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    use crate::alloc::{Allocator, Global};
    use core::fmt;
    use core::iter::{FusedIterator, TrustedLen};
    use core::mem::{self};
    use core::ptr::{self, NonNull};
    use core::slice::{self};

    use super::Vec;

    /// A draining iterator for `Vec<T>`.
    ///
    /// This `struct` is created by [`Vec::drain`].
    /// See its documentation for more.
    ///
    /// # Example
    ///
    /// ```
    /// let mut v = vec![0, 1, 2];
    /// let iter: std::vec::Drain<_> = v.drain(..);
    /// ```
    #[stable(feature = "drain", since = "1.6.0")]
    pub struct Drain<
        'a,
        T: 'a,
        #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator + 'a = Global,
    > {
        /// Index of tail to preserve
        pub(super) tail_start: usize,
        /// Length of tail
        pub(super) tail_len: usize,
        /// Current remaining range to remove
        pub(super) iter: slice::Iter<'a, T>,
        pub(super) vec: NonNull<Vec<T, A>>,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<T: fmt::Debug, A: Allocator> fmt::Debug for Drain<'_, T, A> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_tuple("Drain").field(&self.iter.as_slice()).finish()
        }
    }

    impl<'a, T, A: Allocator> Drain<'a, T, A> {
        /// Returns the remaining items of this iterator as a slice.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut vec = vec!['a', 'b', 'c'];
        /// let mut drain = vec.drain(..);
        /// assert_eq!(drain.as_slice(), &['a', 'b', 'c']);
        /// let _ = drain.next().unwrap();
        /// assert_eq!(drain.as_slice(), &['b', 'c']);
        /// ```
        #[stable(feature = "vec_drain_as_slice", since = "1.46.0")]
        pub fn as_slice(&self) -> &[T] {
            self.iter.as_slice()
        }

        /// Returns a reference to the underlying allocator.
        #[unstable(feature = "allocator_api", issue = "32838")]
        #[inline]
        pub fn allocator(&self) -> &A {
            unsafe { self.vec.as_ref().allocator() }
        }
    }

    #[stable(feature = "vec_drain_as_slice", since = "1.46.0")]
    impl<'a, T, A: Allocator> AsRef<[T]> for Drain<'a, T, A> {
        fn as_ref(&self) -> &[T] {
            self.as_slice()
        }
    }

    #[stable(feature = "drain", since = "1.6.0")]
    unsafe impl<T: Sync, A: Sync + Allocator> Sync for Drain<'_, T, A> {}
    #[stable(feature = "drain", since = "1.6.0")]
    unsafe impl<T: Send, A: Send + Allocator> Send for Drain<'_, T, A> {}

    #[stable(feature = "drain", since = "1.6.0")]
    impl<T, A: Allocator> Iterator for Drain<'_, T, A> {
        type Item = T;

        #[inline]
        fn next(&mut self) -> Option<T> {
            self.iter.next().map(|elt| unsafe { ptr::read(elt as *const _) })
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }

    #[stable(feature = "drain", since = "1.6.0")]
    impl<T, A: Allocator> DoubleEndedIterator for Drain<'_, T, A> {
        #[inline]
        fn next_back(&mut self) -> Option<T> {
            self.iter.next_back().map(|elt| unsafe { ptr::read(elt as *const _) })
        }
    }

    #[stable(feature = "drain", since = "1.6.0")]
    impl<T, A: Allocator> Drop for Drain<'_, T, A> {
        fn drop(&mut self) {
            /// Continues dropping the remaining elements in the `Drain`, then moves back the
            /// un-`Drain`ed elements to restore the original `Vec`.
            struct DropGuard<'r, 'a, T, A: Allocator>(&'r mut Drain<'a, T, A>);

            impl<'r, 'a, T, A: Allocator> Drop for DropGuard<'r, 'a, T, A> {
                fn drop(&mut self) {
                    // Continue the same loop we have below. If the loop already finished, this does
                    // nothing.
                    self.0.for_each(drop);

                    if self.0.tail_len > 0 {
                        unsafe {
                            let source_vec = self.0.vec.as_mut();
                            // memmove back untouched tail, update to new length
                            let start = source_vec.len();
                            let tail = self.0.tail_start;
                            if tail != start {
                                let src = source_vec.as_ptr().add(tail);
                                let dst = source_vec.as_mut_ptr().add(start);
                                ptr::copy(src, dst, self.0.tail_len);
                            }
                            source_vec.set_len(start + self.0.tail_len);
                        }
                    }
                }
            }

            // exhaust self first
            while let Some(item) = self.next() {
                let guard = DropGuard(self);
                drop(item);
                mem::forget(guard);
            }

            // Drop a `DropGuard` to move back the non-drained tail of `self`.
            DropGuard(self);
        }
    }

    #[stable(feature = "drain", since = "1.6.0")]
    impl<T, A: Allocator> ExactSizeIterator for Drain<'_, T, A> {
        fn is_empty(&self) -> bool {
            self.iter.is_empty()
        }
    }

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<T, A: Allocator> TrustedLen for Drain<'_, T, A> {}

    #[stable(feature = "fused", since = "1.26.0")]
    impl<T, A: Allocator> FusedIterator for Drain<'_, T, A> {}