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

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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: https://github.com/rust-lang/rust/issues/63291#issuecomment-2183022955 [1]
Link: https://github.com/rust-lang/rust/pull/129416 [2]
Link: https://github.com/rust-lang/rust/issues/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>
This commit is contained in:
Jubilee Young 2024-08-22 22:03:59 -07:00 committed by Miguel Ojeda
parent 0ff8f3f097
commit 0903b9e2a4
1 changed files with 4 additions and 2 deletions
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6
rust/kernel/alloc/box_ext.rs
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@ -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))]