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

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Message-ID: <CAH5fLgiYfU91QqCs+9L=9y76PKAYiDVva3WNoQE=Ja1ASzua7A@mail.gmail.com>
Date: Mon, 8 Apr 2024 10:00:09 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Miguel Ojeda <ojeda@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Andreas Hindborg <a.hindborg@...sung.com>, Marco Elver <elver@...gle.com>, 
	Kees Cook <keescook@...omium.org>, Coly Li <colyli@...e.de>, Paolo Abeni <pabeni@...hat.com>, 
	Pierre Gondois <pierre.gondois@....com>, Ingo Molnar <mingo@...nel.org>, 
	Jakub Kicinski <kuba@...nel.org>, Wei Yang <richard.weiyang@...il.com>, 
	Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 6/9] rust: list: add iterators

On Thu, Apr 4, 2024 at 4:52 PM Benno Lossin <benno.lossin@...ton.me> wrote:
>
> On 04.04.24 16:41, Alice Ryhl wrote:
> > On Thu, Apr 4, 2024 at 4:36 PM Benno Lossin <benno.lossin@...ton.me> 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.