rust: kernel: stop using ptr_metadata feature

#![feature(dispatch_from_dyn)]

#![feature(new_uninit)]

#![feature(offset_of)]

#![feature(ptr_metadata)]

#![feature(receiver_trait)]

#![feature(unsize)]

    mem::{ManuallyDrop, MaybeUninit},

    ops::{Deref, DerefMut},

    pin::Pin,

    ptr::{NonNull, Pointee},

    ptr::NonNull,

};

use macros::pin_data;

        // 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`].