*RFL import: rust: kernel: stop using ptr_metadata feature

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]>

rust/kernel/lib.rs

@@ -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)]

rust/kernel/sync/arc.rs

@@ -30,7 +30,7 @@ use core::{
  mem::{ManuallyDrop, MaybeUninit},
  ops::{Deref, DerefMut},
  pin::Pin,
- ptr::{NonNull, Pointee},
+ ptr::NonNull,
 };
 use macros::pin_data;
 
@@ -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`].