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From: Alice Ryhl <aliceryhl@google.com>
Date: Thu, 4 Apr 2024 16:41:53 +0200
Message-ID: <CAH5fLggS3qtCy20Y=kuxULUY9Q_4dSzJN5n2WitU362cYuqPLA@mail.gmail.com>
Subject: Re: [PATCH 6/9] rust: list: add iterators
To: Benno Lossin <benno.lossin@proton.me>
Cc: Miguel Ojeda <ojeda@kernel.org>, Andrew Morton <akpm@linux-foundation.org>, 
	Alex Gaynor <alex.gaynor@gmail.com>, Wedson Almeida Filho <wedsonaf@gmail.com>, 
	Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>, 
	=?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, 
	Andreas Hindborg <a.hindborg@samsung.com>, Marco Elver <elver@google.com>, 
	Kees Cook <keescook@chromium.org>, Coly Li <colyli@suse.de>, Paolo Abeni <pabeni@redhat.com>, 
	Pierre Gondois <pierre.gondois@arm.com>, Ingo Molnar <mingo@kernel.org>, 
	Jakub Kicinski <kuba@kernel.org>, Wei Yang <richard.weiyang@gmail.com>, 
	Matthew Wilcox <willy@infradead.org>, linux-kernel@vger.kernel.org, 
	rust-for-linux@vger.kernel.org
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On Thu, Apr 4, 2024 at 4:36=E2=80=AFPM Benno Lossin <benno.lossin@proton.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 =3D 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 =3D ArcBorrow<'a, T>;
> > +
> > +    fn next(&mut self) -> Option<ArcBorrow<'a, T>> {
> > +        if self.current.is_null() {
> > +            return None;
> > +        }
> > +
> > +        let current =3D self.current;
> > +
> > +        // SAFETY: We just checked that `current` is not null, so it i=
s in a list, and hence not
> > +        // dangling. There's no race because the iterator holds an imm=
utable 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.

Alice