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*RFL import: rust: kernel: stop using ptr_metadata feature
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The `byte_sub` method was stabilized in Rust 1.75.0. By using that
method, we no longer need the unstable `ptr_metadata` feature for
implementing `Arc::from_raw`.

This brings us one step closer towards not using unstable compiler
features.

Reviewed-by: Benno Lossin <[email protected]>
Reviewed-by: Martin Rodriguez Reboredo <[email protected]>
Reviewed-by: Trevor Gross <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[ Reworded title. ]
Signed-off-by: Miguel Ojeda <[email protected]>
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Darksonn authored and jannau committed Apr 7, 2024
1 parent c9c4874 commit 8d460ac
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Showing 2 changed files with 7 additions and 10 deletions.
1 change: 0 additions & 1 deletion rust/kernel/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -21,7 +21,6 @@
#![feature(duration_constants)]
#![feature(new_uninit)]
#![feature(offset_of)]
#![feature(ptr_metadata)]
#![feature(receiver_trait)]
#![feature(type_alias_impl_trait)]
#![feature(unsize)]
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16 changes: 7 additions & 9 deletions rust/kernel/sync/arc.rs
Original file line number Diff line number Diff line change
Expand Up @@ -30,7 +30,7 @@ use core::{
mem::{ManuallyDrop, MaybeUninit},
ops::{Deref, DerefMut},
pin::Pin,
ptr::{NonNull, Pointee},
ptr::NonNull,
};
use macros::pin_data;

Expand Down Expand Up @@ -258,22 +258,20 @@ impl<T: ?Sized> Arc<T> {
// binary, so its layout is not so large that it can trigger arithmetic overflow.
let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };

let metadata: <T as Pointee>::Metadata = core::ptr::metadata(ptr);
// SAFETY: The metadata of `T` and `ArcInner<T>` is the same because `ArcInner` is a struct
// with `T` as its last field.
// Pointer casts leave the metadata unchanged. This is okay because the metadata of `T` and
// `ArcInner<T>` is the same since `ArcInner` is a struct with `T` as its last field.
//
// This is documented at:
// <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
let metadata: <ArcInner<T> as Pointee>::Metadata =
unsafe { core::mem::transmute_copy(&metadata) };
let ptr = ptr as *const ArcInner<T>;

// SAFETY: The pointer is in-bounds of an allocation both before and after offsetting the
// pointer, since it originates from a previous call to `Arc::into_raw` and is still valid.
let ptr = unsafe { (ptr as *mut u8).sub(val_offset) as *mut () };
let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
let ptr = unsafe { ptr.byte_sub(val_offset) };

// SAFETY: By the safety requirements we know that `ptr` came from `Arc::into_raw`, so the
// reference count held then will be owned by the new `Arc` object.
unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
unsafe { Self::from_inner(NonNull::new_unchecked(ptr.cast_mut())) }
}

/// Returns an [`ArcBorrow`] from the given [`Arc`].
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