1 //! Defines the `IntoIter` owned iterator for arrays.
5 iter::{ExactSizeIterator, FusedIterator, TrustedLen},
6 mem::{self, MaybeUninit},
11 /// A by-value [array] iterator.
13 /// [array]: ../../std/primitive.array.html
14 #[unstable(feature = "array_value_iter", issue = "65798")]
15 pub struct IntoIter<T, const N: usize> {
16 /// This is the array we are iterating over.
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!
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],
30 /// The elements in `data` that have not been yielded yet.
33 /// - `alive.start <= alive.end`
34 /// - `alive.end <= N`
38 impl<T, const N: usize> IntoIter<T, N> {
39 /// Creates a new iterator over the given `array`.
41 /// *Note*: this method might never get stabilized and/or removed in the
42 /// future as there will likely be another, preferred way of obtaining this
43 /// iterator (either via `IntoIterator` for arrays or via another way).
44 #[unstable(feature = "array_value_iter", issue = "65798")]
45 pub fn new(array: [T; N]) -> Self {
46 // SAFETY: The transmute here is actually safe. The docs of `MaybeUninit`
49 // > `MaybeUninit<T>` is guaranteed to have the same size and alignment
52 // The docs even show a transmute from an array of `MaybeUninit<T>` to
55 // With that, this initialization satisfies the invariants.
57 // FIXME(LukasKalbertodt): actually use `mem::transmute` here, once it
58 // works with const generics:
59 // `mem::transmute::<[T; N], [MaybeUninit<T>; N]>(array)`
61 // Until then, we can use `mem::transmute_copy` to create a bitwise copy
62 // as a different type, then forget `array` so that it is not dropped.
64 let iter = Self { data: mem::transmute_copy(&array), alive: 0..N };
70 /// Returns an immutable slice of all elements that have not been yielded
72 fn as_slice(&self) -> &[T] {
73 // SAFETY: We know that all elements within `alive` are properly initialized.
75 let slice = self.data.get_unchecked(self.alive.clone());
76 MaybeUninit::slice_get_ref(slice)
80 /// Returns a mutable slice of all elements that have not been yielded yet.
81 fn as_mut_slice(&mut self) -> &mut [T] {
82 // SAFETY: We know that all elements within `alive` are properly initialized.
84 let slice = self.data.get_unchecked_mut(self.alive.clone());
85 MaybeUninit::slice_get_mut(slice)
90 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
91 impl<T, const N: usize> Iterator for IntoIter<T, N> {
93 fn next(&mut self) -> Option<Self::Item> {
94 // Get the next index from the front.
96 // Increasing `alive.start` by 1 maintains the invariant regarding
97 // `alive`. However, due to this change, for a short time, the alive
98 // zone is not `data[alive]` anymore, but `data[idx..alive.end]`.
99 self.alive.next().map(|idx| {
100 // Read the element from the array.
101 // SAFETY: `idx` is an index into the former "alive" region of the
102 // array. Reading this element means that `data[idx]` is regarded as
103 // dead now (i.e. do not touch). As `idx` was the start of the
104 // alive-zone, the alive zone is now `data[alive]` again, restoring
106 unsafe { self.data.get_unchecked(idx).read() }
110 fn size_hint(&self) -> (usize, Option<usize>) {
111 let len = self.len();
115 fn count(self) -> usize {
119 fn last(mut self) -> Option<Self::Item> {
124 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
125 impl<T, const N: usize> DoubleEndedIterator for IntoIter<T, N> {
126 fn next_back(&mut self) -> Option<Self::Item> {
127 // Get the next index from the back.
129 // Decreasing `alive.end` by 1 maintains the invariant regarding
130 // `alive`. However, due to this change, for a short time, the alive
131 // zone is not `data[alive]` anymore, but `data[alive.start..=idx]`.
132 self.alive.next_back().map(|idx| {
133 // Read the element from the array.
134 // SAFETY: `idx` is an index into the former "alive" region of the
135 // array. Reading this element means that `data[idx]` is regarded as
136 // dead now (i.e. do not touch). As `idx` was the end of the
137 // alive-zone, the alive zone is now `data[alive]` again, restoring
139 unsafe { self.data.get_unchecked(idx).read() }
144 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
145 impl<T, const N: usize> Drop for IntoIter<T, N> {
147 // SAFETY: This is safe: `as_mut_slice` returns exactly the sub-slice
148 // of elements that have not been moved out yet and that remain
150 unsafe { ptr::drop_in_place(self.as_mut_slice()) }
154 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
155 impl<T, const N: usize> ExactSizeIterator for IntoIter<T, N> {
156 fn len(&self) -> usize {
157 // Will never underflow due to the invariant `alive.start <=
159 self.alive.end - self.alive.start
161 fn is_empty(&self) -> bool {
162 self.alive.is_empty()
166 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
167 impl<T, const N: usize> FusedIterator for IntoIter<T, N> {}
169 // The iterator indeed reports the correct length. The number of "alive"
170 // elements (that will still be yielded) is the length of the range `alive`.
171 // This range is decremented in length in either `next` or `next_back`. It is
172 // always decremented by 1 in those methods, but only if `Some(_)` is returned.
173 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
174 unsafe impl<T, const N: usize> TrustedLen for IntoIter<T, N> {}
176 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
177 impl<T: Clone, const N: usize> Clone for IntoIter<T, N> {
178 fn clone(&self) -> Self {
179 // Note, we don't really need to match the exact same alive range, so
180 // we can just clone into offset 0 regardless of where `self` is.
181 let mut new = Self { data: MaybeUninit::uninit_array(), alive: 0..0 };
183 // Clone all alive elements.
184 for (src, dst) in self.as_slice().iter().zip(&mut new.data) {
185 // Write a clone into the new array, then update its alive range.
186 // If cloning panics, we'll correctly drop the previous items.
187 dst.write(src.clone());
195 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
196 impl<T: fmt::Debug, const N: usize> fmt::Debug for IntoIter<T, N> {
197 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
198 // Only print the elements that were not yielded yet: we cannot
199 // access the yielded elements anymore.
200 f.debug_tuple("IntoIter").field(&self.as_slice()).finish()