rust: alloc: eschew Box<MaybeUninit<T>>::write

Upstream Rust's libs-api team 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 [1]. 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. [ The PR has landed [2] and will be released in Rust 1.82.0 (expected on 2024-10-17), so we could conditionally enable the new unstable feature (`box_uninit_write` [3]) instead, but just for a single `unsafe` block it is probably not worth it. For the time being, I added it to the "nice to have" section of our unstable features list. - Miguel ] Link: rust-lang/rust#63291 (comment) [1] Link: rust-lang/rust#129416 [2] Link: rust-lang/rust#129397 [3] Signed-off-by: Jubilee Young <workingjubilee@gmail.com> Reviewed-by: Alice Ryhl <aliceryhl@google.com> Reviewed-by: Trevor Gross <tmgross@umich.edu> [ Reworded slightly. - Miguel ] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: workingjubilee

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))]