rust: sync: add `Arc::{from_raw, into_raw}`

#![feature(coerce_unsized)]

#![feature(dispatch_from_dyn)]

#![feature(new_uninit)]

#![feature(ptr_metadata)]

#![feature(receiver_trait)]

#![feature(unsize)]

};

use alloc::boxed::Box;

use core::{

    alloc::AllocError,

    alloc::{AllocError, Layout},

    fmt,

    marker::{PhantomData, Unsize},

    mem::{ManuallyDrop, MaybeUninit},

    ops::{Deref, DerefMut},

    pin::Pin,

    ptr::NonNull,

    ptr::{NonNull, Pointee},

};

use macros::pin_data;

        }

    }

    /// Convert the [`Arc`] into a raw pointer.

    ///

    /// The raw pointer has ownership of the refcount that this Arc object owned.

    pub fn into_raw(self) -> *const T {

        let ptr = self.ptr.as_ptr();

        core::mem::forget(self);

        // SAFETY: The pointer is valid.

        unsafe { core::ptr::addr_of!((*ptr).data) }

    }

    /// Recreates an [`Arc`] instance previously deconstructed via [`Arc::into_raw`].

    ///

    /// # Safety

    ///

    /// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it

    /// must not be called more than once for each previous call to [`Arc::into_raw`].

    pub unsafe fn from_raw(ptr: *const T) -> Self {

        let refcount_layout = Layout::new::<bindings::refcount_t>();

        // SAFETY: The caller guarantees that the pointer is valid.

        let val_layout = Layout::for_value(unsafe { &*ptr });

        // SAFETY: We're computing the layout of a real struct that existed when compiling this

        // 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.

        //

        // 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) };

        // 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);

        // 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)) }

    }

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

    ///

    /// This is useful when the argument of a function call is an [`ArcBorrow`] (e.g., in a method