Mention rc::Weak and add an example with a cycle in the speaker notes. (#375)

src/std/rc.md

@@ -15,18 +15,52 @@ fn main() {
 }
 ```
 
-If you need to mutate the data inside an `Rc`, you will need to wrap the data in
-a type such as [`Cell` or `RefCell`][2]. See [`Arc`][3] if you are in a multi-threaded
-context.
+* If you need to mutate the data inside an `Rc`, you will need to wrap the data in
+  a type such as [`Cell` or `RefCell`][2].
+* See [`Arc`][3] if you are in a multi-threaded context.
+* You can *downgrade* a shared pointer into a [`Weak`][4] pointer to create cycles
+  that will get dropped.
 
 [1]: https://doc.rust-lang.org/std/rc/struct.Rc.html
 [2]: https://doc.rust-lang.org/std/cell/index.html
 [3]: ../concurrency/shared_state/arc.md
+[4]: https://doc.rust-lang.org/std/rc/struct.Weak.html
 
 <details>
 
 * Like C++'s `std::shared_ptr`.
 * `clone` is cheap: creates a pointer to the same allocation and increases the reference count.
 * `make_mut` actually clones the inner value if necessary ("clone-on-write") and returns a mutable reference.
+* You can `downgrade()` a `Rc` into a *weakly reference-counted* object to
+  create cycles that will be dropped properly (likely in combination with
+  `RefCell`).
+
+```rust,editable
+use std::rc::{Rc, Weak};
+use std::cell::RefCell;
+
+#[derive(Debug)]
+struct Node {
+    value: i64,
+    parent: Option<Weak<RefCell<Node>>>,
+    children: Vec<Rc<RefCell<Node>>>,
+}
+
+fn main() {
+    let mut root = Rc::new(RefCell::new(Node {
+        value: 42,
+        parent: None,
+        children: vec![],
+    }));
+    let child = Rc::new(RefCell::new(Node {
+        value: 43,
+        children: vec![],
+        parent: Some(Rc::downgrade(&root))
+    }));
+    root.borrow_mut().children.push(child);
+
+    println!("graph: {root:#?}");
+}
+```
 
 </details>