diff --git a/library/alloc/src/collections/btree/map.rs b/library/alloc/src/collections/btree/map.rs
index 426be364a56b0..d69dad70a44e9 100644
--- a/library/alloc/src/collections/btree/map.rs
+++ b/library/alloc/src/collections/btree/map.rs
@@ -1240,6 +1240,162 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
         )
     }
 
+    /// Moves all elements from `other` into `self`, leaving `other` empty.
+    ///
+    /// If a key from `other` is already present in `self`, then the `conflict`
+    /// closure is used to return a value to `self`. The `conflict`
+    /// closure takes in a borrow of `self`'s key, `self`'s value, and `other`'s value
+    /// in that order.
+    ///
+    /// An example of why one might use this method over [`append`]
+    /// is to combine `self`'s value with `other`'s value when their keys conflict.
+    ///
+    /// Similar to [`insert`], though, the key is not overwritten,
+    /// which matters for types that can be `==` without being identical.
+    ///
+    /// [`insert`]: BTreeMap::insert
+    /// [`append`]: BTreeMap::append
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(btree_merge)]
+    /// use std::collections::BTreeMap;
+    ///
+    /// let mut a = BTreeMap::new();
+    /// a.insert(1, String::from("a"));
+    /// a.insert(2, String::from("b"));
+    /// a.insert(3, String::from("c")); // Note: Key (3) also present in b.
+    ///
+    /// let mut b = BTreeMap::new();
+    /// b.insert(3, String::from("d")); // Note: Key (3) also present in a.
+    /// b.insert(4, String::from("e"));
+    /// b.insert(5, String::from("f"));
+    ///
+    /// // concatenate a's value and b's value
+    /// a.merge(b, |_, a_val, b_val| {
+    ///     format!("{a_val}{b_val}")
+    /// });
+    ///
+    /// assert_eq!(a.len(), 5); // all of b's keys in a
+    ///
+    /// assert_eq!(a[&1], "a");
+    /// assert_eq!(a[&2], "b");
+    /// assert_eq!(a[&3], "cd"); // Note: "c" has been combined with "d".
+    /// assert_eq!(a[&4], "e");
+    /// assert_eq!(a[&5], "f");
+    /// ```
+    #[unstable(feature = "btree_merge", issue = "152152")]
+    pub fn merge(&mut self, mut other: Self, mut conflict: impl FnMut(&K, V, V) -> V)
+    where
+        K: Ord,
+        A: Clone,
+    {
+        // Do we have to append anything at all?
+        if other.is_empty() {
+            return;
+        }
+
+        // We can just swap `self` and `other` if `self` is empty.
+        if self.is_empty() {
+            mem::swap(self, &mut other);
+            return;
+        }
+
+        let mut other_iter = other.into_iter();
+        let (first_other_key, first_other_val) = other_iter.next().unwrap();
+
+        // find the first gap that has the smallest key greater than or equal to
+        // the first key from other
+        let mut self_cursor = self.lower_bound_mut(Bound::Included(&first_other_key));
+
+        if let Some((self_key, _)) = self_cursor.peek_next() {
+            match K::cmp(self_key, &first_other_key) {
+                Ordering::Equal => {
+                    // if `f` unwinds, the next entry is already removed leaving
+                    // the tree in valid state.
+                    // FIXME: Once `MaybeDangling` is implemented, we can optimize
+                    // this through using a drop handler and transmutating CursorMutKey<K, V>
+                    // to CursorMutKey<ManuallyDrop<K>, ManuallyDrop<V>> (see PR #152418)
+                    if let Some((k, v)) = self_cursor.remove_next() {
+                        // SAFETY: we remove the K, V out of the next entry,
+                        // apply 'f' to get a new (K, V), and insert it back
+                        // into the next entry that the cursor is pointing at
+                        let v = conflict(&k, v, first_other_val);
+                        unsafe { self_cursor.insert_after_unchecked(k, v) };
+                    }
+                }
+                Ordering::Greater =>
+                // SAFETY: we know our other_key's ordering is less than self_key,
+                // so inserting before will guarantee sorted order
+                unsafe {
+                    self_cursor.insert_before_unchecked(first_other_key, first_other_val);
+                },
+                Ordering::Less => {
+                    unreachable!("Cursor's peek_next should return None.");
+                }
+            }
+        } else {
+            // SAFETY: reaching here means our cursor is at the end
+            // self BTreeMap so we just insert other_key here
+            unsafe {
+                self_cursor.insert_before_unchecked(first_other_key, first_other_val);
+            }
+        }
+
+        for (other_key, other_val) in other_iter {
+            loop {
+                if let Some((self_key, _)) = self_cursor.peek_next() {
+                    match K::cmp(self_key, &other_key) {
+                        Ordering::Equal => {
+                            // if `f` unwinds, the next entry is already removed leaving
+                            // the tree in valid state.
+                            // FIXME: Once `MaybeDangling` is implemented, we can optimize
+                            // this through using a drop handler and transmutating CursorMutKey<K, V>
+                            // to CursorMutKey<ManuallyDrop<K>, ManuallyDrop<V>> (see PR #152418)
+                            if let Some((k, v)) = self_cursor.remove_next() {
+                                // SAFETY: we remove the K, V out of the next entry,
+                                // apply 'f' to get a new (K, V), and insert it back
+                                // into the next entry that the cursor is pointing at
+                                let v = conflict(&k, v, other_val);
+                                unsafe { self_cursor.insert_after_unchecked(k, v) };
+                            }
+                            break;
+                        }
+                        Ordering::Greater => {
+                            // SAFETY: we know our self_key's ordering is greater than other_key,
+                            // so inserting before will guarantee sorted order
+                            unsafe {
+                                self_cursor.insert_before_unchecked(other_key, other_val);
+                            }
+                            break;
+                        }
+                        Ordering::Less => {
+                            // FIXME: instead of doing a linear search here,
+                            // this can be optimized to search the tree by starting
+                            // from self_cursor and going towards the root and then
+                            // back down to the proper node -- that should probably
+                            // be a new method on Cursor*.
+                            self_cursor.next();
+                        }
+                    }
+                } else {
+                    // FIXME: If we get here, that means all of other's keys are greater than
+                    // self's keys. For performance, this should really do a bulk insertion of items
+                    // from other_iter into the end of self `BTreeMap`. Maybe this should be
+                    // a method for Cursor*?
+
+                    // SAFETY: reaching here means our cursor is at the end
+                    // self BTreeMap so we just insert other_key here
+                    unsafe {
+                        self_cursor.insert_before_unchecked(other_key, other_val);
+                    }
+                    break;
+                }
+            }
+        }
+    }
+
     /// Constructs a double-ended iterator over a sub-range of elements in the map.
     /// The simplest way is to use the range syntax `min..max`, thus `range(min..max)` will
     /// yield elements from min (inclusive) to max (exclusive).
diff --git a/library/alloc/src/collections/btree/map/tests.rs b/library/alloc/src/collections/btree/map/tests.rs
index 938e867b85812..73546caa05eac 100644
--- a/library/alloc/src/collections/btree/map/tests.rs
+++ b/library/alloc/src/collections/btree/map/tests.rs
@@ -1,9 +1,9 @@
 use core::assert_matches;
-use std::iter;
 use std::ops::Bound::{Excluded, Included, Unbounded};
 use std::panic::{AssertUnwindSafe, catch_unwind};
 use std::sync::atomic::AtomicUsize;
 use std::sync::atomic::Ordering::SeqCst;
+use std::{cmp, iter};
 
 use super::*;
 use crate::boxed::Box;
@@ -2128,6 +2128,86 @@ create_append_test!(test_append_239, 239);
 #[cfg(not(miri))] // Miri is too slow
 create_append_test!(test_append_1700, 1700);
 
+// a inserts (0, 0)..(8, 8) to its own tree
+// b inserts (5, 5 * 2)..($len, 2 * $len) to its own tree
+// note that between a and b, there are duplicate keys
+// between 5..min($len, 8), so on merge we add the values
+// of these keys together
+// we check that:
+//     - the merged tree 'a' has a length of max(8, $len)
+//     - all keys in 'a' have the correct value associated
+//     - removing and inserting an element into the merged
+//       tree 'a' still keeps it in valid tree form
+macro_rules! create_merge_test {
+    ($name:ident, $len:expr) => {
+        #[test]
+        fn $name() {
+            let mut a = BTreeMap::new();
+            for i in 0..8 {
+                a.insert(i, i);
+            }
+
+            let mut b = BTreeMap::new();
+            for i in 5..$len {
+                b.insert(i, 2 * i);
+            }
+
+            a.merge(b, |_, a_val, b_val| a_val + b_val);
+
+            assert_eq!(a.len(), cmp::max($len, 8));
+
+            for i in 0..cmp::max($len, 8) {
+                if i < 5 {
+                    assert_eq!(a[&i], i);
+                } else {
+                    if i < cmp::min($len, 8) {
+                        assert_eq!(a[&i], i + 2 * i);
+                    } else if i >= $len {
+                        assert_eq!(a[&i], i);
+                    } else {
+                        assert_eq!(a[&i], 2 * i);
+                    }
+                }
+            }
+
+            a.check();
+            assert_eq!(
+                a.remove(&($len - 1)),
+                if $len >= 5 && $len < 8 {
+                    Some(($len - 1) + 2 * ($len - 1))
+                } else {
+                    Some(2 * ($len - 1))
+                }
+            );
+            assert_eq!(a.insert($len - 1, 20), None);
+            a.check();
+        }
+    };
+}
+
+// These are mostly for testing the algorithm that "fixes" the right edge after insertion.
+// Single node, merge conflicting key values.
+create_merge_test!(test_merge_7, 7);
+// Single node.
+create_merge_test!(test_merge_9, 9);
+// Two leafs that don't need fixing.
+create_merge_test!(test_merge_17, 17);
+// Two leafs where the second one ends up underfull and needs stealing at the end.
+create_merge_test!(test_merge_14, 14);
+// Two leafs where the second one ends up empty because the insertion finished at the root.
+create_merge_test!(test_merge_12, 12);
+// Three levels; insertion finished at the root.
+create_merge_test!(test_merge_144, 144);
+// Three levels; insertion finished at leaf while there is an empty node on the second level.
+create_merge_test!(test_merge_145, 145);
+// Tests for several randomly chosen sizes.
+create_merge_test!(test_merge_170, 170);
+create_merge_test!(test_merge_181, 181);
+#[cfg(not(miri))] // Miri is too slow
+create_merge_test!(test_merge_239, 239);
+#[cfg(not(miri))] // Miri is too slow
+create_merge_test!(test_merge_1700, 1700);
+
 #[test]
 #[cfg_attr(not(panic = "unwind"), ignore = "test requires unwinding support")]
 fn test_append_drop_leak() {
@@ -2169,6 +2249,84 @@ fn test_append_ord_chaos() {
     map2.check();
 }
 
+#[test]
+#[cfg_attr(not(panic = "unwind"), ignore = "test requires unwinding support")]
+fn test_merge_drop_leak() {
+    let a = CrashTestDummy::new(0);
+    let b = CrashTestDummy::new(1);
+    let c = CrashTestDummy::new(2);
+    let mut left = BTreeMap::new();
+    let mut right = BTreeMap::new();
+    left.insert(a.spawn(Panic::Never), ());
+    left.insert(b.spawn(Panic::Never), ());
+    left.insert(c.spawn(Panic::Never), ());
+    right.insert(b.spawn(Panic::InDrop), ()); // first duplicate key, dropped during merge
+    right.insert(c.spawn(Panic::Never), ());
+
+    catch_unwind(move || left.merge(right, |_, _, _| ())).unwrap_err();
+    assert_eq!(a.dropped(), 1); // this should not be dropped
+    assert_eq!(b.dropped(), 2); // key is dropped on panic
+    assert_eq!(c.dropped(), 2); // key is dropped on panic
+}
+
+#[test]
+#[cfg_attr(not(panic = "unwind"), ignore = "test requires unwinding support")]
+fn test_merge_conflict_drop_leak() {
+    let a = CrashTestDummy::new(0);
+    let a_val_left = CrashTestDummy::new(0);
+
+    let b = CrashTestDummy::new(1);
+    let b_val_left = CrashTestDummy::new(1);
+    let b_val_right = CrashTestDummy::new(1);
+
+    let c = CrashTestDummy::new(2);
+    let c_val_left = CrashTestDummy::new(2);
+    let c_val_right = CrashTestDummy::new(2);
+
+    let mut left = BTreeMap::new();
+    let mut right = BTreeMap::new();
+
+    left.insert(a.spawn(Panic::Never), a_val_left.spawn(Panic::Never));
+    left.insert(b.spawn(Panic::Never), b_val_left.spawn(Panic::Never));
+    left.insert(c.spawn(Panic::Never), c_val_left.spawn(Panic::Never));
+    right.insert(b.spawn(Panic::Never), b_val_right.spawn(Panic::Never));
+    right.insert(c.spawn(Panic::Never), c_val_right.spawn(Panic::Never));
+
+    // First key that conflicts should
+    catch_unwind(move || {
+        left.merge(right, |_, _, _| panic!("Panic in conflict function"));
+        assert_eq!(left.len(), 1); // only 1 entry should be left
+    })
+    .unwrap_err();
+    assert_eq!(a.dropped(), 1); // should not panic
+    assert_eq!(a_val_left.dropped(), 1); // should not panic
+    assert_eq!(b.dropped(), 2); // should drop from panic (conflict)
+    assert_eq!(b_val_left.dropped(), 1); // should be 2 were it not for Rust issue #47949
+    assert_eq!(b_val_right.dropped(), 1); // should be 2 were it not for Rust issue #47949
+    assert_eq!(c.dropped(), 2); // should drop from panic (conflict)
+    assert_eq!(c_val_left.dropped(), 1); // should be 2 were it not for Rust issue #47949
+    assert_eq!(c_val_right.dropped(), 1); // should be 2 were it not for Rust issue #47949
+}
+
+#[test]
+fn test_merge_ord_chaos() {
+    let mut map1 = BTreeMap::new();
+    map1.insert(Cyclic3::A, ());
+    map1.insert(Cyclic3::B, ());
+    let mut map2 = BTreeMap::new();
+    map2.insert(Cyclic3::A, ());
+    map2.insert(Cyclic3::B, ());
+    map2.insert(Cyclic3::C, ()); // lands first, before A
+    map2.insert(Cyclic3::B, ()); // lands first, before C
+    map1.check();
+    map2.check(); // keys are not unique but still strictly ascending
+    assert_eq!(map1.len(), 2);
+    assert_eq!(map2.len(), 4);
+    map1.merge(map2, |_, _, _| ());
+    assert_eq!(map1.len(), 5);
+    map1.check();
+}
+
 fn rand_data(len: usize) -> Vec<(u32, u32)> {
     let mut rng = DeterministicRng::new();
     Vec::from_iter((0..len).map(|_| (rng.next(), rng.next())))
@@ -2615,3 +2773,25 @@ fn test_id_based_append() {
 
     assert_eq!(lhs.pop_first().unwrap().0.name, "lhs_k".to_string());
 }
+
+#[test]
+fn test_id_based_merge() {
+    let mut lhs = BTreeMap::new();
+    let mut rhs = BTreeMap::new();
+
+    lhs.insert(IdBased { id: 0, name: "lhs_k".to_string() }, "1".to_string());
+    rhs.insert(IdBased { id: 0, name: "rhs_k".to_string() }, "2".to_string());
+
+    lhs.merge(rhs, |_, mut lhs_val, rhs_val| {
+        // confirming that lhs_val comes from lhs tree,
+        // rhs_val comes from rhs tree
+        assert_eq!(lhs_val, String::from("1"));
+        assert_eq!(rhs_val, String::from("2"));
+        lhs_val.push_str(&rhs_val);
+        lhs_val
+    });
+
+    let merged_kv_pair = lhs.pop_first().unwrap();
+    assert_eq!(merged_kv_pair.0.id, 0);
+    assert_eq!(merged_kv_pair.0.name, "lhs_k".to_string());
+}
diff --git a/library/alloctests/tests/lib.rs b/library/alloctests/tests/lib.rs
index e15c86496cf1b..e30bd26307fbf 100644
--- a/library/alloctests/tests/lib.rs
+++ b/library/alloctests/tests/lib.rs
@@ -1,5 +1,6 @@
 #![feature(allocator_api)]
 #![feature(binary_heap_pop_if)]
+#![feature(btree_merge)]
 #![feature(const_heap)]
 #![feature(deque_extend_front)]
 #![feature(iter_array_chunks)]