mv std libs to library/
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236
library/core/src/array/iter.rs
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236
library/core/src/array/iter.rs
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//! Defines the `IntoIter` owned iterator for arrays.
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use crate::{
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fmt,
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iter::{ExactSizeIterator, FusedIterator, TrustedLen},
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mem::{self, MaybeUninit},
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ops::Range,
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ptr,
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};
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/// A by-value [array] iterator.
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///
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/// [array]: ../../std/primitive.array.html
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#[unstable(feature = "array_value_iter", issue = "65798")]
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pub struct IntoIter<T, const N: usize> {
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/// This is the array we are iterating over.
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///
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/// Elements with index `i` where `alive.start <= i < alive.end` have not
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/// been yielded yet and are valid array entries. Elements with indices `i
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/// < alive.start` or `i >= alive.end` have been yielded already and must
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/// not be accessed anymore! Those dead elements might even be in a
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/// completely uninitialized state!
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///
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/// So the invariants are:
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/// - `data[alive]` is alive (i.e. contains valid elements)
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/// - `data[..alive.start]` and `data[alive.end..]` are dead (i.e. the
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/// elements were already read and must not be touched anymore!)
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data: [MaybeUninit<T>; N],
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/// The elements in `data` that have not been yielded yet.
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///
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/// Invariants:
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/// - `alive.start <= alive.end`
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/// - `alive.end <= N`
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alive: Range<usize>,
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}
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impl<T, const N: usize> IntoIter<T, N> {
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/// Creates a new iterator over the given `array`.
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///
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/// *Note*: this method might never get stabilized and/or removed in the
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/// future as there will likely be another, preferred way of obtaining this
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/// iterator (either via `IntoIterator` for arrays or via another way).
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#[unstable(feature = "array_value_iter", issue = "65798")]
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pub fn new(array: [T; N]) -> Self {
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// SAFETY: The transmute here is actually safe. The docs of `MaybeUninit`
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// promise:
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//
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// > `MaybeUninit<T>` is guaranteed to have the same size and alignment
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// > as `T`.
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//
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// The docs even show a transmute from an array of `MaybeUninit<T>` to
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// an array of `T`.
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//
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// With that, this initialization satisfies the invariants.
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// FIXME(LukasKalbertodt): actually use `mem::transmute` here, once it
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// works with const generics:
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// `mem::transmute::<[T; {N}], [MaybeUninit<T>; {N}]>(array)`
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//
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// Until then, we do it manually here. We first create a bitwise copy
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// but cast the pointer so that it is treated as a different type. Then
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// we forget `array` so that it is not dropped.
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let data = unsafe {
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let data = ptr::read(&array as *const [T; N] as *const [MaybeUninit<T>; N]);
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mem::forget(array);
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data
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};
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Self { data, alive: 0..N }
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}
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/// Returns an immutable slice of all elements that have not been yielded
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/// yet.
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fn as_slice(&self) -> &[T] {
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let slice = &self.data[self.alive.clone()];
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// SAFETY: This transmute is safe. As mentioned in `new`, `MaybeUninit` retains
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// the size and alignment of `T`. Furthermore, we know that all
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// elements within `alive` are properly initialized.
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unsafe { mem::transmute::<&[MaybeUninit<T>], &[T]>(slice) }
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}
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/// Returns a mutable slice of all elements that have not been yielded yet.
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fn as_mut_slice(&mut self) -> &mut [T] {
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// This transmute is safe, same as in `as_slice` above.
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let slice = &mut self.data[self.alive.clone()];
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// SAFETY: This transmute is safe. As mentioned in `new`, `MaybeUninit` retains
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// the size and alignment of `T`. Furthermore, we know that all
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// elements within `alive` are properly initialized.
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unsafe { mem::transmute::<&mut [MaybeUninit<T>], &mut [T]>(slice) }
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}
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}
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#[stable(feature = "array_value_iter_impls", since = "1.40.0")]
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impl<T, const N: usize> Iterator for IntoIter<T, N> {
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type Item = T;
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fn next(&mut self) -> Option<Self::Item> {
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if self.alive.start == self.alive.end {
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return None;
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}
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// Bump start index.
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//
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// From the check above we know that `alive.start != alive.end`.
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// Combine this with the invariant `alive.start <= alive.end`, we know
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// that `alive.start < alive.end`. Increasing `alive.start` by 1
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// maintains the invariant regarding `alive`. However, due to this
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// change, for a short time, the alive zone is not `data[alive]`
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// anymore, but `data[idx..alive.end]`.
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let idx = self.alive.start;
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self.alive.start += 1;
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// Read the element from the array.
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// SAFETY: This is safe: `idx` is an index
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// into the "alive" region of the array. Reading this element means
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// that `data[idx]` is regarded as dead now (i.e. do not touch). As
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// `idx` was the start of the alive-zone, the alive zone is now
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// `data[alive]` again, restoring all invariants.
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let out = unsafe { self.data.get_unchecked(idx).read() };
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Some(out)
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}
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fn size_hint(&self) -> (usize, Option<usize>) {
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let len = self.len();
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(len, Some(len))
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}
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fn count(self) -> usize {
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self.len()
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}
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fn last(mut self) -> Option<Self::Item> {
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self.next_back()
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}
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}
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#[stable(feature = "array_value_iter_impls", since = "1.40.0")]
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impl<T, const N: usize> DoubleEndedIterator for IntoIter<T, N> {
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fn next_back(&mut self) -> Option<Self::Item> {
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if self.alive.start == self.alive.end {
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return None;
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}
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// Decrease end index.
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//
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// From the check above we know that `alive.start != alive.end`.
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// Combine this with the invariant `alive.start <= alive.end`, we know
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// that `alive.start < alive.end`. As `alive.start` cannot be negative,
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// `alive.end` is at least 1, meaning that we can safely decrement it
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// by one. This also maintains the invariant `alive.start <=
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// alive.end`. However, due to this change, for a short time, the alive
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// zone is not `data[alive]` anymore, but `data[alive.start..alive.end
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// + 1]`.
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self.alive.end -= 1;
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// Read the element from the array.
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// SAFETY: This is safe: `alive.end` is an
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// index into the "alive" region of the array. Compare the previous
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// comment that states that the alive region is
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// `data[alive.start..alive.end + 1]`. Reading this element means that
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// `data[alive.end]` is regarded as dead now (i.e. do not touch). As
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// `alive.end` was the end of the alive-zone, the alive zone is now
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// `data[alive]` again, restoring all invariants.
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let out = unsafe { self.data.get_unchecked(self.alive.end).read() };
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Some(out)
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}
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}
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#[stable(feature = "array_value_iter_impls", since = "1.40.0")]
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impl<T, const N: usize> Drop for IntoIter<T, N> {
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fn drop(&mut self) {
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// SAFETY: This is safe: `as_mut_slice` returns exactly the sub-slice
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// of elements that have not been moved out yet and that remain
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// to be dropped.
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unsafe { ptr::drop_in_place(self.as_mut_slice()) }
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}
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}
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#[stable(feature = "array_value_iter_impls", since = "1.40.0")]
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impl<T, const N: usize> ExactSizeIterator for IntoIter<T, N> {
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fn len(&self) -> usize {
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// Will never underflow due to the invariant `alive.start <=
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// alive.end`.
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self.alive.end - self.alive.start
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}
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fn is_empty(&self) -> bool {
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self.alive.is_empty()
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}
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}
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#[stable(feature = "array_value_iter_impls", since = "1.40.0")]
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impl<T, const N: usize> FusedIterator for IntoIter<T, N> {}
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// The iterator indeed reports the correct length. The number of "alive"
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// elements (that will still be yielded) is the length of the range `alive`.
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// This range is decremented in length in either `next` or `next_back`. It is
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// always decremented by 1 in those methods, but only if `Some(_)` is returned.
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#[stable(feature = "array_value_iter_impls", since = "1.40.0")]
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unsafe impl<T, const N: usize> TrustedLen for IntoIter<T, N> {}
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#[stable(feature = "array_value_iter_impls", since = "1.40.0")]
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impl<T: Clone, const N: usize> Clone for IntoIter<T, N> {
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fn clone(&self) -> Self {
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// SAFETY: each point of unsafety is documented inside the unsafe block
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unsafe {
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// This creates a new uninitialized array. Note that the `assume_init`
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// refers to the array, not the individual elements. And it is Ok if
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// the array is in an uninitialized state as all elements may be
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// uninitialized (all bit patterns are valid). Compare the
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// `MaybeUninit` docs for more information.
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let mut new_data: [MaybeUninit<T>; N] = MaybeUninit::uninit().assume_init();
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// Clone all alive elements.
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for idx in self.alive.clone() {
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// The element at `idx` in the old array is alive, so we can
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// safely call `get_ref()`. We then clone it, and write the
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// clone into the new array.
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let clone = self.data.get_unchecked(idx).get_ref().clone();
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new_data.get_unchecked_mut(idx).write(clone);
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}
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Self { data: new_data, alive: self.alive.clone() }
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}
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}
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}
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#[stable(feature = "array_value_iter_impls", since = "1.40.0")]
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impl<T: fmt::Debug, const N: usize> fmt::Debug for IntoIter<T, N> {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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// Only print the elements that were not yielded yet: we cannot
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// access the yielded elements anymore.
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f.debug_tuple("IntoIter").field(&self.as_slice()).finish()
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}
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}
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