1 // SPDX-License-Identifier: Apache-2.0 OR MIT
2 
3 use crate::alloc::{Allocator, Global};
4 use core::fmt;
5 use core::iter::{FusedIterator, TrustedLen};
6 use core::mem;
7 use core::ptr::{self, NonNull};
8 use core::slice::{self};
9 
10 use 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")]
24 pub 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")]
39 impl<T: fmt::Debug, A: Allocator> fmt::Debug for Drain<'_, T, A> {
fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result40     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
41         f.debug_tuple("Drain").field(&self.iter.as_slice()).finish()
42     }
43 }
44 
45 impl<'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")]
as_slice(&self) -> &[T]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]
allocator(&self) -> &A67     pub fn allocator(&self) -> &A {
68         unsafe { self.vec.as_ref().allocator() }
69     }
70 }
71 
72 #[stable(feature = "vec_drain_as_slice", since = "1.46.0")]
73 impl<'a, T, A: Allocator> AsRef<[T]> for Drain<'a, T, A> {
as_ref(&self) -> &[T]74     fn as_ref(&self) -> &[T] {
75         self.as_slice()
76     }
77 }
78 
79 #[stable(feature = "drain", since = "1.6.0")]
80 unsafe impl<T: Sync, A: Sync + Allocator> Sync for Drain<'_, T, A> {}
81 #[stable(feature = "drain", since = "1.6.0")]
82 unsafe impl<T: Send, A: Send + Allocator> Send for Drain<'_, T, A> {}
83 
84 #[stable(feature = "drain", since = "1.6.0")]
85 impl<T, A: Allocator> Iterator for Drain<'_, T, A> {
86     type Item = T;
87 
88     #[inline]
next(&mut self) -> Option<T>89     fn next(&mut self) -> Option<T> {
90         self.iter.next().map(|elt| unsafe { ptr::read(elt as *const _) })
91     }
92 
size_hint(&self) -> (usize, Option<usize>)93     fn size_hint(&self) -> (usize, Option<usize>) {
94         self.iter.size_hint()
95     }
96 }
97 
98 #[stable(feature = "drain", since = "1.6.0")]
99 impl<T, A: Allocator> DoubleEndedIterator for Drain<'_, T, A> {
100     #[inline]
next_back(&mut self) -> Option<T>101     fn next_back(&mut self) -> Option<T> {
102         self.iter.next_back().map(|elt| unsafe { ptr::read(elt as *const _) })
103     }
104 }
105 
106 #[stable(feature = "drain", since = "1.6.0")]
107 impl<T, A: Allocator> Drop for Drain<'_, T, A> {
drop(&mut self)108     fn drop(&mut self) {
109         /// Moves back the un-`Drain`ed elements to restore the original `Vec`.
110         struct DropGuard<'r, 'a, T, A: Allocator>(&'r mut Drain<'a, T, A>);
111 
112         impl<'r, 'a, T, A: Allocator> Drop for DropGuard<'r, 'a, T, A> {
113             fn drop(&mut self) {
114                 if self.0.tail_len > 0 {
115                     unsafe {
116                         let source_vec = self.0.vec.as_mut();
117                         // memmove back untouched tail, update to new length
118                         let start = source_vec.len();
119                         let tail = self.0.tail_start;
120                         if tail != start {
121                             let src = source_vec.as_ptr().add(tail);
122                             let dst = source_vec.as_mut_ptr().add(start);
123                             ptr::copy(src, dst, self.0.tail_len);
124                         }
125                         source_vec.set_len(start + self.0.tail_len);
126                     }
127                 }
128             }
129         }
130 
131         let iter = mem::replace(&mut self.iter, (&mut []).iter());
132         let drop_len = iter.len();
133 
134         let mut vec = self.vec;
135 
136         if mem::size_of::<T>() == 0 {
137             // ZSTs have no identity, so we don't need to move them around, we only need to drop the correct amount.
138             // this can be achieved by manipulating the Vec length instead of moving values out from `iter`.
139             unsafe {
140                 let vec = vec.as_mut();
141                 let old_len = vec.len();
142                 vec.set_len(old_len + drop_len + self.tail_len);
143                 vec.truncate(old_len + self.tail_len);
144             }
145 
146             return;
147         }
148 
149         // ensure elements are moved back into their appropriate places, even when drop_in_place panics
150         let _guard = DropGuard(self);
151 
152         if drop_len == 0 {
153             return;
154         }
155 
156         // as_slice() must only be called when iter.len() is > 0 because
157         // vec::Splice modifies vec::Drain fields and may grow the vec which would invalidate
158         // the iterator's internal pointers. Creating a reference to deallocated memory
159         // is invalid even when it is zero-length
160         let drop_ptr = iter.as_slice().as_ptr();
161 
162         unsafe {
163             // drop_ptr comes from a slice::Iter which only gives us a &[T] but for drop_in_place
164             // a pointer with mutable provenance is necessary. Therefore we must reconstruct
165             // it from the original vec but also avoid creating a &mut to the front since that could
166             // invalidate raw pointers to it which some unsafe code might rely on.
167             let vec_ptr = vec.as_mut().as_mut_ptr();
168             let drop_offset = drop_ptr.sub_ptr(vec_ptr);
169             let to_drop = ptr::slice_from_raw_parts_mut(vec_ptr.add(drop_offset), drop_len);
170             ptr::drop_in_place(to_drop);
171         }
172     }
173 }
174 
175 #[stable(feature = "drain", since = "1.6.0")]
176 impl<T, A: Allocator> ExactSizeIterator for Drain<'_, T, A> {
is_empty(&self) -> bool177     fn is_empty(&self) -> bool {
178         self.iter.is_empty()
179     }
180 }
181 
182 #[unstable(feature = "trusted_len", issue = "37572")]
183 unsafe impl<T, A: Allocator> TrustedLen for Drain<'_, T, A> {}
184 
185 #[stable(feature = "fused", since = "1.26.0")]
186 impl<T, A: Allocator> FusedIterator for Drain<'_, T, A> {}
187