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

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Message-ID: <D8PPIYIJCNX8.13VPQULEI0ALN@proton.me>
Date: Tue, 25 Mar 2025 22:40:33 +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 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 :)

---
Cheers,
Benno