Re: [PATCH 1/6] rust: types: Add Ownable/Owned types

[Alice Ryhl <aliceryhl@google.com>]

On Sun, Feb 2, 2025 at 2:06 PM Asahi Lina <lina@asahilina.net> wrote:
>
> By analogy to AlwaysRefCounted and ARef, an Ownable type is a (typically
> C FFI) type that *may* be owned by Rust, but need not be. Unlike
> AlwaysRefCounted, this mechanism expects the reference to be unique
> within Rust, and does not allow cloning.
>
> Conceptually, this is similar to a KBox<T>, except that it delegates
> resource management to the T instead of using a generic allocator.
>
> Signed-off-by: Asahi Lina <lina@asahilina.net>

Overall LGTM.

> +/// A subtrait of Ownable that asserts that an `Owned<T>` Rust reference is not only unique
> +/// within Rust and keeps the `T` alive, but also guarantees that the C code follows the
> +/// usual mutable reference requirements. That is, the kernel will never mutate the
> +/// `T` (excluding internal mutability that follows the usual rules) while Rust owns it.
> +///
> +/// When this type is implemented for an [`Ownable`] type, it allows `Owned<T>` to be
> +/// dereferenced into a &mut T.
> +///
> +/// # Safety
> +///
> +/// Implementers must ensure that the kernel never mutates the underlying type while
> +/// Rust owns it.
> +pub unsafe trait OwnableMut: Ownable {}

Giving out mutable references allows users to call core::mem::swap on
the object. We must require that this is allowed.

> +impl<T: Ownable> Owned<T> {
> +    /// Creates a new instance of [`Owned`].
> +    ///
> +    /// It takes over ownership of the underlying object.
> +    ///
> +    /// # Safety
> +    ///
> +    /// Callers must ensure that the underlying object is acquired and can be considered owned by
> +    /// Rust.
> +    pub(crate) unsafe fn from_raw(ptr: NonNull<T>) -> Self {
> +        // INVARIANT: The safety requirements guarantee that the new instance now owns the
> +        // reference.
> +        Self {
> +            ptr,
> +            _p: PhantomData,
> +        }
> +    }
> +
> +    /// Consumes the `Owned`, returning a raw pointer.
> +    ///
> +    /// This function does not actually relinquish ownership of the object.
> +    /// After calling this function, the caller is responsible for ownership previously managed
> +    /// by the `Owned`.
> +    #[allow(dead_code)]
> +    pub(crate) fn into_raw(me: Self) -> NonNull<T> {

I would just make these methods public, like the ARef ones. Then you
can drop the #[allow(dead_code)] annotation.

Alice