Rollup merge of #89796 - jkugelman:must-use-non-mutating-verb-methods, r=joshtriplett

Add #[must_use] to non-mutating verb methods

These are methods that could be misconstrued to mutate their input, similar to #89694. I gave each one a different custom message.

I wrote that `upgrade` and `downgrade` don't modify the input pointers. Logically they don't, but technically they do...

Parent issue: #89692

r? ```@joshtriplett```

library/alloc/src/rc.rs

@@ -2230,6 +2230,8 @@ impl<T: ?Sized> Weak<T> {
     ///
     /// assert!(weak_five.upgrade().is_none());
     /// ```
+    #[must_use = "this returns a new `Rc`, \
+                  without modifying the original weak pointer"]
     #[stable(feature = "rc_weak", since = "1.4.0")]
     pub fn upgrade(&self) -> Option<Rc<T>> {
         let inner = self.inner()?;