alloc/vec/

into_iter.rs

use core::iter::{
    FusedIterator, InPlaceIterable, SourceIter, TrustedFused, TrustedLen,
    TrustedRandomAccessNoCoerce,
};
use core::marker::PhantomData;
use core::mem::{ManuallyDrop, MaybeUninit, SizedTypeProperties};
use core::num::NonZero;
#[cfg(not(no_global_oom_handling))]
use core::ops::Deref;
use core::panic::UnwindSafe;
use core::ptr::{self, NonNull};
use core::{array, fmt, slice};

#[cfg(not(no_global_oom_handling))]
use super::AsVecIntoIter;
use crate::alloc::{Allocator, Global};
#[cfg(not(no_global_oom_handling))]
use crate::collections::VecDeque;
use crate::raw_vec::RawVec;

macro non_null {
    (mut $place:expr, $t:ident) => {{
        #![allow(unused_unsafe)] // we're sometimes used within an unsafe block
        // ignore-tidy-undocumented-unsafe
        unsafe { &mut *((&raw mut $place) as *mut NonNull<$t>) }
    }},
    ($place:expr, $t:ident) => {{
        #![allow(unused_unsafe)] // we're sometimes used within an unsafe block
        // ignore-tidy-undocumented-unsafe
        unsafe { *((&raw const $place) as *const NonNull<$t>) }
    }},
}

/// An iterator that moves out of a vector.
///
/// This `struct` is created by the `into_iter` method on [`Vec`](super::Vec)
/// (provided by the [`IntoIterator`] trait).
///
/// # Example
///
/// ```
/// let v = vec![0, 1, 2];
/// let iter: std::vec::IntoIter<_> = v.into_iter();
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_insignificant_dtor]
pub struct IntoIter<
    T,
    #[stable(feature = "vec_into_iter_allocator", since = "CURRENT_RUSTC_VERSION")] A: Allocator = Global,
> {
    pub(super) buf: NonNull<T>,
    pub(super) phantom: PhantomData<T>,
    pub(super) cap: usize,
    // the drop impl reconstructs a RawVec from buf, cap and alloc
    // to avoid dropping the allocator twice we need to wrap it into ManuallyDrop
    pub(super) alloc: ManuallyDrop<A>,
    pub(super) ptr: NonNull<T>,
    /// If T is a ZST, this is actually ptr+len. This encoding is picked so that
    /// ptr == end is a quick test for the Iterator being empty, that works
    /// for both ZST and non-ZST.
    /// For non-ZSTs the pointer is treated as `NonNull<T>`
    pub(super) end: *const T,
}

// Manually mirroring what `Vec` has,
// because otherwise we get `T: RefUnwindSafe` from `NonNull`.
#[stable(feature = "catch_unwind", since = "1.9.0")]
impl<T: UnwindSafe, A: Allocator + UnwindSafe> UnwindSafe for IntoIter<T, A> {}

#[stable(feature = "vec_intoiter_debug", since = "1.13.0")]
impl<T: fmt::Debug, A: Allocator> fmt::Debug for IntoIter<T, A> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("IntoIter").field(&self.as_slice()).finish()
    }
}

impl<T, A: Allocator> IntoIter<T, A> {
    /// Returns the remaining items of this iterator as a slice.
    ///
    /// # Examples
    ///
    /// ```
    /// let vec = vec!['a', 'b', 'c'];
    /// let mut into_iter = vec.into_iter();
    /// assert_eq!(into_iter.as_slice(), &['a', 'b', 'c']);
    /// let _ = into_iter.next().unwrap();
    /// assert_eq!(into_iter.as_slice(), &['b', 'c']);
    /// ```
    #[stable(feature = "vec_into_iter_as_slice", since = "1.15.0")]
    pub fn as_slice(&self) -> &[T] {
        // ignore-tidy-undocumented-unsafe
        unsafe { slice::from_raw_parts(self.ptr.as_ptr(), self.len()) }
    }

    /// Returns the remaining items of this iterator as a mutable slice.
    ///
    /// # Examples
    ///
    /// ```
    /// let vec = vec!['a', 'b', 'c'];
    /// let mut into_iter = vec.into_iter();
    /// assert_eq!(into_iter.as_slice(), &['a', 'b', 'c']);
    /// into_iter.as_mut_slice()[2] = 'z';
    /// assert_eq!(into_iter.next().unwrap(), 'a');
    /// assert_eq!(into_iter.next().unwrap(), 'b');
    /// assert_eq!(into_iter.next().unwrap(), 'z');
    /// ```
    #[stable(feature = "vec_into_iter_as_slice", since = "1.15.0")]
    pub fn as_mut_slice(&mut self) -> &mut [T] {
        // ignore-tidy-undocumented-unsafe
        unsafe { &mut *self.as_raw_mut_slice() }
    }

    /// Returns a reference to the underlying allocator.
    #[stable(feature = "vec_into_iter_allocator", since = "CURRENT_RUSTC_VERSION")]
    #[inline]
    pub fn allocator(&self) -> &A {
        &self.alloc
    }

    fn as_raw_mut_slice(&mut self) -> *mut [T] {
        self.ptr.as_ptr().cast_slice(self.len())
    }

    /// Drops remaining elements and relinquishes the backing allocation.
    ///
    /// This method guarantees it won't panic before relinquishing the backing
    /// allocation.
    ///
    /// This is roughly equivalent to the following, but more efficient
    ///
    /// ```
    /// # let mut vec = Vec::<u8>::with_capacity(10);
    /// # let ptr = vec.as_mut_ptr();
    /// # let mut into_iter = vec.into_iter();
    /// let mut into_iter = std::mem::replace(&mut into_iter, Vec::new().into_iter());
    /// (&mut into_iter).for_each(drop);
    /// std::mem::forget(into_iter);
    /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
    /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
    /// # drop(unsafe { Vec::<u8>::from_raw_parts(ptr, 0, 10) });
    /// ```
    ///
    /// This method is used by in-place iteration, refer to the vec::in_place_collect
    /// documentation for an overview.
    #[cfg(not(no_global_oom_handling))]
    pub(super) fn forget_allocation_drop_remaining(&mut self) {
        let remaining = self.as_raw_mut_slice();

        // overwrite the individual fields instead of creating a new
        // struct and then overwriting &mut self.
        // this creates less assembly
        self.cap = 0;
        self.buf = RawVec::new().non_null();
        self.ptr = self.buf;
        self.end = self.buf.as_ptr();

        // Dropping the remaining elements can panic, so this needs to be
        // done only after updating the other fields.
        // ignore-tidy-undocumented-unsafe
        unsafe {
            ptr::drop_in_place(remaining);
        }
    }

    /// Forgets to Drop the remaining elements while still allowing the backing allocation to be freed.
    ///
    /// This method does not consume `self`, and leaves deallocation to `impl Drop for IntoIter`.
    /// If consuming `self` is possible, consider calling
    /// [`Self::forget_remaining_elements_and_dealloc()`] instead.
    pub(crate) fn forget_remaining_elements(&mut self) {
        // For the ZST case, it is crucial that we mutate `end` here, not `ptr`.
        // `ptr` must stay aligned, while `end` may be unaligned.
        self.end = self.ptr.as_ptr();
    }

    /// Forgets to Drop the remaining elements and frees the backing allocation.
    /// Consuming version of [`Self::forget_remaining_elements()`].
    ///
    /// This can be used in place of `drop(self)` when `self` is known to be exhausted,
    /// to avoid producing a needless `drop_in_place::<[T]>()`.
    #[inline]
    pub(crate) fn forget_remaining_elements_and_dealloc(self) {
        let mut this = ManuallyDrop::new(self);
        // SAFETY: `this` is in ManuallyDrop, so it will not be double-freed.
        unsafe {
            this.dealloc_only();
        }
    }

    /// Frees the allocation, without checking or dropping anything else.
    ///
    /// The safe version of this method is [`Self::forget_remaining_elements_and_dealloc()`].
    /// This function exists only to share code between that method and the `impl Drop`.
    ///
    /// # Safety
    ///
    /// This function must only be called with an [`IntoIter`] that is not going to be dropped
    /// or otherwise used in any way, either because it is being forgotten or because its `Drop`
    /// is already executing; otherwise a double-free will occur, and possibly a read from freed
    /// memory if there are any remaining elements.
    #[inline]
    unsafe fn dealloc_only(&mut self) {
        // SAFETY: our caller promises not to touch `*self` again.
        let alloc = unsafe { ManuallyDrop::take(&mut self.alloc) };
        // SAFETY: We're using this to deallocate a preexisting `RawVec`.
        let _ = unsafe { RawVec::from_nonnull_in(self.buf, self.cap, alloc) };
    }

    #[cfg(not(no_global_oom_handling))]
    #[inline]
    pub(crate) fn into_vecdeque(self) -> VecDeque<T, A> {
        // Keep our `Drop` impl from dropping the elements and the allocator
        let mut this = ManuallyDrop::new(self);

        let buf = this.buf.as_ptr();
        let initialized = if T::IS_ZST || this.len() == 0 {
            // All the pointers are the same for ZSTs, so it's fine to
            // say that they're all at the beginning of the "allocation".
            // For non-ZSTs, we have length 0, so we can choose the (empty)
            // range to be at the start of the buffer.
            //
            // Due to `0` ≤ `this.len()` ≤ `this.cap`, the range is well-formed,
            // and due to the argument above it spans exactly the elements of
            // this iterator. Because `init.start` = `0`, it follows that either
            // `init.start` < `cap` or `cap` = `init.start` = `0`; thus the range
            // satisfies the requirements of `from_contiguous_raw_parts_in`.
            0..this.len()
        } else {
            // SAFETY: `this.ptr` and `this.end` are created via offsets of `this.buf`,
            // so they point to the same allocation. We have `this.buf` ≤ `this.ptr` ≤ `this.end`,
            // so this cannot wrap, and will produce a well-formed range that spans exactly
            // the elements of this iterator.
            //
            // Additionally, due to `end ≤ buf + cap`, we have `init.start` ≤ `init.end` ≤ `cap`.
            // Due to the length check above, `init.start < cap`, so the range satisfies the
            // requirements of `from_contiguous_raw_parts_in`.
            unsafe { this.ptr.offset_from_unsigned(this.buf)..this.end.offset_from_unsigned(buf) }
        };

        let cap = this.cap;
        // SAFETY: `this` is forgotten afterwards, so we can move out the allocator.
        let alloc = unsafe { ManuallyDrop::take(&mut this.alloc) };

        // SAFETY: This allocation originally came from a `Vec`, so it satisfies all
        // requirements for the `buf` pointer with capacity `cap` allocated in `alloc`.
        // Correctness of `initialized` was shown above.
        unsafe { VecDeque::from_contiguous_raw_parts_in(buf, initialized, cap, alloc) }
    }
}

#[stable(feature = "vec_intoiter_as_ref", since = "1.46.0")]
impl<T, A: Allocator> AsRef<[T]> for IntoIter<T, A> {
    fn as_ref(&self) -> &[T] {
        self.as_slice()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: Send, A: Allocator + Send> Send for IntoIter<T, A> {}
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: Sync, A: Allocator + Sync> Sync for IntoIter<T, A> {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, A: Allocator> Iterator for IntoIter<T, A> {
    type Item = T;

    #[inline]
    fn next(&mut self) -> Option<T> {
        let ptr = if T::IS_ZST {
            if self.ptr.as_ptr() == self.end as *mut T {
                return None;
            }
            // `ptr` has to stay where it is to remain aligned, so we reduce the length by 1 by
            // reducing the `end`.
            self.end = self.end.wrapping_byte_sub(1);
            self.ptr
        } else {
            if self.ptr == non_null!(self.end, T) {
                return None;
            }
            let old = self.ptr;
            // ignore-tidy-undocumented-unsafe
            self.ptr = unsafe { old.add(1) };
            old
        };
        // ignore-tidy-undocumented-unsafe
        Some(unsafe { ptr.read() })
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let exact = if T::IS_ZST {
            self.end.addr().wrapping_sub(self.ptr.as_ptr().addr())
        } else {
            // ignore-tidy-undocumented-unsafe
            unsafe { non_null!(self.end, T).offset_from_unsigned(self.ptr) }
        };
        (exact, Some(exact))
    }

    #[inline]
    fn advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
        let step_size = self.len().min(n);
        let to_drop = self.ptr.as_ptr().cast_slice(step_size);
        if T::IS_ZST {
            // See `next` for why we sub `end` here.
            self.end = self.end.wrapping_byte_sub(step_size);
        } else {
            // SAFETY: the min() above ensures that step_size is in bounds
            self.ptr = unsafe { self.ptr.add(step_size) };
        }
        // SAFETY: the min() above ensures that step_size is in bounds
        unsafe {
            ptr::drop_in_place(to_drop);
        }
        NonZero::new(n - step_size).map_or(Ok(()), Err)
    }

    #[inline]
    fn count(self) -> usize {
        self.len()
    }

    #[inline]
    fn last(mut self) -> Option<T> {
        self.next_back()
    }

    #[inline]
    fn next_chunk<const N: usize>(&mut self) -> Result<[T; N], core::array::IntoIter<T, N>> {
        let mut raw_ary = [const { MaybeUninit::uninit() }; N];

        let len = self.len();

        if T::IS_ZST {
            if len < N {
                self.forget_remaining_elements();
                // SAFETY: ZSTs can be conjured ex nihilo, only the amount has to be correct
                return Err(unsafe { array::IntoIter::new_unchecked(raw_ary, 0..len) });
            }

            self.end = self.end.wrapping_byte_sub(N);
            // SAFETY: ditto
            return Ok(unsafe { raw_ary.transpose().assume_init() });
        }

        if len < N {
            // SAFETY: `len` indicates that this many elements are available and we
            // just checked that it fits into the array.
            unsafe {
                ptr::copy_nonoverlapping(self.ptr.as_ptr(), raw_ary.as_mut_ptr() as *mut T, len);
                self.forget_remaining_elements();
                return Err(array::IntoIter::new_unchecked(raw_ary, 0..len));
            }
        }

        // SAFETY: `len` is larger than the array size. Copy a fixed amount here to fully initialize
        // the array.
        unsafe {
            ptr::copy_nonoverlapping(self.ptr.as_ptr(), raw_ary.as_mut_ptr() as *mut T, N);
            self.ptr = self.ptr.add(N);
            Ok(raw_ary.transpose().assume_init())
        }
    }

    #[inline]
    fn fold<B, F>(mut self, mut accum: B, mut f: F) -> B
    where
        F: FnMut(B, Self::Item) -> B,
    {
        if T::IS_ZST {
            while self.ptr.as_ptr() != self.end.cast_mut() {
                // SAFETY: we just checked that `self.ptr` is in bounds.
                let tmp = unsafe { self.ptr.read() };
                // See `next` for why we subtract from `end` here.
                self.end = self.end.wrapping_byte_sub(1);
                accum = f(accum, tmp);
            }
        } else {
            // SAFETY: `self.end` can only be null if `T` is a ZST.
            while self.ptr != non_null!(self.end, T) {
                // SAFETY: we just checked that `self.ptr` is in bounds.
                let tmp = unsafe { self.ptr.read() };
                // SAFETY: the maximum this can be is `self.end`.
                // Increment `self.ptr` first to avoid double dropping in the event of a panic.
                self.ptr = unsafe { self.ptr.add(1) };
                accum = f(accum, tmp);
            }
        }

        // There are in fact no remaining elements to forget, but by doing this we can avoid
        // potentially generating a needless loop to drop the elements that cannot exist at
        // this point.
        self.forget_remaining_elements_and_dealloc();

        accum
    }

    #[inline]
    fn try_fold<B, F, R>(&mut self, mut accum: B, mut f: F) -> R
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> R,
        R: core::ops::Try<Output = B>,
    {
        if T::IS_ZST {
            while self.ptr.as_ptr() != self.end.cast_mut() {
                // SAFETY: we just checked that `self.ptr` is in bounds.
                let tmp = unsafe { self.ptr.read() };
                // See `next` for why we subtract from `end` here.
                self.end = self.end.wrapping_byte_sub(1);
                accum = f(accum, tmp)?;
            }
        } else {
            // SAFETY: `self.end` can only be null if `T` is a ZST.
            while self.ptr != non_null!(self.end, T) {
                // SAFETY: we just checked that `self.ptr` is in bounds.
                let tmp = unsafe { self.ptr.read() };
                // SAFETY: the maximum this can be is `self.end`.
                // Increment `self.ptr` first to avoid double dropping in the event of a panic.
                self.ptr = unsafe { self.ptr.add(1) };
                accum = f(accum, tmp)?;
            }
        }
        R::from_output(accum)
    }

    unsafe fn __iterator_get_unchecked(&mut self, i: usize) -> Self::Item
    where
        Self: TrustedRandomAccessNoCoerce,
    {
        // SAFETY: the caller must guarantee that `i` is in bounds of the
        // `Vec<T>`, so `i` cannot overflow an `isize`, and the `self.ptr.add(i)`
        // is guaranteed to pointer to an element of the `Vec<T>` and
        // thus guaranteed to be valid to dereference.
        //
        // Also note the implementation of `Self: TrustedRandomAccess` requires
        // that `T: Copy` so reading elements from the buffer doesn't invalidate
        // them for `Drop`.
        unsafe { self.ptr.add(i).read() }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, A: Allocator> DoubleEndedIterator for IntoIter<T, A> {
    #[inline]
    fn next_back(&mut self) -> Option<T> {
        if T::IS_ZST {
            if self.ptr.as_ptr() == self.end as *mut _ {
                return None;
            }
            // See above for why 'ptr.offset' isn't used
            self.end = self.end.wrapping_byte_sub(1);
            // Note that even though this is next_back() we're reading from `self.ptr`, not
            // `self.end`. We track our length using the byte offset from `self.ptr` to `self.end`,
            // so the end pointer may not be suitably aligned for T.
            // ignore-tidy-undocumented-unsafe
            Some(unsafe { ptr::read(self.ptr.as_ptr()) })
        } else {
            if self.ptr == non_null!(self.end, T) {
                return None;
            }
            // ignore-tidy-undocumented-unsafe
            unsafe {
                self.end = self.end.sub(1);
                Some(ptr::read(self.end))
            }
        }
    }

    #[inline]
    fn next_chunk_back<const N: usize>(&mut self) -> Result<[T; N], core::array::IntoIter<T, N>> {
        let mut raw_ary = [const { MaybeUninit::uninit() }; N];

        let len = self.len();

        if T::IS_ZST {
            if len < N {
                self.forget_remaining_elements();
                // SAFETY: ZSTs can be conjured ex nihilo, only the amount has to be correct
                return Err(unsafe { array::IntoIter::new_unchecked(raw_ary, N - len..N) });
            }

            self.end = self.end.wrapping_byte_sub(N);
            // SAFETY: ditto
            return Ok(unsafe { MaybeUninit::array_assume_init(raw_ary) });
        }

        if len < N {
            // SAFETY: `len` indicates that this many elements are available
            // and we just checked that it fits into the array.
            unsafe {
                ptr::copy_nonoverlapping(self.ptr.as_ptr(), raw_ary.as_mut_ptr() as *mut T, len);
                self.forget_remaining_elements();
                return Err(array::IntoIter::new_unchecked(raw_ary, 0..len));
            }
        }

        // SAFETY: `len` is larger than the array size. Copy a fixed amount here to fully initialize
        // the array.
        unsafe {
            ptr::copy_nonoverlapping(
                self.ptr.add(len - N).as_ptr(),
                raw_ary.as_mut_ptr() as *mut T,
                N,
            );
            self.end = self.end.sub(N);
            Ok(MaybeUninit::array_assume_init(raw_ary))
        }
    }

    #[inline]
    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
        let step_size = self.len().min(n);
        if T::IS_ZST {
            // SAFETY: same as for advance_by()
            self.end = self.end.wrapping_byte_sub(step_size);
        } else {
            // SAFETY: same as for advance_by()
            self.end = unsafe { self.end.sub(step_size) };
        }
        let to_drop = if T::IS_ZST {
            // ZST may cause unalignment
            ptr::NonNull::<T>::dangling().as_ptr().cast_slice(step_size)
        } else {
            self.end.cast::<T>().cast_mut().cast_slice(step_size)
        };
        // SAFETY: same as for advance_by()
        unsafe {
            ptr::drop_in_place(to_drop);
        }
        NonZero::new(n - step_size).map_or(Ok(()), Err)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, A: Allocator> ExactSizeIterator for IntoIter<T, A> {
    fn is_empty(&self) -> bool {
        if T::IS_ZST {
            self.ptr.as_ptr() == self.end as *mut _
        } else {
            self.ptr == non_null!(self.end, T)
        }
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<T, A: Allocator> FusedIterator for IntoIter<T, A> {}

#[doc(hidden)]
#[unstable(issue = "none", feature = "trusted_fused")]
unsafe impl<T, A: Allocator> TrustedFused for IntoIter<T, A> {}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<T, A: Allocator> TrustedLen for IntoIter<T, A> {}

#[stable(feature = "default_iters", since = "1.70.0")]
impl<T, A> Default for IntoIter<T, A>
where
    A: Allocator + Default,
{
    /// Creates an empty `vec::IntoIter`.
    ///
    /// ```
    /// # use std::vec;
    /// let iter: vec::IntoIter<u8> = Default::default();
    /// assert_eq!(iter.len(), 0);
    /// assert_eq!(iter.as_slice(), &[]);
    /// ```
    fn default() -> Self {
        super::Vec::new_in(Default::default()).into_iter()
    }
}

#[doc(hidden)]
#[unstable(issue = "none", feature = "std_internals")]
#[unsafe(rustc_allow_lifetime_dependent_specialization)]
trait NonDrop {}

// T: Copy as approximation for !Drop since get_unchecked does not advance self.ptr
// and thus we can't implement drop-handling
#[unstable(issue = "none", feature = "std_internals")]
impl<T: Copy> NonDrop for T {}

#[doc(hidden)]
#[unstable(issue = "none", feature = "std_internals")]
// TrustedRandomAccess (without NoCoerce) must not be implemented because
// subtypes/supertypes of `T` might not be `NonDrop`
unsafe impl<T, A: Allocator> TrustedRandomAccessNoCoerce for IntoIter<T, A>
where
    T: NonDrop,
{
    const MAY_HAVE_SIDE_EFFECT: bool = false;
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "vec_into_iter_clone", since = "1.8.0")]
impl<T: Clone, A: Allocator + Clone> Clone for IntoIter<T, A> {
    fn clone(&self) -> Self {
        self.as_slice().to_vec_in(self.alloc.deref().clone()).into_iter()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<#[may_dangle] T, A: Allocator> Drop for IntoIter<T, A> {
    fn drop(&mut self) {
        struct DropGuard<'a, T, A: Allocator>(&'a mut IntoIter<T, A>);

        impl<T, A: Allocator> Drop for DropGuard<'_, T, A> {
            fn drop(&mut self) {
                // ignore-tidy-undocumented-unsafe
                unsafe {
                    self.0.dealloc_only();
                }
            }
        }

        let guard = DropGuard(self);
        // destroy the remaining elements
        // ignore-tidy-undocumented-unsafe
        unsafe {
            ptr::drop_in_place(guard.0.as_raw_mut_slice());
        }
        // now `guard` will be dropped and do the rest
    }
}

// In addition to the SAFETY invariants of the following three unsafe traits
// also refer to the vec::in_place_collect module documentation to get an overview
#[unstable(issue = "none", feature = "inplace_iteration")]
#[doc(hidden)]
unsafe impl<T, A: Allocator> InPlaceIterable for IntoIter<T, A> {
    const EXPAND_BY: Option<NonZero<usize>> = NonZero::new(1);
    const MERGE_BY: Option<NonZero<usize>> = NonZero::new(1);
}

#[unstable(issue = "none", feature = "inplace_iteration")]
#[doc(hidden)]
unsafe impl<T, A: Allocator> SourceIter for IntoIter<T, A> {
    type Source = Self;

    #[inline]
    unsafe fn as_inner(&mut self) -> &mut Self::Source {
        self
    }
}

#[cfg(not(no_global_oom_handling))]
unsafe impl<T> AsVecIntoIter for IntoIter<T> {
    type Item = T;

    fn as_into_iter(&mut self) -> &mut IntoIter<Self::Item> {
        self
    }
}