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`. Instead, we can use
`MaybeUninit<T>::write`, so Rust for Linux can drop the feature after
stabilization. That will happen after merging, as the FCP has completed:
rust-lang/rust#63291 (comment)

This is required before stabilization because remaining-unstable API
will be divided into new features. This code doesn't know about those
yet. It can't: they haven't landed, as the relevant PR is blocked on
rustc's CI testing Rust-for-Linux without this patch.

Signed-off-by: Jubilee Young <workingjubilee@gmail.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Link: https://lore.kernel.org/r/20240823050359.1737893-1-workingjubilee@gmail.com

rust/kernel/alloc/box_ext.rs

@@ -21,8 +21,10 @@ 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))]