library/alloc/src/vec/drain.rs

   1 use crate::alloc::{Allocator, Global};
   2 use core::fmt;
   3 use core::iter::{FusedIterator, TrustedLen};
   4 use core::mem::{self};
   5 use core::ptr::{self, NonNull};
   6 use core::slice::{self};
   7
   8 use super::Vec;
   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")]
  22 pub 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")]
  37 impl<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
  43 impl<'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     #[stable(feature = "vec_drain_as_slice", since = "1.46.0")]
  56     pub fn as_slice(&self) -> &[T] {
  57         self.iter.as_slice()
  58     }
  59
  60     /// Returns a reference to the underlying allocator.
  61     #[unstable(feature = "allocator_api", issue = "32838")]
  62     #[inline]
  63     pub fn allocator(&self) -> &A {
  64         unsafe { self.vec.as_ref().allocator() }
  65     }
  66 }
  67
  68 #[stable(feature = "vec_drain_as_slice", since = "1.46.0")]
  69 impl<'a, T, A: Allocator> AsRef<[T]> for Drain<'a, T, A> {
  70     fn as_ref(&self) -> &[T] {
  71         self.as_slice()
  72     }
  73 }
  74
  75 #[stable(feature = "drain", since = "1.6.0")]
  76 unsafe impl<T: Sync, A: Sync + Allocator> Sync for Drain<'_, T, A> {}
  77 #[stable(feature = "drain", since = "1.6.0")]
  78 unsafe impl<T: Send, A: Send + Allocator> Send for Drain<'_, T, A> {}
  79
  80 #[stable(feature = "drain", since = "1.6.0")]
  81 impl<T, A: Allocator> Iterator for Drain<'_, T, A> {
  82     type Item = T;
  83
  84     #[inline]
  85     fn next(&mut self) -> Option<T> {
  86         self.iter.next().map(|elt| unsafe { ptr::read(elt as *const _) })
  87     }
  88
  89     fn size_hint(&self) -> (usize, Option<usize>) {
  90         self.iter.size_hint()
  91     }
  92 }
  93
  94 #[stable(feature = "drain", since = "1.6.0")]
  95 impl<T, A: Allocator> DoubleEndedIterator for Drain<'_, T, A> {
  96     #[inline]
  97     fn next_back(&mut self) -> Option<T> {
  98         self.iter.next_back().map(|elt| unsafe { ptr::read(elt as *const _) })
  99     }
 100 }
 101
 102 #[stable(feature = "drain", since = "1.6.0")]
 103 impl<T, A: Allocator> Drop for Drain<'_, T, A> {
 104     fn drop(&mut self) {
 105         /// Continues dropping the remaining elements in the `Drain`, then moves back the
 106         /// un-`Drain`ed elements to restore the original `Vec`.
 107         struct DropGuard<'r, 'a, T, A: Allocator>(&'r mut Drain<'a, T, A>);
 108
 109         impl<'r, 'a, T, A: Allocator> Drop for DropGuard<'r, 'a, T, A> {
 110             fn drop(&mut self) {
 111                 // Continue the same loop we have below. If the loop already finished, this does
 112                 // nothing.
 113                 self.0.for_each(drop);
 114
 115                 if self.0.tail_len > 0 {
 116                     unsafe {
 117                         let source_vec = self.0.vec.as_mut();
 118                         // memmove back untouched tail, update to new length
 119                         let start = source_vec.len();
 120                         let tail = self.0.tail_start;
 121                         if tail != start {
 122                             let src = source_vec.as_ptr().add(tail);
 123                             let dst = source_vec.as_mut_ptr().add(start);
 124                             ptr::copy(src, dst, self.0.tail_len);
 125                         }
 126                         source_vec.set_len(start + self.0.tail_len);
 127                     }
 128                 }
 129             }
 130         }
 131
 132         // exhaust self first
 133         while let Some(item) = self.next() {
 134             let guard = DropGuard(self);
 135             drop(item);
 136             mem::forget(guard);
 137         }
 138
 139         // Drop a `DropGuard` to move back the non-drained tail of `self`.
 140         DropGuard(self);
 141     }
 142 }
 143
 144 #[stable(feature = "drain", since = "1.6.0")]
 145 impl<T, A: Allocator> ExactSizeIterator for Drain<'_, T, A> {
 146     fn is_empty(&self) -> bool {
 147         self.iter.is_empty()
 148     }
 149 }
 150
 151 #[unstable(feature = "trusted_len", issue = "37572")]
 152 unsafe impl<T, A: Allocator> TrustedLen for Drain<'_, T, A> {}
 153
 154 #[stable(feature = "fused", since = "1.26.0")]
 155 impl<T, A: Allocator> FusedIterator for Drain<'_, T, A> {}