linux-kernel - Re: [PATCH v7 7/7] rust: enable `clippy::ref_as

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Message-ID: <D8QJMH5UR6VG.2OT5MXJJQU5QT@proton.me>
Date: Wed, 26 Mar 2025 22:15:43 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Tamir Duberstein <tamird@...il.com>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nicolas@...sle.eu>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rafael@...nel.org>, Brendan Higgins <brendan.higgins@...ux.dev>, David Gow <davidgow@...gle.com>, Rae Moar <rmoar@...gle.com>, Bjorn Helgaas <bhelgaas@...gle.com>, Luis Chamberlain <mcgrof@...nel.org>, Russ Weight <russ.weight@...ux.dev>, Rob Herring <robh@...nel.org>, Saravana Kannan <saravanak@...gle.com>, Abdiel Janulgue <abdiel.janulgue@...il.com>, Daniel Almeida <daniel.almeida@...labora.com>, Robin Murphy <robin.murphy@....com>, Maarten Lankhorst
	<maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, FUJITA Tomonori <fujita.tomonori@...il.com>, linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, linux-kselftest@...r.kernel.org, kunit-dev@...glegroups.com, linux-pci@...r.kernel.org, linux-block@...r.kernel.org, devicetree@...r.kernel.org, dri-devel@...ts.freedesktop.org, netdev@...r.kernel.org
Subject: Re: [PATCH v7 7/7] rust: enable `clippy::ref_as_ptr` lint

On Wed Mar 26, 2025 at 11:09 PM CET, Tamir Duberstein wrote:
> On Wed, Mar 26, 2025 at 5:09 PM Benno Lossin <benno.lossin@...ton.me> wrote:
>> On Wed Mar 26, 2025 at 8:06 PM CET, Tamir Duberstein wrote:
>> > On Wed, Mar 26, 2025 at 1:36 PM Benno Lossin <benno.lossin@...ton.me> wrote:
>> >> On Wed Mar 26, 2025 at 5:57 PM CET, Tamir Duberstein wrote:
>> >> >
>> >> > Yeah, we should do this - but again: not relevant in this discussion.
>> >>
>> >> I think it's pretty relevant.
>> >
>> > It's not relevant because we're no longer talking about transmuting
>> > pointer to pointer. The two options are:
>> > 1. transmute reference to reference.
>> > 2. coerce reference to pointer, `as` cast pointer to pointer (triggers
>> > `ptr_as_ptr`), reborrow pointer to reference.
>> >
>> > If anyone can help me understand why (2) is better than (1), I'd
>> > certainly appreciate it.
>>
>> I am very confident that (2) is correct. With (1) I'm not sure (see
>> above), so that's why I mentioned it.
>
> Can you help me understand why you're confident about (2) but not (1)?

My explanation from above explains why I'm not confident about (1):

    For ptr-to-int transmutes, I know that they will probably remove
    provenance, hence I am a bit cautious about using them for ptr-to-ptr or
    ref-to-ref.

The reason I'm confident about (2) is that that is the canonical way to
cast the type of a reference pointing to an `!Sized` value.

---
Cheers,
Benno