rust: Eschew Box<MaybeUninit<T>>::write

T-libs-api has consensus for stabilizing some of `feature(new_uninit)`,
but not for `Box<MaybeUninit<T>>::write`. The replacement is trivial, so
let's just do that, and RfL can simply move off the feature after it is
stabilized. That will happen immediately after this unblocks Rust CI,
as the relevant FCP has completed:

rust-lang/rust#63291 (comment)

This is required in advance of the stabilization because any remaining
unstable API will be carved up into subfeatures as a direct result of
stabilizing the majority of it. This code doesn't know about those yet.
It can't, as they haven't landed, as building this code blocks Rust's CI
but the expected-to-be-stabilized API can't be stabilized as long as
this code requires that feature.

Signed-off-by: Jubilee Young <workingjubilee@gmail.com>

rust/kernel/alloc/box_ext.rs

@@ -37,8 +37,10 @@ pub trait BoxExt<T>: Sized {
 
 impl<T> BoxExt<T> for Box<T> {
     fn new(x: T, flags: Flags) -> Result<Self, AllocError> {
-        let b = <Self as BoxExt<_>>::new_uninit(flags)?;
-        Ok(Box::write(b, x))
+        let mut b = <Self as BoxExt<_>>::new_uninit(flags)?;
+        b.write(x);
+        // SAFETY: We just wrote to it.
+        Ok(unsafe { b.assume_init() })
     }
 
     #[cfg(any(test, testlib))]