Re: [PATCH 7/9] rust: list: add cursor

[Alice Ryhl]

On Thu, Apr 4, 2024 at 3:28 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
>
> On 03.04.24 14:49, Alice Ryhl wrote:
> > On Wed, Apr 3, 2024 at 2:19 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
> >> On 02.04.24 14:17, Alice Ryhl wrote:
> >>> +impl<'a, T: ?Sized + ListItem<ID>, const ID: u64> Cursor<'a, T, ID> {
> >>> +    /// Access the current element of this cursor.
> >>> +    pub fn current(&self) -> ArcBorrow<'_, T> {
> >>> +        // SAFETY: The `current` pointer points a value in the list.
> >>> +        let me = unsafe { T::view_value(ListLinks::from_fields(self.current)) };
> >>> +        // SAFETY:
> >>> +        // * All values in a list are stored in an `Arc`.
> >>> +        // * The value cannot be removed from the list for the duration of the lifetime annotated
> >>> +        //   on the returned `ArcBorrow`, because removing it from the list would require mutable
> >>> +        //   access to the cursor or the list. However, the `ArcBorrow` holds an immutable borrow
> >>> +        //   on the cursor, which in turn holds an immutable borrow on the list, so any such
> >>
> >> The cursor has a mutable borrow on the list.
> >>
> >>
> >>> +        //   mutable access requires first releasing the immutable borrow on the cursor.
> >>> +        // * Values in a list never have a `UniqueArc` reference.
> >>
> >> Is there some type invariant guaranteeing this?
> >
> > The List owns a ListArc reference to the value. It would be unsound
> > for there to also be a UniqueArc reference to it.
>
> I think it would be good to add the existence of the `ListArc` as an
> explanation.

Will do. I'll do the other suggestions as well.

Alice