Re: [PATCH v4] rust: adding UniqueRefCounted and UniqueRef types

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Mar 05, 2025 at 11:31:44AM +0000, Oliver Mangold wrote:
> Add `UniqueRef` as a variant of `ARef` that is guaranteed to be unique.
> This is useful when mutable access to the underlying type is required
> and we can guarantee uniqueness, and when APIs that would normally take
> an `ARef` require uniqueness.
> 
> Signed-off-by: Oliver Mangold <oliver.mangold@pm.me>

[...]

> +impl<T: UniqueRefCounted> Deref for UniqueRef<T> {
> +    type Target = T;
> +
> +    fn deref(&self) -> &Self::Target {
> +        // SAFETY: The type invariants guarantee that the object is valid.
> +        unsafe { self.ptr.as_ref() }
> +    }
> +}

What stops people from doing this?

let my_unique: UniqueRef<T> = ...;
let my_ref: &T = &*my_unique;
let my_shared: ARef<T> = ARef::from(my_ref);

Now it is no longer unique.

> +impl<T: UniqueRefCounted> DerefMut for UniqueRef<T> {
> +    fn deref_mut(&mut self) -> &mut Self::Target {
> +        // SAFETY: The type invariants guarantee that the object is valid.
> +        unsafe { self.ptr.as_mut() }
> +    }
> +}

This DerefMut will make it almost impossible for C types to implement
UniqueRefCounted because it is incompatible with pinning. You probably
want `T: UniqueRefCounted + Unpin` here.

For `T: !Unpin` (i.e. almost all C types), you can at most produce an
`Pin<&mut Self>`.

Alice