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//! Defines the `IntoIter` owned iterator for arrays.
use crate::{
fmt,
iter::{self, ExactSizeIterator, FusedIterator, TrustedLen},
mem::{self, MaybeUninit},
ops::Range,
ptr,
};
/// A by-value [array] iterator.
#[stable(feature = "array_value_iter", since = "1.51.0")]
#[rustc_insignificant_dtor]
pub struct IntoIter<T, const N: usize> {
/// This is the array we are iterating over.
///
/// Elements with index `i` where `alive.start <= i < alive.end` have not
/// been yielded yet and are valid array entries. Elements with indices `i
/// < alive.start` or `i >= alive.end` have been yielded already and must
/// not be accessed anymore! Those dead elements might even be in a
/// completely uninitialized state!
///
/// So the invariants are:
/// - `data[alive]` is alive (i.e. contains valid elements)
/// - `data[..alive.start]` and `data[alive.end..]` are dead (i.e. the
/// elements were already read and must not be touched anymore!)
data: [MaybeUninit<T>; N],
/// The elements in `data` that have not been yielded yet.
///
/// Invariants:
/// - `alive.start <= alive.end`
/// - `alive.end <= N`
alive: Range<usize>,
}
impl<T, const N: usize> IntoIter<T, N> {
/// Creates a new iterator over the given `array`.
///
/// *Note*: this method might be deprecated in the future,
/// since [`IntoIterator`] is now implemented for arrays.
///
/// # Examples
///
/// ```
/// use std::array;
///
/// for value in array::IntoIter::new([1, 2, 3, 4, 5]) {
/// // The type of `value` is an `i32` here, instead of `&i32`
/// let _: i32 = value;
/// }
///
/// // Since Rust 1.53, arrays implement IntoIterator directly:
/// for value in [1, 2, 3, 4, 5] {
/// // The type of `value` is an `i32` here, instead of `&i32`
/// let _: i32 = value;
/// }
/// ```
#[stable(feature = "array_value_iter", since = "1.51.0")]
pub fn new(array: [T; N]) -> Self {
// SAFETY: The transmute here is actually safe. The docs of `MaybeUninit`
// promise:
//
// > `MaybeUninit<T>` is guaranteed to have the same size and alignment
// > as `T`.
//
// The docs even show a transmute from an array of `MaybeUninit<T>` to
// an array of `T`.
//
// With that, this initialization satisfies the invariants.
// FIXME(LukasKalbertodt): actually use `mem::transmute` here, once it
// works with const generics:
// `mem::transmute::<[T; N], [MaybeUninit<T>; N]>(array)`
//
// Until then, we can use `mem::transmute_copy` to create a bitwise copy
// as a different type, then forget `array` so that it is not dropped.
unsafe {
let iter = Self { data: mem::transmute_copy(&array), alive: 0..N };
mem::forget(array);
iter
}
}
/// Returns an immutable slice of all elements that have not been yielded
/// yet.
#[stable(feature = "array_value_iter", since = "1.51.0")]
pub fn as_slice(&self) -> &[T] {
// SAFETY: We know that all elements within `alive` are properly initialized.
unsafe {
let slice = self.data.get_unchecked(self.alive.clone());
MaybeUninit::slice_assume_init_ref(slice)
}
}
/// Returns a mutable slice of all elements that have not been yielded yet.
#[stable(feature = "array_value_iter", since = "1.51.0")]
pub fn as_mut_slice(&mut self) -> &mut [T] {
// SAFETY: We know that all elements within `alive` are properly initialized.
unsafe {
let slice = self.data.get_unchecked_mut(self.alive.clone());
MaybeUninit::slice_assume_init_mut(slice)
}
}
}
#[stable(feature = "array_value_iter_impls", since = "1.40.0")]
impl<T, const N: usize> Iterator for IntoIter<T, N> {
type Item = T;
fn next(&mut self) -> Option<Self::Item> {
// Get the next index from the front.
//
// Increasing `alive.start` by 1 maintains the invariant regarding
// `alive`. However, due to this change, for a short time, the alive
// zone is not `data[alive]` anymore, but `data[idx..alive.end]`.
self.alive.next().map(|idx| {
// Read the element from the array.
// SAFETY: `idx` is an index into the former "alive" region of the
// array. Reading this element means that `data[idx]` is regarded as
// dead now (i.e. do not touch). As `idx` was the start of the
// alive-zone, the alive zone is now `data[alive]` again, restoring
// all invariants.
unsafe { self.data.get_unchecked(idx).assume_init_read() }
})
}
fn size_hint(&self) -> (usize, Option<usize>) {
let len = self.len();
(len, Some(len))
}
#[inline]
fn fold<Acc, Fold>(mut self, init: Acc, mut fold: Fold) -> Acc
where
Fold: FnMut(Acc, Self::Item) -> Acc,
{
let data = &mut self.data;
self.alive.by_ref().fold(init, |acc, idx| {
// SAFETY: idx is obtained by folding over the `alive` range, which implies the
// value is currently considered alive but as the range is being consumed each value
// we read here will only be read once and then considered dead.
fold(acc, unsafe { data.get_unchecked(idx).assume_init_read() })
})
}
fn count(self) -> usize {
self.len()
}
fn last(mut self) -> Option<Self::Item> {
self.next_back()
}
}
#[stable(feature = "array_value_iter_impls", since = "1.40.0")]
impl<T, const N: usize> DoubleEndedIterator for IntoIter<T, N> {
fn next_back(&mut self) -> Option<Self::Item> {
// Get the next index from the back.
//
// Decreasing `alive.end` by 1 maintains the invariant regarding
// `alive`. However, due to this change, for a short time, the alive
// zone is not `data[alive]` anymore, but `data[alive.start..=idx]`.
self.alive.next_back().map(|idx| {
// Read the element from the array.
// SAFETY: `idx` is an index into the former "alive" region of the
// array. Reading this element means that `data[idx]` is regarded as
// dead now (i.e. do not touch). As `idx` was the end of the
// alive-zone, the alive zone is now `data[alive]` again, restoring
// all invariants.
unsafe { self.data.get_unchecked(idx).assume_init_read() }
})
}
}
#[stable(feature = "array_value_iter_impls", since = "1.40.0")]
impl<T, const N: usize> Drop for IntoIter<T, N> {
fn drop(&mut self) {
// SAFETY: This is safe: `as_mut_slice` returns exactly the sub-slice
// of elements that have not been moved out yet and that remain
// to be dropped.
unsafe { ptr::drop_in_place(self.as_mut_slice()) }
}
}
#[stable(feature = "array_value_iter_impls", since = "1.40.0")]
impl<T, const N: usize> ExactSizeIterator for IntoIter<T, N> {
fn len(&self) -> usize {
// Will never underflow due to the invariant `alive.start <=
// alive.end`.
self.alive.end - self.alive.start
}
fn is_empty(&self) -> bool {
self.alive.is_empty()
}
}
#[stable(feature = "array_value_iter_impls", since = "1.40.0")]
impl<T, const N: usize> FusedIterator for IntoIter<T, N> {}
// The iterator indeed reports the correct length. The number of "alive"
// elements (that will still be yielded) is the length of the range `alive`.
// This range is decremented in length in either `next` or `next_back`. It is
// always decremented by 1 in those methods, but only if `Some(_)` is returned.
#[stable(feature = "array_value_iter_impls", since = "1.40.0")]
unsafe impl<T, const N: usize> TrustedLen for IntoIter<T, N> {}
#[stable(feature = "array_value_iter_impls", since = "1.40.0")]
impl<T: Clone, const N: usize> Clone for IntoIter<T, N> {
fn clone(&self) -> Self {
// Note, we don't really need to match the exact same alive range, so
// we can just clone into offset 0 regardless of where `self` is.
let mut new = Self { data: MaybeUninit::uninit_array(), alive: 0..0 };
// Clone all alive elements.
for (src, dst) in iter::zip(self.as_slice(), &mut new.data) {
// Write a clone into the new array, then update its alive range.
// If cloning panics, we'll correctly drop the previous items.
dst.write(src.clone());
new.alive.end += 1;
}
new
}
}
#[stable(feature = "array_value_iter_impls", since = "1.40.0")]
impl<T: fmt::Debug, const N: usize> fmt::Debug for IntoIter<T, N> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
// Only print the elements that were not yielded yet: we cannot
// access the yielded elements anymore.
f.debug_tuple("IntoIter").field(&self.as_slice()).finish()
}
}