Re: [PATCH 6/9] rust: list: add iterators

[Alice Ryhl]

On Thu, Apr 4, 2024 at 4:52 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
>
> On 04.04.24 16:41, Alice Ryhl wrote:
> > On Thu, Apr 4, 2024 at 4:36 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
> >>
> >> On 02.04.24 14:17, Alice Ryhl wrote:
> >>> +/// the end of the list. The `stop` pointer points at the first value in the same list, or it is
> >>> +/// null if the list is empty.
> >>> +#[derive(Clone)]
> >>> +pub struct Iter<'a, T: ?Sized + ListItem<ID>, const ID: u64 = 0> {
> >>> +    current: *mut ListLinksFields,
> >>> +    stop: *mut ListLinksFields,
> >>> +    _ty: PhantomData<&'a ListArc<T, ID>>,
> >>> +}
> >>> +
> >>> +impl<'a, T: ?Sized + ListItem<ID>, const ID: u64> Iterator for Iter<'a, T, ID> {
> >>> +    type Item = ArcBorrow<'a, T>;
> >>> +
> >>> +    fn next(&mut self) -> Option<ArcBorrow<'a, T>> {
> >>> +        if self.current.is_null() {
> >>> +            return None;
> >>> +        }
> >>> +
> >>> +        let current = self.current;
> >>> +
> >>> +        // SAFETY: We just checked that `current` is not null, so it is in a list, and hence not
> >>> +        // dangling. There's no race because the iterator holds an immutable borrow to the list.
> >>
> >> This (that the iterator holds an immutable borrow) is not true (there
> >> is no `&List` field in `Iter`), but you can make that an invariant
> >> instead.
> >
> > What I mean is that the borrow-checker will consider the `List` to be
> > borrowed by `Iter`. Whether or not there is a real reference or not
> > doesn't matter.
>
> Yes, but that is implementation detail of the safe function
> `List::iter`, so I think it must also be captured by an invariant.

I will add an invariant.