add prepend, improve doc and add tracking issue number

Author: antonilol

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

@@ -1971,7 +1971,28 @@ impl<T, A: Allocator> VecDeque<T, A> {
         unsafe { self.buffer_write(self.to_physical_idx(len), value) }
     }
 
-    /// Prepends all elements from the iterator to the front of the deque, as if [`push_front`][VecDeque::push_front] was called with the elements of the iterator.
+    /// Prepends all contents of the iterator to the front of the deque.
+    /// The order of the contents is preserved.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(deque_extend_front)]
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut deque = VecDeque::from([4, 5, 6]);
+    /// deque.prepend([1, 2, 3]);
+    /// assert_eq!(deque, [1, 2, 3, 4, 5, 6]);
+    /// ```
+    #[unstable(feature = "deque_extend_front", issue = "146975")]
+    #[track_caller]
+    pub fn prepend<I: IntoIterator<Item = T, IntoIter: DoubleEndedIterator>>(&mut self, other: I) {
+        self.extend_front(other.into_iter().rev())
+    }
+
+    /// Prepends all contents of the iterator to the front of the deque,
+    /// as if [`push_front`][VecDeque::push_front] was called repeatedly with
+    /// the values yielded by the iterator.
     ///
     /// # Examples
     ///
@@ -1983,7 +2004,19 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// deque.extend_front([3, 2, 1]);
     /// assert_eq!(deque, [1, 2, 3, 4, 5, 6]);
     /// ```
-    #[unstable(feature = "deque_extend_front", issue = "none")]
+    ///
+    /// This behaves like [`push_front`][VecDeque::push_front] was called repeatedly:
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut deque = VecDeque::from([4, 5, 6]);
+    /// for v in [3, 2, 1] {
+    ///     deque.push_front(v);
+    /// }
+    /// assert_eq!(deque, [1, 2, 3, 4, 5, 6]);
+    /// ```
+    #[unstable(feature = "deque_extend_front", issue = "146975")]
     #[track_caller]
     pub fn extend_front<I: IntoIterator<Item = T>>(&mut self, iter: I) {
         <Self as SpecExtendFront<T, I::IntoIter>>::spec_extend_front(self, iter.into_iter());