diff --git a/library/alloc/src/vec.rs b/library/alloc/src/vec.rs
index b20ccd388d1f1..85ca8ea4738e7 100644
--- a/library/alloc/src/vec.rs
+++ b/library/alloc/src/vec.rs
@@ -243,8 +243,8 @@ use crate::raw_vec::RawVec;
 /// * It would penalize the general case, incurring an additional branch
 ///   on every access.
 ///
-/// `Vec` will never automatically shrink itself, even if completely empty. This
-/// ensures no unnecessary allocations or deallocations occur. Emptying a `Vec`
+/// `Vec` will not automatically shrink itself, even if completely empty, when doing so
+/// would cause unnecessary allocations or deallocations to occur. Emptying a `Vec`
 /// and then filling it back up to the same [`len`] should incur no calls to
 /// the allocator. If you wish to free up unused memory, use
 /// [`shrink_to_fit`].
@@ -1257,6 +1257,14 @@ impl<T> Vec<T> {
     #[inline]
     #[stable(feature = "append", since = "1.4.0")]
     pub fn append(&mut self, other: &mut Self) {
+        // Reuses other if doing so allows us to turn a certain reallocation within self
+        // into a potential, future reallocation in other.
+        // The capacity limit ensures that we are not stealing a large preallocation from `other`
+        // that is not commensurate with the avoided reallocation in self.
+        if self.len == 0 && self.capacity() < other.len && other.capacity() / 2 <= other.len {
+            mem::swap(self, other);
+            return;
+        }
         unsafe {
             self.append_elements(other.as_slice() as _);
             other.set_len(0);