alloc/collections/vec_deque/
spec_extend.rsuse core::iter::TrustedLen;
use core::slice;
use super::VecDeque;
use crate::alloc::Allocator;
use crate::vec;
pub(super) trait SpecExtend<T, I> {
    #[track_caller]
    fn spec_extend(&mut self, iter: I);
}
impl<T, I, A: Allocator> SpecExtend<T, I> for VecDeque<T, A>
where
    I: Iterator<Item = T>,
{
    #[track_caller]
    default fn spec_extend(&mut self, mut iter: I) {
        while let Some(element) = iter.next() {
            let (lower, _) = iter.size_hint();
            self.reserve(lower.saturating_add(1));
            unsafe { self.push_unchecked(element) };
            while self.len < self.capacity() {
                let Some(element) = iter.next() else {
                    return;
                };
                unsafe { self.push_unchecked(element) };
            }
        }
    }
}
impl<T, I, A: Allocator> SpecExtend<T, I> for VecDeque<T, A>
where
    I: TrustedLen<Item = T>,
{
    #[track_caller]
    default fn spec_extend(&mut self, iter: I) {
        let (low, high) = iter.size_hint();
        if let Some(additional) = high {
            debug_assert_eq!(
                low,
                additional,
                "TrustedLen iterator's size hint is not exact: {:?}",
                (low, high)
            );
            self.reserve(additional);
            let written = unsafe {
                self.write_iter_wrapping(self.to_physical_idx(self.len), iter, additional)
            };
            debug_assert_eq!(
                additional, written,
                "The number of items written to VecDeque doesn't match the TrustedLen size hint"
            );
        } else {
            panic!("capacity overflow");
        }
    }
}
impl<T, A: Allocator> SpecExtend<T, vec::IntoIter<T>> for VecDeque<T, A> {
    #[track_caller]
    fn spec_extend(&mut self, mut iterator: vec::IntoIter<T>) {
        let slice = iterator.as_slice();
        self.reserve(slice.len());
        unsafe {
            self.copy_slice(self.to_physical_idx(self.len), slice);
            self.len += slice.len();
        }
        iterator.forget_remaining_elements();
    }
}
impl<'a, T: 'a, I, A: Allocator> SpecExtend<&'a T, I> for VecDeque<T, A>
where
    I: Iterator<Item = &'a T>,
    T: Copy,
{
    #[track_caller]
    default fn spec_extend(&mut self, iterator: I) {
        self.spec_extend(iterator.copied())
    }
}
impl<'a, T: 'a, A: Allocator> SpecExtend<&'a T, slice::Iter<'a, T>> for VecDeque<T, A>
where
    T: Copy,
{
    #[track_caller]
    fn spec_extend(&mut self, iterator: slice::Iter<'a, T>) {
        let slice = iterator.as_slice();
        self.reserve(slice.len());
        unsafe {
            self.copy_slice(self.to_physical_idx(self.len), slice);
            self.len += slice.len();
        }
    }
}