Rollup merge of #89833 - jkugelman:must-use-rc-downgrade, r=joshtriplett

Add #[must_use] to Rc::downgrade

Missed this in previous PR https://github.com/rust-lang/rust/pull/89796#issuecomment-941456006

Parent issue: #89692

r? ```@joshtriplett```

1 files changed, 3 insertions, 1 deletions

diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
index 2950ec421da..c0ca068de9c 100644
--- a/library/alloc/src/rc.rs
+++ b/library/alloc/src/rc.rs
@@ -41,7 +41,7 @@
 //! use std::rc::Rc;
 //!
 //! let my_rc = Rc::new(());
-//! Rc::downgrade(&my_rc);
+//! let my_weak = Rc::downgrade(&my_rc);
 //! ```
 //!
 //! `Rc<T>`'s implementations of traits like `Clone` may also be called using
@@ -889,6 +889,8 @@ impl<T: ?Sized> Rc<T> {
     ///
     /// let weak_five = Rc::downgrade(&five);
     /// ```
+    #[must_use = "this returns a new `Weak` pointer, \
+                  without modifying the original `Rc`"]
     #[stable(feature = "rc_weak", since = "1.4.0")]
     pub fn downgrade(this: &Self) -> Weak<T> {
         this.inner().inc_weak();