Add BTreeSet entry APIs to match HashSet

* `fn get_or_insert(&mut self, value: T) -> &T`
* `fn get_or_insert_with<Q: ?Sized, F>(&mut self, value: &Q, f: F) -> &T`
* `fn entry(&mut self, value: T) -> Entry<'_, T, A>` (+ `Entry` APIs)

Author: cuviper

library/alloc/src/collections/btree/map.rs

@@ -308,11 +308,38 @@ impl<K, A: Allocator + Clone> BTreeMap<K, SetValZST, A> {
                     alloc: (*map.alloc).clone(),
                     _marker: PhantomData,
                 }
-                .insert(SetValZST::default());
+                .insert(SetValZST);
                 None
             }
         }
     }
+
+    pub(super) fn get_or_insert_with<Q: ?Sized, F>(&mut self, q: &Q, f: F) -> &K
+    where
+        K: Borrow<Q> + Ord,
+        Q: Ord,
+        F: FnOnce(&Q) -> K,
+    {
+        let (map, dormant_map) = DormantMutRef::new(self);
+        let root_node =
+            map.root.get_or_insert_with(|| Root::new((*map.alloc).clone())).borrow_mut();
+        match root_node.search_tree(q) {
+            Found(handle) => handle.into_kv_mut().0,
+            GoDown(handle) => {
+                let key = f(q);
+                assert!(*key.borrow() == *q, "new value is not equal");
+                VacantEntry {
+                    key,
+                    handle: Some(handle),
+                    dormant_map,
+                    alloc: (*map.alloc).clone(),
+                    _marker: PhantomData,
+                }
+                .insert_entry(SetValZST)
+                .into_key()
+            }
+        }
+    }
 }
 
 /// An iterator over the entries of a `BTreeMap`.

library/alloc/src/collections/btree/map/entry.rs

@@ -449,6 +449,11 @@ impl<'a, K: Ord, V, A: Allocator + Clone> OccupiedEntry<'a, K, V, A> {
         self.handle.reborrow().into_kv().0
     }
 
+    /// Converts the entry into a reference to its key.
+    pub(crate) fn into_key(self) -> &'a K {
+        self.handle.into_kv_mut().0
+    }
+
     /// Take ownership of the key and value from the map.
     ///
     /// # Examples

library/alloc/src/collections/btree/set.rs

@@ -7,12 +7,17 @@ use core::iter::{FusedIterator, Peekable};
 use core::mem::ManuallyDrop;
 use core::ops::{BitAnd, BitOr, BitXor, Bound, RangeBounds, Sub};
 
-use super::map::{BTreeMap, Keys};
+use super::map::{self, BTreeMap, Keys};
 use super::merge_iter::MergeIterInner;
 use super::set_val::SetValZST;
 use crate::alloc::{Allocator, Global};
 use crate::vec::Vec;
 
+mod entry;
+
+#[unstable(feature = "btree_set_entry", issue = "none")]
+pub use self::entry::{Entry, OccupiedEntry, VacantEntry};
+
 /// An ordered set based on a B-Tree.
 ///
 /// See [`BTreeMap`]'s documentation for a detailed discussion of this collection's performance
@@ -928,6 +933,79 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
         self.map.replace(value)
     }
 
+    /// Inserts the given `value` into the set if it is not present, then
+    /// returns a reference to the value in the set.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(btree_set_entry)]
+    ///
+    /// use std::collections::BTreeSet;
+    ///
+    /// let mut set = BTreeSet::from([1, 2, 3]);
+    /// assert_eq!(set.len(), 3);
+    /// assert_eq!(set.get_or_insert(2), &2);
+    /// assert_eq!(set.get_or_insert(100), &100);
+    /// assert_eq!(set.len(), 4); // 100 was inserted
+    /// ```
+    #[inline]
+    #[unstable(feature = "btree_set_entry", issue = "none")]
+    pub fn get_or_insert(&mut self, value: T) -> &T
+    where
+        T: Ord,
+    {
+        self.map.entry(value).insert_entry(SetValZST).into_key()
+    }
+
+    /// Inserts a value computed from `f` into the set if the given `value` is
+    /// not present, then returns a reference to the value in the set.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(btree_set_entry)]
+    ///
+    /// use std::collections::BTreeSet;
+    ///
+    /// let mut set: BTreeSet<String> = ["cat", "dog", "horse"]
+    ///     .iter().map(|&pet| pet.to_owned()).collect();
+    ///
+    /// assert_eq!(set.len(), 3);
+    /// for &pet in &["cat", "dog", "fish"] {
+    ///     let value = set.get_or_insert_with(pet, str::to_owned);
+    ///     assert_eq!(value, pet);
+    /// }
+    /// assert_eq!(set.len(), 4); // a new "fish" was inserted
+    /// ```
+    #[inline]
+    #[unstable(feature = "btree_set_entry", issue = "none")]
+    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,
+    {
+        self.map.get_or_insert_with(value, f)
+    }
+
+    /// Gets the given value's corresponding entry in the set for in-place manipulation.
+    ///
+    /// # Examples
+    ///
+    /// TODO
+    #[inline]
+    #[unstable(feature = "btree_set_entry", issue = "none")]
+    pub fn entry(&mut self, value: T) -> Entry<'_, T, A>
+    where
+        T: Ord,
+    {
+        match self.map.entry(value) {
+            map::Entry::Occupied(entry) => Entry::Occupied(OccupiedEntry { inner: entry }),
+            map::Entry::Vacant(entry) => Entry::Vacant(VacantEntry { inner: entry }),
+        }
+    }
+
     /// If the set contains an element equal to the value, removes it from the
     /// set and drops it. Returns whether such an element was present.
     ///