From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-4322.protonmail.ch (mail-4322.protonmail.ch [185.70.43.22]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0F2531ACEAF; Wed, 26 Mar 2025 10:31:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=185.70.43.22 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1742985069; cv=none; b=OyoKMIq4yuQ4n1TqiucWr0O/RuBdnFqIXQwgi6UAbFdEE17lWZekzfU3Yxx94EscrLm/BVrzjHlH2yRKriTh/2XmOwCfo+/dDVZSSHsBIAyQmo6hRcHHDeOa7F74+6/YGa+E5tBm87q5WiT2nRBOtbeSRnbNXr29aXAb2AO71wM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1742985069; c=relaxed/simple; bh=pSoI1WGaycGSmBtCOUNQSjGw+nskVB7jyyqdbGzX9pE=; h=Date:To:From:Cc:Subject:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=Ncvpb7w7tbUILWIH+762f9hHpsIRPMeKu4tBtTYdkWEhGHNSKL9ULCDREtHtfJUpCSscC6GBi1QXuydvV9bkBTtu93ZLO6jhWA9kmhKCL4PfhvXlpVU9H5f19vEY5CQfKT6j0VSJoTiobUv9AW/HCTyFbGd86UlUwefu/Nlow5A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=proton.me; spf=pass smtp.mailfrom=proton.me; dkim=pass (2048-bit key) header.d=proton.me header.i=@proton.me header.b=ViqpvZj2; arc=none smtp.client-ip=185.70.43.22 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=proton.me Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=proton.me Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=proton.me header.i=@proton.me header.b="ViqpvZj2" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=proton.me; s=protonmail; t=1742985064; x=1743244264; bh=PD6QsKlbd/UTEIZFjdWL2INF7cdUTBh6ExeiHs8aHeE=; h=Date:To:From:Cc:Subject:Message-ID:In-Reply-To:References: Feedback-ID:From:To:Cc:Date:Subject:Reply-To:Feedback-ID: Message-ID:BIMI-Selector:List-Unsubscribe:List-Unsubscribe-Post; b=ViqpvZj2DYShZqRsjkXRHya7dm+a8V1pJaSqmByjF0GxUAhLHBw9zK8OGj2tJfKYn W22bKXuOek30uCw7TONhJ+SsJ5O2t36d68I0KUmw1yXcEV0uuTtKPD0yOVbeuOSTeB eGeVXA9Js4rg17xXT76DaF91riSkaGXNVfpX6+zdJxwt16NqaSXUZGFh6s1LLN5KEU gflg5QvG69rpxM1yFrnnlcWKeat+CI7hMkQ/PiLfWdwH2ouVQcYV6hfwWt4oXPjVxq OP5L7EM+1YJYcwmU6Bqj6wY6i2ytPo1CNgJx1tbMtBTTsAOpafptEC3pI24CS7+euR W1EGIj+3YmK+Q== Date: Wed, 26 Mar 2025 10:30:56 +0000 To: Tamir Duberstein From: Benno Lossin Cc: Masahiro Yamada , Nathan Chancellor , Nicolas Schier , Miguel Ojeda , Alex Gaynor , Boqun Feng , Gary Guo , =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= , Andreas Hindborg , Alice Ryhl , Trevor Gross , Danilo Krummrich , Greg Kroah-Hartman , "Rafael J. Wysocki" , Brendan Higgins , David Gow , Rae Moar , Bjorn Helgaas , Luis Chamberlain , Russ Weight , Rob Herring , Saravana Kannan , Abdiel Janulgue , Daniel Almeida , Robin Murphy , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , FUJITA Tomonori , linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, linux-kselftest@vger.kernel.org, kunit-dev@googlegroups.com, linux-pci@vger.kernel.org, linux-block@vger.kernel.org, devicetree@vger.kernel.org, dri-devel@lists.freedesktop.org, netdev@vger.kernel.org Subject: Re: [PATCH v7 7/7] rust: enable `clippy::ref_as_ptr` lint Message-ID: In-Reply-To: References: <20250325-ptr-as-ptr-v7-0-87ab452147b9@gmail.com> <20250325-ptr-as-ptr-v7-7-87ab452147b9@gmail.com> Feedback-ID: 71780778:user:proton X-Pm-Message-ID: e50c9b550854ec00eb95a773d66c999d39fa5c32 Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable On Wed Mar 26, 2025 at 12:54 AM CET, Tamir Duberstein wrote: > On Tue, Mar 25, 2025 at 6:40=E2=80=AFPM Benno Lossin wrote: >> On Tue Mar 25, 2025 at 11:33 PM CET, Tamir Duberstein wrote: >> > On Tue, Mar 25, 2025 at 6:11=E2=80=AFPM Benno Lossin 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 Sel= f>(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] =3D bytes; >> > unsafe { &*bytes.cast() } >> > >> > but that doesn't compile because of the implicit Sized bound on pointe= r::cast. >> >> This is AFAIK one of the only places where we cannot get rid of the `as` >> cast. So: >> >> let bytes: *const [u8] =3D bytes; >> // CAST: `BStr` transparently wraps `[u8]`. >> let bytes =3D 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