Tracking Issue for `BTreeSet` entry APIs

Feature gate: #![feature(btree_set_entry)]

This is a tracking issue for Entry and entry-like methods on BTreeSet.

Public API

impl<T, A: Allocator + Clone> BTreeSet<T, A> {
    pub fn get_or_insert(&mut self, value: T) -> &T
    where
        T: Ord,
    {...}
    pub fn get_or_insert_with<Q: ?Sized, F>(&mut self, value: &Q, f: F) -> &T
    where
        T: Borrow<Q> + Ord,
        Q: Ord,
        F: FnOnce(&Q) -> T,
    {...}

    pub fn entry(&mut self, value: T) -> Entry<'_, T, A>
    where
        T: Ord,
    {...}
}

pub enum Entry<'a, T, A: Allocator + Clone = Global> {
    Occupied(OccupiedEntry<'a, T, A>),
    Vacant(VacantEntry<'a, T, A>),
}
pub struct OccupiedEntry<'a, T, A: Allocator + Clone = Global> {...}
pub struct VacantEntry<'a, T, A: Allocator + Clone = Global> {...}

impl<T: Debug + Ord, A: Allocator + Clone> Debug for Entry<'_, T, A> {...}
impl<T: Debug + Ord, A: Allocator + Clone> Debug for OccupiedEntry<'_, T, A> {...}
impl<T: Debug + Ord, A: Allocator + Clone> Debug for VacantEntry<'_, T, A> {...}

impl<'a, T: Ord, A: Allocator + Clone> Entry<'a, T, A> {
    pub fn insert(self) -> OccupiedEntry<'a, T, A> {...}
    pub fn or_insert(self) {...}
    pub fn get(&self) -> &T {...}
}

impl<'a, T: Ord, A: Allocator + Clone> OccupiedEntry<'a, T, A> {
    pub fn get(&self) -> &T {...}
    pub fn remove(self) -> T {...}
}

impl<'a, T: Ord, A: Allocator + Clone> VacantEntry<'a, T, A> {
    pub fn get(&self) -> &T {...}
    pub fn into_value(self) -> T {...}
    pub fn insert(self) {...}
}

Steps / History

• Implementation: #133548
• Final comment period (FCP)^1
• Stabilization PR

Unresolved Questions

• None yet.

See also #60896 for HashSet.

This is a tracking issue for the BTreeSet entry APIs, with implementation recorded as complete in #133548. Start by reviewing that implementation and the stabilization guidance linked in the issue; done means completing the remaining FCP and stabilization steps, but no files or tests are named here.