linux-kernel - Re: [PATCH] rust: enable `clippy::ptr_as

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Message-ID: <CAJ-ks9kFze-neAT7mHcDWSLvV_AsAS0NGeWeoXLUkqnyEDwSag@mail.gmail.com>
Date: Fri, 7 Mar 2025 12:07:06 -0500
From: Tamir Duberstein <tamird@...il.com>
To: Benno Lossin <benno.lossin@...ton.me>
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>, 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
Subject: Re: [PATCH] rust: enable `clippy::ptr_as_ptr` lint

On Fri, Mar 7, 2025 at 11:58 AM Benno Lossin <benno.lossin@...ton.me> wrote:
>
> On Fri Mar 7, 2025 at 5:41 PM CET, Tamir Duberstein wrote:
> > In Rust 1.51.0, Clippy introduced the `ignored_unit_patterns` lint [1]:
>
> You link to the `ptr_as_ptr` lint though, is this a typo?

Oops, yep, a typo.

> >> Though `as` casts between raw pointers are not terrible,
> >> `pointer::cast` is safer because it cannot accidentally change the
> >> pointer's mutability, nor cast the pointer to other types like `usize`.
> >
> > There are a few classes of changes required:
> > - Modules generated by bindgen are marked
> >   `#[allow(clippy::ptr_as_ptr)]`.
> > - Inferred casts (` as _`) are replaced with `.cast()`.
> > - Ascribed casts (` as *... T`) are replaced with `.cast::<T>()`.
> > - Multistep casts from references (` as *const _ as *const T`) are
> >   replaced with `let x: *const _ = &x;` and `.cast()` or `.cast::<T>()`
> >   according to the previous rules. The intermediate `let` binding is
> >   required because `(x as *const _).cast::<T>()` results in inference
> >   failure.
> > - Native literal C strings are replaced with `c_str!().as_char_ptr()`.
>
> These all seem very nice, thanks! I think it would also be a good idea
> to enable `ptr_cast_constness` [1], since those are the other kind of
> `as` usage that we should be doing via `cast_mut`/`cast_const`.

There's also `as_ptr_cast_mut` which could be good to enable.

> There are still some legitimate uses of `as` casts, when unsizing
> values. I don't know if these two lints can trigger on those, it would
> be nice if they don't. So those should continue to use `as`, but other
> than that, there shouldn't be any `as` in our code :)
>
> [1]: https://rust-lang.github.io/rust-clippy/master/index.html#ptr_cast_constness
>
> (I will take a look at the patch itself a bit later)

Thanks!