From: Alice Ryhl <aliceryhl@google.com>
To: Trevor Gross <tmgross@umich.edu>
Cc: "Miguel Ojeda" <ojeda@kernel.org>,
"Wedson Almeida Filho" <wedsonaf@gmail.com>,
"Alex Gaynor" <alex.gaynor@gmail.com>,
"Boqun Feng" <boqun.feng@gmail.com>,
"Gary Guo" <gary@garyguo.net>,
"Björn Roy Baron" <bjorn3_gh@protonmail.com>,
"Benno Lossin" <benno.lossin@proton.me>,
rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] rust: stop using ptr_metadata feature
Date: Thu, 8 Feb 2024 15:31:15 +0100 [thread overview]
Message-ID: <CAH5fLgg07Fx0FrtK2nsEj=DR96WuhqVrWyJ-bBhXqFr9Caz6FA@mail.gmail.com> (raw)
In-Reply-To: <CALNs47t63qAU2OpokvN6KykT0_NKpDLTBQGhLm-+zsvR1pcUAQ@mail.gmail.com>
On Mon, Feb 5, 2024 at 10:02 PM Trevor Gross <tmgross@umich.edu> wrote:
>
> On Mon, Feb 5, 2024 at 3:19 AM Alice Ryhl <aliceryhl@google.com> wrote:
> > // SAFETY: The metadata of `T` and `ArcInner<T>` is the same because `ArcInner` is a struct
> > // with `T` as its last field.
> > //
> > // This is documented at:
> > // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
>
> The comment should be reworded, no more metadata and no unsafe block
> so it doesn't have to be SAFETY.
How about this?
// Pointer casts leave the metadata unchanged. This is okay because
the metadata of `T` and
// `ArcInner<T>` is the same since `ArcInner` is a struct with `T` as
its last field.
//
// This is documented at:
// <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
> > - let metadata: <ArcInner<T> as Pointee>::Metadata =
> > - unsafe { core::mem::transmute_copy(&metadata) };
> > + let ptr = ptr as *mut ArcInner<T>;
>
> Nit: this could be `.cast::<ArcInner<T>>().cast_mut()` to make the
> intentional mutability change clear.
The `.cast()` method can't be used here. It only works for sized types.
Alice
prev parent reply other threads:[~2024-02-08 14:31 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-02-05 9:18 [PATCH] rust: stop using ptr_metadata feature Alice Ryhl
2024-02-05 12:25 ` Benno Lossin
2024-02-05 13:54 ` Martin Rodriguez Reboredo
2024-02-05 21:02 ` Trevor Gross
2024-02-08 14:31 ` Alice Ryhl [this message]
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