Rollup merge of #89778 - jkugelman:must-use-as_type-conversions, r=joshtriplett

Add #[must_use] to as_type conversions

Clippy missed these:

```rust
alloc::string::String   fn as_mut_str(&mut self) -> &mut str;
core::mem::NonNull<T>   unsafe fn as_uninit_mut<'a>(&mut self) -> &'a MaybeUninit<T>;
str                     unsafe fn as_bytes_mut(&mut self) -> &mut [u8];
str                     fn as_mut_ptr(&mut self) -> *mut u8;
```

Parent issue: #89692

r? ````@joshtriplett````

library/alloc/src/rc.rs

@@ -2093,6 +2093,7 @@ impl<T: ?Sized> Weak<T> {
     /// ```
     ///
     /// [`null`]: ptr::null
+    #[must_use]
     #[stable(feature = "rc_as_ptr", since = "1.45.0")]
     pub fn as_ptr(&self) -> *const T {
         let ptr: *mut RcBox<T> = NonNull::as_ptr(self.ptr);