diff --git a/library/alloc/src/collections/btree/append.rs b/library/alloc/src/collections/btree/append.rs
new file mode 100644
index 0000000000000..e0362b2f37df7
--- /dev/null
+++ b/library/alloc/src/collections/btree/append.rs
@@ -0,0 +1,124 @@
+use super::map::MIN_LEN;
+use super::merge_iter::MergeIterInner;
+use super::node::{self, ForceResult::*, Root};
+use core::iter::FusedIterator;
+
+impl<K, V> Root<K, V> {
+    /// Appends all key-value pairs from the union of two ascending iterators,
+    /// incrementing a `length` variable along the way. The latter makes it
+    /// easier for the caller to avoid a leak when a drop handler panicks.
+    ///
+    /// If both iterators produce the same key, this method drops the pair from
+    /// the left iterator and appends the pair from the right iterator.
+    ///
+    /// If you want the tree to end up in a strictly ascending order, like for
+    /// a `BTreeMap`, both iterators should produce keys in strictly ascending
+    /// order, each greater than all keys in the tree, including any keys
+    /// already in the tree upon entry.
+    pub fn append_from_sorted_iters<I>(&mut self, left: I, right: I, length: &mut usize)
+    where
+        K: Ord,
+        I: Iterator<Item = (K, V)> + FusedIterator,
+    {
+        // We prepare to merge `left` and `right` into a sorted sequence in linear time.
+        let iter = MergeIter(MergeIterInner::new(left, right));
+
+        // Meanwhile, we build a tree from the sorted sequence in linear time.
+        self.bulk_push(iter, length)
+    }
+
+    /// Pushes all key-value pairs to the end of the tree, incrementing a
+    /// `length` variable along the way. The latter makes it easier for the
+    /// caller to avoid a leak when the iterator panicks.
+    fn bulk_push<I>(&mut self, iter: I, length: &mut usize)
+    where
+        I: Iterator<Item = (K, V)>,
+    {
+        let mut cur_node = self.node_as_mut().last_leaf_edge().into_node();
+        // Iterate through all key-value pairs, pushing them into nodes at the right level.
+        for (key, value) in iter {
+            // Try to push key-value pair into the current leaf node.
+            if cur_node.len() < node::CAPACITY {
+                cur_node.push(key, value);
+            } else {
+                // No space left, go up and push there.
+                let mut open_node;
+                let mut test_node = cur_node.forget_type();
+                loop {
+                    match test_node.ascend() {
+                        Ok(parent) => {
+                            let parent = parent.into_node();
+                            if parent.len() < node::CAPACITY {
+                                // Found a node with space left, push here.
+                                open_node = parent;
+                                break;
+                            } else {
+                                // Go up again.
+                                test_node = parent.forget_type();
+                            }
+                        }
+                        Err(_) => {
+                            // We are at the top, create a new root node and push there.
+                            open_node = self.push_internal_level();
+                            break;
+                        }
+                    }
+                }
+
+                // Push key-value pair and new right subtree.
+                let tree_height = open_node.height() - 1;
+                let mut right_tree = Root::new_leaf();
+                for _ in 0..tree_height {
+                    right_tree.push_internal_level();
+                }
+                open_node.push(key, value, right_tree);
+
+                // Go down to the right-most leaf again.
+                cur_node = open_node.forget_type().last_leaf_edge().into_node();
+            }
+
+            // Increment length every iteration, to make sure the map drops
+            // the appended elements even if advancing the iterator panicks.
+            *length += 1;
+        }
+        self.fix_right_edge();
+    }
+
+    fn fix_right_edge(&mut self) {
+        // Handle underfull nodes, start from the top.
+        let mut cur_node = self.node_as_mut();
+        while let Internal(internal) = cur_node.force() {
+            // Check if right-most child is underfull.
+            let mut last_edge = internal.last_edge();
+            let right_child_len = last_edge.reborrow().descend().len();
+            if right_child_len < MIN_LEN {
+                // We need to steal.
+                let mut last_kv = match last_edge.left_kv() {
+                    Ok(left) => left,
+                    Err(_) => unreachable!(),
+                };
+                last_kv.bulk_steal_left(MIN_LEN - right_child_len);
+                last_edge = last_kv.right_edge();
+            }
+
+            // Go further down.
+            cur_node = last_edge.descend();
+        }
+    }
+}
+
+// An iterator for merging two sorted sequences into one
+struct MergeIter<K, V, I: Iterator<Item = (K, V)>>(MergeIterInner<I>);
+
+impl<K: Ord, V, I> Iterator for MergeIter<K, V, I>
+where
+    I: Iterator<Item = (K, V)> + FusedIterator,
+{
+    type Item = (K, V);
+
+    /// If two keys are equal, returns the key-value pair from the right source.
+    fn next(&mut self) -> Option<(K, V)> {
+        let (a_next, b_next) = self.0.nexts(|a: &(K, V), b: &(K, V)| K::cmp(&a.0, &b.0));
+        b_next.or(a_next)
+    }
+}
diff --git a/library/alloc/src/collections/btree/map.rs b/library/alloc/src/collections/btree/map.rs
index 07c23d29e20a6..49122f53d33ad 100644
--- a/library/alloc/src/collections/btree/map.rs
+++ b/library/alloc/src/collections/btree/map.rs
@@ -9,7 +9,6 @@ use core::ops::{Index, RangeBounds};
 use core::ptr;
 
 use super::borrow::DormantMutRef;
-use super::merge_iter::MergeIterInner;
 use super::node::{self, marker, ForceResult::*, Handle, NodeRef};
 use super::search::{self, SearchResult::*};
 use super::unwrap_unchecked;
@@ -458,9 +457,6 @@ impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for RangeMut<'_, K, V> {
     }
 }
 
-// An iterator for merging two sorted sequences into one
-struct MergeIter<K, V, I: Iterator<Item = (K, V)>>(MergeIterInner<I>);
-
 impl<K: Ord, V> BTreeMap<K, V> {
     /// Makes a new empty BTreeMap.
     ///
@@ -908,13 +904,10 @@ impl<K: Ord, V> BTreeMap<K, V> {
             return;
         }
 
-        // First, we merge `self` and `other` into a sorted sequence in linear time.
         let self_iter = mem::take(self).into_iter();
         let other_iter = mem::take(other).into_iter();
-        let iter = MergeIter(MergeIterInner::new(self_iter, other_iter));
-
-        // Second, we build a tree from the sorted sequence in linear time.
-        self.from_sorted_iter(iter);
+        let root = BTreeMap::ensure_is_owned(&mut self.root);
+        root.append_from_sorted_iters(self_iter, other_iter, &mut self.length)
     }
 
     /// Constructs a double-ended iterator over a sub-range of elements in the map.
@@ -1039,78 +1032,6 @@ impl<K: Ord, V> BTreeMap<K, V> {
         }
     }
 
-    fn from_sorted_iter<I: Iterator<Item = (K, V)>>(&mut self, iter: I) {
-        let root = Self::ensure_is_owned(&mut self.root);
-        let mut cur_node = root.node_as_mut().last_leaf_edge().into_node();
-        // Iterate through all key-value pairs, pushing them into nodes at the right level.
-        for (key, value) in iter {
-            // Try to push key-value pair into the current leaf node.
-            if cur_node.len() < node::CAPACITY {
-                cur_node.push(key, value);
-            } else {
-                // No space left, go up and push there.
-                let mut open_node;
-                let mut test_node = cur_node.forget_type();
-                loop {
-                    match test_node.ascend() {
-                        Ok(parent) => {
-                            let parent = parent.into_node();
-                            if parent.len() < node::CAPACITY {
-                                // Found a node with space left, push here.
-                                open_node = parent;
-                                break;
-                            } else {
-                                // Go up again.
-                                test_node = parent.forget_type();
-                            }
-                        }
-                        Err(_) => {
-                            // We are at the top, create a new root node and push there.
-                            open_node = root.push_internal_level();
-                            break;
-                        }
-                    }
-                }
-
-                // Push key-value pair and new right subtree.
-                let tree_height = open_node.height() - 1;
-                let mut right_tree = node::Root::new_leaf();
-                for _ in 0..tree_height {
-                    right_tree.push_internal_level();
-                }
-                open_node.push(key, value, right_tree);
-
-                // Go down to the right-most leaf again.
-                cur_node = open_node.forget_type().last_leaf_edge().into_node();
-            }
-
-            self.length += 1;
-        }
-        Self::fix_right_edge(root)
-    }
-
-    fn fix_right_edge(root: &mut node::Root<K, V>) {
-        // Handle underfull nodes, start from the top.
-        let mut cur_node = root.node_as_mut();
-        while let Internal(internal) = cur_node.force() {
-            // Check if right-most child is underfull.
-            let mut last_edge = internal.last_edge();
-            let right_child_len = last_edge.reborrow().descend().len();
-            if right_child_len < MIN_LEN {
-                // We need to steal.
-                let mut last_kv = match last_edge.left_kv() {
-                    Ok(left) => left,
-                    Err(_) => unreachable!(),
-                };
-                last_kv.bulk_steal_left(MIN_LEN - right_child_len);
-                last_edge = last_kv.right_edge();
-            }
-
-            // Go further down.
-            cur_node = last_edge.descend();
-        }
-    }
-
     /// Splits the collection into two at the given key. Returns everything after the given key,
     /// including the key.
     ///
@@ -2220,18 +2141,5 @@ impl<K, V> BTreeMap<K, V> {
     }
 }
 
-impl<K: Ord, V, I> Iterator for MergeIter<K, V, I>
-where
-    I: Iterator<Item = (K, V)> + ExactSizeIterator + FusedIterator,
-{
-    type Item = (K, V);
-
-    /// If two keys are equal, returns the key/value-pair from the right source.
-    fn next(&mut self) -> Option<(K, V)> {
-        let (a_next, b_next) = self.0.nexts(|a: &(K, V), b: &(K, V)| K::cmp(&a.0, &b.0));
-        b_next.or(a_next)
-    }
-}
-
 #[cfg(test)]
 mod tests;
diff --git a/library/alloc/src/collections/btree/map/tests.rs b/library/alloc/src/collections/btree/map/tests.rs
index d5b1c600d93eb..89a6e327422bb 100644
--- a/library/alloc/src/collections/btree/map/tests.rs
+++ b/library/alloc/src/collections/btree/map/tests.rs
@@ -1667,6 +1667,33 @@ create_append_test!(test_append_239, 239);
 #[cfg(not(miri))] // Miri is too slow
 create_append_test!(test_append_1700, 1700);
 
+#[test]
+fn test_append_drop_leak() {
+    static DROPS: AtomicUsize = AtomicUsize::new(0);
+
+    struct D;
+
+    impl Drop for D {
+        fn drop(&mut self) {
+            if DROPS.fetch_add(1, Ordering::SeqCst) == 0 {
+                panic!("panic in `drop`");
+            }
+        }
+    }
+
+    let mut left = BTreeMap::new();
+    let mut right = BTreeMap::new();
+    left.insert(0, D);
+    left.insert(1, D); // first to be dropped during append
+    left.insert(2, D);
+    right.insert(1, D);
+    right.insert(2, D);
+
+    catch_unwind(move || left.append(&mut right)).unwrap_err();
+
+    assert_eq!(DROPS.load(Ordering::SeqCst), 4); // Rust issue #47949 ate one little piggy
+}
+
 fn rand_data(len: usize) -> Vec<(u32, u32)> {
     assert!(len * 2 <= 70029); // from that point on numbers repeat
     let mut rng = DeterministicRng::new();
diff --git a/library/alloc/src/collections/btree/merge_iter.rs b/library/alloc/src/collections/btree/merge_iter.rs
index 88e6f86c2c6f5..7f23d93b990f5 100644
--- a/library/alloc/src/collections/btree/merge_iter.rs
+++ b/library/alloc/src/collections/btree/merge_iter.rs
@@ -2,27 +2,25 @@ use core::cmp::Ordering;
 use core::fmt::{self, Debug};
 use core::iter::FusedIterator;
 
-/// Core of an iterator that merges the output of two ascending iterators,
+/// Core of an iterator that merges the output of two strictly ascending iterators,
 /// for instance a union or a symmetric difference.
-pub struct MergeIterInner<I>
-where
-    I: Iterator,
-{
+pub struct MergeIterInner<I: Iterator> {
     a: I,
     b: I,
     peeked: Option<Peeked<I>>,
 }
 
-/// Benchmarks faster than wrapping both iterators in a Peekable.
+/// Benchmarks faster than wrapping both iterators in a Peekable,
+/// probably because we can afford to impose a FusedIterator bound.
 #[derive(Clone, Debug)]
 enum Peeked<I: Iterator> {
     A(I::Item),
     B(I::Item),
 }
 
-impl<I> Clone for MergeIterInner<I>
+impl<I: Iterator> Clone for MergeIterInner<I>
 where
-    I: Clone + Iterator,
+    I: Clone,
     I::Item: Clone,
 {
     fn clone(&self) -> Self {
@@ -30,20 +28,17 @@ where
     }
 }
 
-impl<I> Debug for MergeIterInner<I>
+impl<I: Iterator> Debug for MergeIterInner<I>
 where
-    I: Iterator + Debug,
+    I: Debug,
     I::Item: Debug,
 {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.debug_tuple("MergeIterInner").field(&self.a).field(&self.b).finish()
+        f.debug_tuple("MergeIterInner").field(&self.a).field(&self.b).field(&self.peeked).finish()
     }
 }
 
-impl<I> MergeIterInner<I>
-where
-    I: ExactSizeIterator + FusedIterator,
-{
+impl<I: Iterator> MergeIterInner<I> {
     /// Creates a new core for an iterator merging a pair of sources.
     pub fn new(a: I, b: I) -> Self {
         MergeIterInner { a, b, peeked: None }
@@ -52,13 +47,17 @@ where
     /// Returns the next pair of items stemming from the pair of sources
     /// being merged. If both returned options contain a value, that value
     /// is equal and occurs in both sources. If one of the returned options
-    /// contains a value, that value doesn't occur in the other source.
-    /// If neither returned option contains a value, iteration has finished
-    /// and subsequent calls will return the same empty pair.
+    /// contains a value, that value doesn't occur in the other source (or
+    /// the sources are not strictly ascending). If neither returned option
+    /// contains a value, iteration has finished and subsequent calls will
+    /// return the same empty pair.
     pub fn nexts<Cmp: Fn(&I::Item, &I::Item) -> Ordering>(
         &mut self,
         cmp: Cmp,
-    ) -> (Option<I::Item>, Option<I::Item>) {
+    ) -> (Option<I::Item>, Option<I::Item>)
+    where
+        I: FusedIterator,
+    {
         let mut a_next;
         let mut b_next;
         match self.peeked.take() {
@@ -86,7 +85,10 @@ where
     }
 
     /// Returns a pair of upper bounds for the `size_hint` of the final iterator.
-    pub fn lens(&self) -> (usize, usize) {
+    pub fn lens(&self) -> (usize, usize)
+    where
+        I: ExactSizeIterator,
+    {
         match self.peeked {
             Some(Peeked::A(_)) => (1 + self.a.len(), self.b.len()),
             Some(Peeked::B(_)) => (self.a.len(), 1 + self.b.len()),
diff --git a/library/alloc/src/collections/btree/mod.rs b/library/alloc/src/collections/btree/mod.rs
index 7bf1706dd6d57..ebcbb0e467c46 100644
--- a/library/alloc/src/collections/btree/mod.rs
+++ b/library/alloc/src/collections/btree/mod.rs
@@ -1,3 +1,4 @@
+mod append;
 mod borrow;
 pub mod map;
 mod mem;