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