Rollup merge of rust-lang#89769 - jkugelman:must-use-maybe-uninit-new, r=joshtriplett

Add #[must_use] to MaybeUninit::new

As discussed in rust-lang#89729 (comment).

Parent issue: rust-lang#89692

r? ``@joshtriplett``

Author: matthiaskrgr

library/core/src/mem/maybe_uninit.rs

@@ -291,6 +291,7 @@ impl<T> MaybeUninit<T> {
     /// [`assume_init`]: MaybeUninit::assume_init
     #[stable(feature = "maybe_uninit", since = "1.36.0")]
     #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")]
+    #[must_use = "use `forget` to avoid running Drop code"]
     #[inline(always)]
     pub const fn new(val: T) -> MaybeUninit<T> {
         MaybeUninit { value: ManuallyDrop::new(val) }