Tracking Issue for new `VecDeque` methods `prepend`, `extend_front`, `splice`, `extend_from_within` and `prepend_from_within`

Feature gate: #![feature(deque_extend_front)]

This is a tracking issue for 5 new methods on VecDeque similar to ones already on Vec or their counterparts that push to the front instead of back.

ACP: https://github.com/rust-lang/libs-team/issues/658

Public API

// alloc::collections::vec_deque

impl<T> VecDeque<T> {
    // Preserves order of elements.
    pub fn prepend<I>(&mut self, other: I)
    where
        I: IntoIterator<Item = T>,
        I::Iter: DoubleEndedIterator
    {
        self.extend_front(other.into_iter().rev())
    }

    // Reverses order of elements.
    pub fn extend_front<I>(&mut self, other: I)
    where
        I: IntoIterator<Item = T>;

    pub fn splice<R, I>(&mut self, range: R, replace_with: I) -> Splice<'_, I::IntoIter, A>
    where
        R: RangeBounds<usize>,
        I: IntoIterator<Item = T>;
}

impl<T: Clone> VecDeque<T> {
    pub fn extend_from_within<R>(&mut self, src: R)
    where
        R: RangeBounds<usize>;

    // Preserves order of elements.
    pub fn prepend_from_within<R>(&mut self, src: R)
    where
        R: RangeBounds<usize>;
}

Steps / History

(Remember to update the S-tracking-* label when checking boxes.)

• Implementation:
  • extend_front, prepend: #146861
    • specialization of deque1.prepend(deque2.drain(range)): #150595
    • part of #150597
  • extend_from_within, prepend_from_within: #147161
  • splice: #147247
• Final comment period (FCP)^1
• Stabilization PR

Unresolved Questions

• Exact moving/allocation behavior of VecDeque::splice in the less optimal conditions (see "This is optimal if:" in the doc), see https://github.com/rust-lang/rust/pull/147247/files#r2507005344

Start with the public API in alloc::collections::vec_deque and review the implementation and discussion in PR #147247, especially the unresolved moving and allocation behavior for splice. Done means resolving remaining stabilization questions, completing the final comment period, and opening a stabilization PR.