library/core/src/array/iter.rs

   1 //! Defines the `IntoIter` owned iterator for arrays.
   2
   3 use crate::{
   4     fmt,
   5     iter::{ExactSizeIterator, FusedIterator, TrustedLen},
   6     mem::{self, MaybeUninit},
   7     ops::Range,
   8     ptr,
   9 };
  10
  11 /// A by-value [array] iterator.
  12 ///
  13 /// [array]: ../../std/primitive.array.html
  14 #[stable(feature = "array_value_iter", since = "1.51.0")]
  15 pub struct IntoIter<T, const N: usize> {
  16     /// This is the array we are iterating over.
  17     ///
  18     /// Elements with index `i` where `alive.start <= i < alive.end` have not
  19     /// been yielded yet and are valid array entries. Elements with indices `i
  20     /// < alive.start` or `i >= alive.end` have been yielded already and must
  21     /// not be accessed anymore! Those dead elements might even be in a
  22     /// completely uninitialized state!
  23     ///
  24     /// So the invariants are:
  25     /// - `data[alive]` is alive (i.e. contains valid elements)
  26     /// - `data[..alive.start]` and `data[alive.end..]` are dead (i.e. the
  27     ///   elements were already read and must not be touched anymore!)
  28     data: [MaybeUninit<T>; N],
  29
  30     /// The elements in `data` that have not been yielded yet.
  31     ///
  32     /// Invariants:
  33     /// - `alive.start <= alive.end`
  34     /// - `alive.end <= N`
  35     alive: Range<usize>,
  36 }
  37
  38 impl<T, const N: usize> IntoIter<T, N> {
  39     /// Creates a new iterator over the given `array`.
  40     ///
  41     /// *Note*: this method might be deprecated in the future,
  42     /// after [`IntoIterator` is implemented for arrays][array-into-iter].
  43     ///
  44     /// # Examples
  45     ///
  46     /// ```
  47     /// use std::array;
  48     ///
  49     /// for value in array::IntoIter::new([1, 2, 3, 4, 5]) {
  50     ///     // The type of `value` is a `i32` here, instead of `&i32`
  51     ///     let _: i32 = value;
  52     /// }
  53     /// ```
  54     /// [array-into-iter]: https://github.com/rust-lang/rust/pull/65819
  55     #[stable(feature = "array_value_iter", since = "1.51.0")]
  56     pub fn new(array: [T; N]) -> Self {
  57         // SAFETY: The transmute here is actually safe. The docs of `MaybeUninit`
  58         // promise:
  59         //
  60         // > `MaybeUninit<T>` is guaranteed to have the same size and alignment
  61         // > as `T`.
  62         //
  63         // The docs even show a transmute from an array of `MaybeUninit<T>` to
  64         // an array of `T`.
  65         //
  66         // With that, this initialization satisfies the invariants.
  67
  68         // FIXME(LukasKalbertodt): actually use `mem::transmute` here, once it
  69         // works with const generics:
  70         //     `mem::transmute::<[T; N], [MaybeUninit<T>; N]>(array)`
  71         //
  72         // Until then, we can use `mem::transmute_copy` to create a bitwise copy
  73         // as a different type, then forget `array` so that it is not dropped.
  74         unsafe {
  75             let iter = Self { data: mem::transmute_copy(&array), alive: 0..N };
  76             mem::forget(array);
  77             iter
  78         }
  79     }
  80
  81     /// Returns an immutable slice of all elements that have not been yielded
  82     /// yet.
  83     #[stable(feature = "array_value_iter", since = "1.51.0")]
  84     pub fn as_slice(&self) -> &[T] {
  85         // SAFETY: We know that all elements within `alive` are properly initialized.
  86         unsafe {
  87             let slice = self.data.get_unchecked(self.alive.clone());
  88             MaybeUninit::slice_assume_init_ref(slice)
  89         }
  90     }
  91
  92     /// Returns a mutable slice of all elements that have not been yielded yet.
  93     #[stable(feature = "array_value_iter", since = "1.51.0")]
  94     pub fn as_mut_slice(&mut self) -> &mut [T] {
  95         // SAFETY: We know that all elements within `alive` are properly initialized.
  96         unsafe {
  97             let slice = self.data.get_unchecked_mut(self.alive.clone());
  98             MaybeUninit::slice_assume_init_mut(slice)
  99         }
 100     }
 101 }
 102
 103 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
 104 impl<T, const N: usize> Iterator for IntoIter<T, N> {
 105     type Item = T;
 106     fn next(&mut self) -> Option<Self::Item> {
 107         // Get the next index from the front.
 108         //
 109         // Increasing `alive.start` by 1 maintains the invariant regarding
 110         // `alive`. However, due to this change, for a short time, the alive
 111         // zone is not `data[alive]` anymore, but `data[idx..alive.end]`.
 112         self.alive.next().map(|idx| {
 113             // Read the element from the array.
 114             // SAFETY: `idx` is an index into the former "alive" region of the
 115             // array. Reading this element means that `data[idx]` is regarded as
 116             // dead now (i.e. do not touch). As `idx` was the start of the
 117             // alive-zone, the alive zone is now `data[alive]` again, restoring
 118             // all invariants.
 119             unsafe { self.data.get_unchecked(idx).assume_init_read() }
 120         })
 121     }
 122
 123     fn size_hint(&self) -> (usize, Option<usize>) {
 124         let len = self.len();
 125         (len, Some(len))
 126     }
 127
 128     fn count(self) -> usize {
 129         self.len()
 130     }
 131
 132     fn last(mut self) -> Option<Self::Item> {
 133         self.next_back()
 134     }
 135 }
 136
 137 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
 138 impl<T, const N: usize> DoubleEndedIterator for IntoIter<T, N> {
 139     fn next_back(&mut self) -> Option<Self::Item> {
 140         // Get the next index from the back.
 141         //
 142         // Decreasing `alive.end` by 1 maintains the invariant regarding
 143         // `alive`. However, due to this change, for a short time, the alive
 144         // zone is not `data[alive]` anymore, but `data[alive.start..=idx]`.
 145         self.alive.next_back().map(|idx| {
 146             // Read the element from the array.
 147             // SAFETY: `idx` is an index into the former "alive" region of the
 148             // array. Reading this element means that `data[idx]` is regarded as
 149             // dead now (i.e. do not touch). As `idx` was the end of the
 150             // alive-zone, the alive zone is now `data[alive]` again, restoring
 151             // all invariants.
 152             unsafe { self.data.get_unchecked(idx).assume_init_read() }
 153         })
 154     }
 155 }
 156
 157 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
 158 impl<T, const N: usize> Drop for IntoIter<T, N> {
 159     fn drop(&mut self) {
 160         // SAFETY: This is safe: `as_mut_slice` returns exactly the sub-slice
 161         // of elements that have not been moved out yet and that remain
 162         // to be dropped.
 163         unsafe { ptr::drop_in_place(self.as_mut_slice()) }
 164     }
 165 }
 166
 167 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
 168 impl<T, const N: usize> ExactSizeIterator for IntoIter<T, N> {
 169     fn len(&self) -> usize {
 170         // Will never underflow due to the invariant `alive.start <=
 171         // alive.end`.
 172         self.alive.end - self.alive.start
 173     }
 174     fn is_empty(&self) -> bool {
 175         self.alive.is_empty()
 176     }
 177 }
 178
 179 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
 180 impl<T, const N: usize> FusedIterator for IntoIter<T, N> {}
 181
 182 // The iterator indeed reports the correct length. The number of "alive"
 183 // elements (that will still be yielded) is the length of the range `alive`.
 184 // This range is decremented in length in either `next` or `next_back`. It is
 185 // always decremented by 1 in those methods, but only if `Some(_)` is returned.
 186 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
 187 unsafe impl<T, const N: usize> TrustedLen for IntoIter<T, N> {}
 188
 189 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
 190 impl<T: Clone, const N: usize> Clone for IntoIter<T, N> {
 191     fn clone(&self) -> Self {
 192         // Note, we don't really need to match the exact same alive range, so
 193         // we can just clone into offset 0 regardless of where `self` is.
 194         let mut new = Self { data: MaybeUninit::uninit_array(), alive: 0..0 };
 195
 196         // Clone all alive elements.
 197         for (src, dst) in self.as_slice().iter().zip(&mut new.data) {
 198             // Write a clone into the new array, then update its alive range.
 199             // If cloning panics, we'll correctly drop the previous items.
 200             dst.write(src.clone());
 201             new.alive.end += 1;
 202         }
 203
 204         new
 205     }
 206 }
 207
 208 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
 209 impl<T: fmt::Debug, const N: usize> fmt::Debug for IntoIter<T, N> {
 210     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 211         // Only print the elements that were not yielded yet: we cannot
 212         // access the yielded elements anymore.
 213         f.debug_tuple("IntoIter").field(&self.as_slice()).finish()
 214     }
 215 }