iter.rs.html -- source
Description: Source to the Rust file `src/libcore/array/iter.rs`.
Keywords: rust, rustlang, rust-lang

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    //! Defines the `IntoIter` owned iterator for arrays.

    use crate::{
        fmt,
        iter::{ExactSizeIterator, FusedIterator, TrustedLen},
        mem::{self, MaybeUninit},
        ops::Range,
        ptr,
    };
    use super::LengthAtMost32;

    /// A by-value [array] iterator.
    ///
    /// [array]: ../../std/primitive.array.html
    #[unstable(feature = "array_value_iter", issue = "65798")]
    pub struct IntoIter<T, const N: usize>
    where
        [T; N]: LengthAtMost32,
    {
        /// 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}>
    where
        [T; N]: LengthAtMost32,
    {
        /// Creates a new iterator over the given `array`.
        ///
        /// *Note*: this method might never get stabilized and/or removed in the
        /// future as there will likely be another, preferred way of obtaining this
        /// iterator (either via `IntoIterator` for arrays or via another way).
        #[unstable(feature = "array_value_iter", issue = "65798")]
        pub fn new(array: [T; N]) -> Self {
            // 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 do it manually here. We first create a bitwise copy
            // but cast the pointer so that it is treated as a different type. Then
            // we forget `array` so that it is not dropped.
            let data = unsafe {
                let data = ptr::read(&array as *const [T; N] as *const [MaybeUninit<T>; N]);
                mem::forget(array);
                data
            };

            Self {
                data,
                alive: 0..N,
            }
        }

        /// Returns an immutable slice of all elements that have not been yielded
        /// yet.
        fn as_slice(&self) -> &[T] {
            // This transmute is safe. As mentioned in `new`, `MaybeUninit` retains
            // the size and alignment of `T`. Furthermore, we know that all
            // elements within `alive` are properly initialized.
            let slice = &self.data[self.alive.clone()];
            unsafe {
                mem::transmute::<&[MaybeUninit<T>], &[T]>(slice)
            }
        }
    }

    #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
    impl<T, const N: usize> Iterator for IntoIter<T, {N}>
    where
        [T; N]: LengthAtMost32,
    {
        type Item = T;
        fn next(&mut self) -> Option<Self::Item> {
            if self.alive.start == self.alive.end {
                return None;
            }

            // Bump start index.
            //
            // From the check above we know that `alive.start != alive.end`.
            // Combine this with the invariant `alive.start <= alive.end`, we know
            // that `alive.start < alive.end`. 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]`.
            let idx = self.alive.start;
            self.alive.start += 1;

            // Read the element from the array. This is safe: `idx` is an index
            // into the "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.
            let out = unsafe { self.data.get_unchecked(idx).read() };

            Some(out)
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            let len = self.len();
            (len, Some(len))
        }

        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}>
    where
        [T; N]: LengthAtMost32,
    {
        fn next_back(&mut self) -> Option<Self::Item> {
            if self.alive.start == self.alive.end {
                return None;
            }

            // Decrease end index.
            //
            // From the check above we know that `alive.start != alive.end`.
            // Combine this with the invariant `alive.start <= alive.end`, we know
            // that `alive.start < alive.end`. As `alive.start` cannot be negative,
            // `alive.end` is at least 1, meaning that we can safely decrement it
            // by one. This also maintains the invariant `alive.start <=
            // alive.end`. However, due to this change, for a short time, the alive
            // zone is not `data[alive]` anymore, but `data[alive.start..alive.end
            // + 1]`.
            self.alive.end -= 1;

            // Read the element from the array. This is safe: `alive.end` is an
            // index into the "alive" region of the array. Compare the previous
            // comment that states that the alive region is
            // `data[alive.start..alive.end + 1]`. Reading this element means that
            // `data[alive.end]` is regarded as dead now (i.e. do not touch). As
            // `alive.end` was the end of the alive-zone, the alive zone is now
            // `data[alive]` again, restoring all invariants.
            let out = unsafe { self.data.get_unchecked(self.alive.end).read() };

            Some(out)
        }
    }

    #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
    impl<T, const N: usize> Drop for IntoIter<T, {N}>
    where
        [T; N]: LengthAtMost32,
    {
        fn drop(&mut self) {
            // We simply drop each element via `for_each`. This should not incur
            // any significant runtime overhead and avoids adding another `unsafe`
            // block.
            self.by_ref().for_each(drop);
        }
    }

    #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
    impl<T, const N: usize> ExactSizeIterator for IntoIter<T, {N}>
    where
        [T; N]: LengthAtMost32,
    {
        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}>
    where
        [T; N]: LengthAtMost32,
    {}

    // 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}>
    where
        [T; N]: LengthAtMost32,
    {}

    #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
    impl<T: Clone, const N: usize> Clone for IntoIter<T, {N}>
    where
        [T; N]: LengthAtMost32,
    {
        fn clone(&self) -> Self {
            unsafe {
                // This creates a new uninitialized array. Note that the `assume_init`
                // refers to the array, not the individual elements. And it is Ok if
                // the array is in an uninitialized state as all elements may be
                // uninitialized (all bit patterns are valid). Compare the
                // `MaybeUninit` docs for more information.
                let mut new_data: [MaybeUninit<T>; N] = MaybeUninit::uninit().assume_init();

                // Clone all alive elements.
                for idx in self.alive.clone() {
                    // The element at `idx` in the old array is alive, so we can
                    // safely call `get_ref()`. We then clone it, and write the
                    // clone into the new array.
                    let clone = self.data.get_unchecked(idx).get_ref().clone();
                    new_data.get_unchecked_mut(idx).write(clone);
                }

                Self {
                    data: new_data,
                    alive: self.alive.clone(),
                }
            }
        }
    }

    #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
    impl<T: fmt::Debug, const N: usize> fmt::Debug for IntoIter<T, {N}>
    where
        [T; N]: LengthAtMost32,
    {
        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()
        }
    }