Update example on push_mut

Author: balt-dev

library/alloc/src/vec/mod.rs

@@ -2514,11 +2514,28 @@ impl<T, A: Allocator> Vec<T, A> {
     /// ```
     /// #![feature(push_mut)]
     ///
-    /// let mut vec = vec![];
-    /// // Due to current borrow checker limitations (see -Zpolonius), this is the only way to spell this right now.
-    /// let last = if let Some(v) = vec.last_mut() { v } else { vec.push_mut(0) };
-    /// *last += 6;
-    /// assert_eq!(vec, [6]);
+    /// # #[allow(unused)]
+    /// #[derive(PartialEq, Eq, Debug)]
+    /// struct Item { identifier: &'static str, count: usize }
+    /// 
+    /// impl Default for Item {
+    ///     fn default() -> Self {
+    ///         return Self { identifier: "stone", count: 64 }
+    ///     }
+    /// }
+    ///
+    /// let mut items = vec![];
+    ///
+    /// // We can mutate the just-pushed value without having to fetch it again
+    /// for count in [15, 35, 61] {
+    ///     let item = items.push_mut(Item::default());
+    ///     item.count = count;
+    /// }
+    ///
+    /// assert_eq!(
+    ///     items,
+    ///     [Item { identifier: "stone", count: 15 }, Item { identifier: "stone", count: 35 }, Item { identifier: "stone", count: 61 }]
+    /// );
     /// ```
     ///
     /// # Time complexity
@@ -2563,8 +2580,12 @@ impl<T, A: Allocator> Vec<T, A> {
     ///
     /// Takes *O*(1) time.
     #[inline]
+
+    // Since there's currently no way to have multiple unstable attributes on the same item, I compromised.
+    // Uncomment/delete the respective attribute when its respective issue stabilizes, since this falls under both.
     #[unstable(feature = "push_mut", issue = "135974")]
     // #[unstable(feature = "vec_push_within_capacity", issue = "100486")]
+
     #[must_use = "if you don't need a reference to the value, use Vec::push_within_capacity instead"]
     pub fn push_mut_within_capacity(&mut self, value: T) -> Result<&mut T, T> {
         if self.len == self.buf.capacity() {