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From: Alice Ryhl <aliceryhl@google.com>
Date: Thu, 4 Apr 2024 15:40:37 +0200
Message-ID: <CAH5fLghDBzaDf38FQtt=DROmJ7qV+cXCxDn3nAmeaZibW20Trg@mail.gmail.com>
Subject: Re: [PATCH 7/9] rust: list: add cursor
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 3:28=E2=80=AFPM Benno Lossin <benno.lossin@proton.me=
> wrote:
>
> On 03.04.24 14:49, Alice Ryhl wrote:
> > On Wed, Apr 3, 2024 at 2:19=E2=80=AFPM Benno Lossin <benno.lossin@proto=
n.me> 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 =3D unsafe { T::view_value(ListLinks::from_fields(sel=
f.current)) };
> >>> +        // SAFETY:
> >>> +        // * All values in a list are stored in an `Arc`.
> >>> +        // * The value cannot be removed from the list for the durat=
ion of the lifetime annotated
> >>> +        //   on the returned `ArcBorrow`, because removing it from t=
he list would require mutable
> >>> +        //   access to the cursor or the list. However, the `ArcBorr=
ow` 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 b=
orrow 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