rust: add ARef::into_raw

Add a method for `ARef` that is analogous to `Arc::into_raw`. It is the
inverse operation of `ARef::from_raw`, and allows you to convert the
`ARef` back into a raw pointer while retaining ownership of the
refcount.

This new function will be used by [1] for converting the type in an
`ARef` using `ARef::from_raw(ARef::into_raw(me).cast())`. The author has
also needed the same function for other use-cases in the past, but [1]
is the first to go upstream.

This was implemented independently by Kartik and Alice. The two versions
were merged by Alice, so all mistakes are Alice's.

Link: https://lore.kernel.org/r/20240801-vma-v3-1-db6c1c0afda9@google.com [1]
Closes: Rust-for-Linux#1044
Signed-off-by: Kartik Prajapati <kartikprajapati987@gmail.com>
Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

rust/kernel/types.rs

@@ -7,7 +7,7 @@ use alloc::boxed::Box;
 use core::{
     cell::UnsafeCell,
     marker::{PhantomData, PhantomPinned},
-    mem::MaybeUninit,
+    mem::{ManuallyDrop, MaybeUninit},
     ops::{Deref, DerefMut},
     pin::Pin,
     ptr::NonNull,
@@ -396,6 +396,35 @@ impl<T: AlwaysRefCounted> ARef<T> {
             _p: PhantomData,
         }
     }
+
+    /// Consumes the `ARef`, returning a raw pointer.
+    ///
+    /// This function does not change the refcount. After calling this function, the caller is
+    /// responsible for the refcount previously managed by the `ARef`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use core::ptr::NonNull;
+    /// use kernel::types::{ARef, AlwaysRefCounted};
+    ///
+    /// struct Empty {}
+    ///
+    /// unsafe impl AlwaysRefCounted for Empty {
+    ///     fn inc_ref(&self) {}
+    ///     unsafe fn dec_ref(_obj: NonNull<Self>) {}
+    /// }
+    ///
+    /// let mut data = Empty {};
+    /// let ptr = NonNull::<Empty>::new(&mut data as *mut _).unwrap();
+    /// let data_ref: ARef<Empty> = unsafe { ARef::from_raw(ptr) };
+    /// let raw_ptr: NonNull<Empty> = ARef::into_raw(data_ref);
+    ///
+    /// assert_eq!(ptr, raw_ptr);
+    /// ```
+    pub fn into_raw(me: Self) -> NonNull<T> {
+        ManuallyDrop::new(me).ptr
+    }
 }
 
 impl<T: AlwaysRefCounted> Clone for ARef<T> {