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(name:"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: &T| unsafe { ptr::read(src: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: &T| unsafe { ptr::read(src: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> {}
254