Rollup merge of #121287 - zachs18:rc-into-raw-must-use, r=cuviper

Clarify/add `must_use` message for Rc/Arc/Weak::into_raw.

The current `#[must_use]` messages for `{sync,rc}::Weak::into_raw` ("`self` will be dropped if the result is not used") are misleading, as `self` is consumed and will *not* be dropped.

This PR changes their `#[must_use]` message to the same as `Arc::into_raw`'s[ current `#[must_use]` message](https://github.com/rust-lang/rust/blob/d5735645753e990a72446094f703df9b5e421555/library/alloc/src/sync.rs#L1482) ("losing the pointer will leak memory"), and also adds it to `Rc::into_raw`, which is not currently `#[must_use]`.

library/alloc/src/rc.rs

@@ -1344,6 +1344,7 @@ impl<T: ?Sized, A: Allocator> Rc<T, A> {
     /// let x_ptr = Rc::into_raw(x);
     /// assert_eq!(unsafe { &*x_ptr }, "hello");
     /// ```
+    #[must_use = "losing the pointer will leak memory"]
     #[stable(feature = "rc_raw", since = "1.17.0")]
     #[rustc_never_returns_null_ptr]
     pub fn into_raw(this: Self) -> *const T {
@@ -2985,7 +2986,7 @@ impl<T: ?Sized, A: Allocator> Weak<T, A> {
     ///
     /// [`from_raw`]: Weak::from_raw
     /// [`as_ptr`]: Weak::as_ptr
-    #[must_use = "`self` will be dropped if the result is not used"]
+    #[must_use = "losing the pointer will leak memory"]
     #[stable(feature = "weak_into_raw", since = "1.45.0")]
     pub fn into_raw(self) -> *const T {
         let result = self.as_ptr();

library/alloc/src/sync.rs

@@ -2719,7 +2719,7 @@ impl<T: ?Sized, A: Allocator> Weak<T, A> {
     ///
     /// [`from_raw`]: Weak::from_raw
     /// [`as_ptr`]: Weak::as_ptr
-    #[must_use = "`self` will be dropped if the result is not used"]
+    #[must_use = "losing the pointer will leak memory"]
     #[stable(feature = "weak_into_raw", since = "1.45.0")]
     pub fn into_raw(self) -> *const T {
         let result = self.as_ptr();