Re: [PATCH v3 04/10] rust: list: add struct with prev/next pointers

[Benno Lossin <benno.lossin@proton.me>]

On 23.07.24 10:22, Alice Ryhl wrote:
> +/// The prev/next pointers for an item in a linked list.
> +///
> +/// # Invariants
> +///
> +/// The fields are null if and only if this item is not in a list.
> +#[repr(transparent)]
> +pub struct ListLinks<const ID: u64 = 0> {
> +    #[allow(dead_code)]
> +    inner: Opaque<ListLinksFields>,

Do you really need `Opaque`? Or would `UnsafeCell` be enough? (If it is
enough and you change this, be aware that `Opaque` is `!Unpin`, so if
you intend for `ListLinks` to also be `!Unpin`, then you need a
`PhantomPinned`)

> +}
> +
> +// SAFETY: The next/prev fields of a ListLinks can be moved across thread boundaries.

Why? This is not a justification.

> +unsafe impl<const ID: u64> Send for ListLinks<ID> {}
> +// SAFETY: The type is opaque so immutable references to a ListLinks are useless. Therefore, it's
> +// okay to have immutable access to a ListLinks from several threads at once.

You don't need to argue via `Opaque`, the type doesn't expose any
`&self` functions, so there are no functions to consider.

---
Cheers,
Benno

> +unsafe impl<const ID: u64> Sync for ListLinks<ID> {}
> +
> +impl<const ID: u64> ListLinks<ID> {
> +    /// Creates a new initializer for this type.
> +    pub fn new() -> impl PinInit<Self> {
> +        // INVARIANT: Pin-init initializers can't be used on an existing `Arc`, so this value will
> +        // not be constructed in an `Arc` that already has a `ListArc`.
> +        ListLinks {
> +            inner: Opaque::new(ListLinksFields {
> +                prev: ptr::null_mut(),
> +                next: ptr::null_mut(),
> +            }),
> +        }
> +    }
> +}
> 
> --
> 2.45.2.1089.g2a221341d9-goog
>