Collections: improve the documentation of drain members

library/alloc/src/collections/binary_heap.rs

@@ -746,9 +746,12 @@ impl<T: Ord> BinaryHeap<T> {
         self.rebuild_tail(start);
     }
 
-    /// Returns an iterator which retrieves elements in heap order.
-    /// The retrieved elements are removed from the original heap.
-    /// The remaining elements will be removed on drop in heap order.
+    /// Clears the binary heap, returning an iterator over the removed elements
+    /// in heap order. If the iterator is dropped before being fully consumed,
+    /// it drops the remaining elements in heap order.
+    ///
+    /// The returned iterator keeps a mutable borrow on the heap to optimize
+    /// its implementation.
     ///
     /// Note:
     /// * `.drain_sorted()` is *O*(*n* \* log(*n*)); much slower than `.drain()`.
@@ -1158,9 +1161,12 @@ impl<T> BinaryHeap<T> {
         self.len() == 0
     }
 
-    /// Clears the binary heap, returning an iterator over the removed elements.
+    /// Clears the binary heap, returning an iterator over the removed elements
+    /// in arbitrary order. If the iterator is dropped before being fully
+    /// consumed, it drops the remaining elements in arbitrary order.
     ///
-    /// The elements are removed in arbitrary order.
+    /// The returned iterator keeps a mutable borrow on the heap to optimize
+    /// its implementation.
     ///
     /// # Examples
     ///

library/alloc/src/collections/vec_deque/mod.rs

@@ -1215,21 +1215,25 @@ impl<T, A: Allocator> VecDeque<T, A> {
         unsafe { IterMut::new(ring, tail, head, PhantomData) }
     }
 
-    /// Creates a draining iterator that removes the specified range in the
-    /// deque and yields the removed items.
+    /// Removes the specified range from the deque in bulk, returning all
+    /// removed elements as an iterator. If the iterator is dropped before
+    /// being fully consumed, it drops the remaining removed elements.
     ///
-    /// Note 1: The element range is removed even if the iterator is not
-    /// consumed until the end.
+    /// The returned iterator keeps a mutable borrow on the queue to optimize
+    /// its implementation.
     ///
-    /// Note 2: It is unspecified how many elements are removed from the deque,
-    /// if the `Drain` value is not dropped, but the borrow it holds expires
-    /// (e.g., due to `mem::forget`).
     ///
     /// # Panics
     ///
     /// Panics if the starting point is greater than the end point or if
     /// the end point is greater than the length of the deque.
     ///
+    /// # Leaking
+    ///
+    /// If the returned iterator goes out of scope without being dropped (due to
+    /// [`mem::forget`], for example), the deque may have lost and leaked
+    /// elements arbitrarily, including elements outside the range.
+    ///
     /// # Examples
     ///
     /// ```
@@ -1240,7 +1244,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// assert_eq!(drained, [3]);
     /// assert_eq!(deque, [1, 2]);
     ///
-    /// // A full range clears all contents
+    /// // A full range clears all contents, like `clear()` does
     /// deque.drain(..);
     /// assert!(deque.is_empty());
     /// ```

library/alloc/src/string.rs

@@ -1628,17 +1628,24 @@ impl String {
         self.vec.clear()
     }
 
-    /// Creates a draining iterator that removes the specified range in the `String`
-    /// and yields the removed `chars`.
+    /// Removes the specified range from the string in bulk, returning all
+    /// removed characters as an iterator.
     ///
-    /// Note: The element range is removed even if the iterator is not
-    /// consumed until the end.
+    /// The returned iterator keeps a mutable borrow on the string to optimize
+    /// its implementation.
     ///
     /// # Panics
     ///
     /// Panics if the starting point or end point do not lie on a [`char`]
     /// boundary, or if they're out of bounds.
     ///
+    /// # Leaking
+    ///
+    /// If the returned iterator goes out of scope without being dropped (due to
+    /// [`core::mem::forget`], for example), the string may still contain a copy
+    /// of any drained characters, or may have lost characters arbitrarily,
+    /// including characters outside the range.
+    ///
     /// # Examples
     ///
     /// Basic usage:
@@ -1652,7 +1659,7 @@ impl String {
     /// assert_eq!(t, "α is alpha, ");
     /// assert_eq!(s, "β is beta");
     ///
-    /// // A full range clears the string
+    /// // A full range clears the string, like `clear()` does
     /// s.drain(..);
     /// assert_eq!(s, "");
     /// ```

library/alloc/src/vec/mod.rs

@@ -1799,19 +1799,24 @@ impl<T, A: Allocator> Vec<T, A> {
         self.len += count;
     }
 
-    /// Creates a draining iterator that removes the specified range in the vector
-    /// and yields the removed items.
+    /// Removes the specified range from the vector in bulk, returning all
+    /// removed elements as an iterator. If the iterator is dropped before
+    /// being fully consumed, it drops the remaining removed elements.
     ///
-    /// When the iterator **is** dropped, all elements in the range are removed
-    /// from the vector, even if the iterator was not fully consumed. If the
-    /// iterator **is not** dropped (with [`mem::forget`] for example), it is
-    /// unspecified how many elements are removed.
+    /// The returned iterator keeps a mutable borrow on the vector to optimize
+    /// its implementation.
     ///
     /// # Panics
     ///
     /// Panics if the starting point is greater than the end point or if
     /// the end point is greater than the length of the vector.
     ///
+    /// # Leaking
+    ///
+    /// If the returned iterator goes out of scope without being dropped (due to
+    /// [`mem::forget`], for example), the vector may have lost and leaked
+    /// elements arbitrarily, including elements outside the range.
+    ///
     /// # Examples
     ///
     /// ```
@@ -1820,7 +1825,7 @@ impl<T, A: Allocator> Vec<T, A> {
     /// assert_eq!(v, &[1]);
     /// assert_eq!(u, &[2, 3]);
     ///
-    /// // A full range clears the vector
+    /// // A full range clears the vector, like `clear()` does
     /// v.drain(..);
     /// assert_eq!(v, &[]);
     /// ```

library/std/src/collections/hash/map.rs

@@ -547,6 +547,10 @@ impl<K, V, S> HashMap<K, V, S> {
     /// Clears the map, returning all key-value pairs as an iterator. Keeps the
     /// allocated memory for reuse.
     ///
+    /// If the returned iterator is dropped before being fully consumed, it
+    /// drops the remaining key-value pairs. The returned iterator keeps a
+    /// mutable borrow on the vector to optimize its implementation.
+    ///
     /// # Examples
     ///
     /// ```

library/std/src/collections/hash/set.rs

@@ -227,7 +227,12 @@ impl<T, S> HashSet<T, S> {
         self.base.is_empty()
     }
 
-    /// Clears the set, returning all elements in an iterator.
+    /// Clears the set, returning all elements as an iterator. Keeps the
+    /// allocated memory for reuse.
+    ///
+    /// If the returned iterator is dropped before being fully consumed, it
+    /// drops the remaining elements. The returned iterator keeps a mutable
+    /// borrow on the vector to optimize its implementation.
     ///
     /// # Examples
     ///