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

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Message-ID: <D8Q4MSXXZ7OI.1NC226MO02VSN@proton.me>
Date: Wed, 26 Mar 2025 10:30:56 +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 12:54 AM CET, Tamir Duberstein wrote:
> On Tue, Mar 25, 2025 at 6:40 PM Benno Lossin <benno.lossin@...ton.me> wrote:
>> On Tue Mar 25, 2025 at 11:33 PM CET, Tamir Duberstein wrote:
>> > On Tue, Mar 25, 2025 at 6:11 PM Benno Lossin <benno.lossin@...ton.me> wrote:
>> >> On Tue Mar 25, 2025 at 9:07 PM CET, Tamir Duberstein wrote:
>> >> > diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
>> >> > index 40034f77fc2f..6233af50bab7 100644
>> >> > --- a/rust/kernel/str.rs
>> >> > +++ b/rust/kernel/str.rs
>> >> > @@ -29,7 +29,7 @@ pub const fn is_empty(&self) -> bool {
>> >> >      #[inline]
>> >> >      pub const fn from_bytes(bytes: &[u8]) -> &Self {
>> >> >          // SAFETY: `BStr` is transparent to `[u8]`.
>> >> > -        unsafe { &*(bytes as *const [u8] as *const BStr) }
>> >> > +        unsafe { &*(core::mem::transmute::<*const [u8], *const Self>(bytes)) }
>> >>
>> >> Hmm I'm not sure about using `transmute` here. Yes the types are
>> >> transparent, but I don't think that we should use it here.
>> >
>> > What's your suggestion? I initially tried
>> >
>> > let bytes: *const [u8] = bytes;
>> > unsafe { &*bytes.cast() }
>> >
>> > but that doesn't compile because of the implicit Sized bound on pointer::cast.
>>
>> This is AFAIK one of the only places where we cannot get rid of the `as`
>> cast. So:
>>
>>     let bytes: *const [u8] = bytes;
>>     // CAST: `BStr` transparently wraps `[u8]`.
>>     let bytes = bytes as *const BStr;
>>     // SAFETY: `bytes` is derived from a reference.
>>     unsafe { &*bytes }
>>
>> IMO a `transmute` is worse than an `as` cast :)
>
> Hmm, looking at this again we can just transmute ref-to-ref and avoid
> pointers entirely. We're already doing that in
> `CStr::from_bytes_with_nul_unchecked`
>
> Why is transmute worse than an `as` cast?

It's right in the docs: "`transmute` should be the absolute last
resort." [1]. IIRC, Gary was a bit more lenient in its use, but I think
we should avoid it as much as possible such that people copying code or
taking inspiration also don't use it.

So for both cases I'd prefer an `as` cast.

[1]: https://doc.rust-lang.org/std/mem/fn.transmute.html

---
Cheers,
Benno