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> { 40 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")] 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 72 #[stable(feature = "vec_drain_as_slice", since = "1.46.0")] 73 impl<'a, T, A: Allocator> AsRef<[T]> for Drain<'a, T, A> { 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] 89 fn next(&mut self) -> Option<T> { 90 self.iter.next().map(|elt| unsafe { ptr::read(elt as *const _) }) 91 } 92 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] 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> { 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> { 177 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