restore the original `Vec`.
179 struct DropGuard<'r, 'a, T, A: Allocator>(&'r mut Drain<'a, T, A>);
180
181 impl<'r, 'a, T, A: Allocator> Drop for DropGuard<'r, 'a, T, A> {
182 fn drop(&mut self) {
183 if self.0.tail_len > 0 {
184 unsafe {
185 let source_vec = self.0.vec.as_mut();
186 // memmove back untouched tail, update to new length
187 let start = source_vec.len();
188 let tail = self.0.tail_start;
189 if tail != start {
190 let src = source_vec.as_ptr().add(tail);
191 let dst = source_vec.as_mut_ptr().add(start);
192 ptr::copy(src, dst, self.0.tail_len);
193 }
194 source_vec.set_len(start + self.0.tail_len);
195 }
196 }
197 }
198 }
199
200 let iter = mem::take(&mut self.iter);
201 let drop_len = iter.len();
202
203 let mut vec = self.vec;
204
205 if T::IS_ZST {
206 // ZSTs have no identity, so we don't need to move them around, we only need to drop the correct amount.
207 // this can be achieved by manipulating the Vec length instead of moving values out from `iter`.
208 unsafe {
209 let vec = vec.as_mut();
210 let old_len = vec.len();
211 vec.set_len(old_len + drop_len + self.tail_len);
212 vec.truncate(old_len + self.tail_len);
213 }
214
215 return;
216 }
217
218 // ensure elements are moved back into their appropriate places, even when drop_in_place panics
219 let _guard = DropGuard(self);
220
221 if drop_len == 0 {
222 return;
223 }
224
225 // as_slice() must only be called when iter.len() is > 0 because
226 // it also gets touched by vec::Splice which may turn it into a dangling pointer
227 // which would make it and the vec pointer point to different allocations which would
228 // lead to invalid pointer arithmetic below.
229 let drop_ptr = iter.as_slice().as_ptr();
230
231 unsafe {
232 // drop_ptr comes from a slice::Iter which only gives us a &[T] but for drop_in_place
233 // a pointer with mutable provenance is necessary. Therefore we must reconstruct
234 // it from the original vec but also avoid creating a &mut to the front since that could
235 // invalidate raw pointers to it which some unsafe code might rely on.
236 let vec_ptr = vec.as_mut().as_mut_ptr();
237 let drop_offset = drop_ptr.sub_ptr(vec_ptr);
238 let to_drop = ptr::slice_from_raw_parts_mut(vec_ptr.add(drop_offset), drop_len);
239 ptr::drop_in_place(to_drop);
240 }
241 }
242}
243
244#[stable(feature = "drain", since = "1.6.0")]
245impl<T, A: Allocator> ExactSizeIterator for Drain<'_, T, A> {
246 fn is_empty(&self) -> bool {
247 self.iter.is_empty()
248 }
249}
250
251#[unstable(feature = "trusted_len", issue = "37572")]
252unsafe impl<T, A: Allocator> TrustedLen for Drain<'_, T, A> {}
253
254#[stable(feature = "fused", since = "1.26.0")]
255impl<T, A: Allocator> FusedIterator for Drain<'_, T, A> {}