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From: Alice Ryhl <aliceryhl@google.com>
Date: Thu, 8 Feb 2024 15:31:15 +0100
Message-ID: <CAH5fLgg07Fx0FrtK2nsEj=DR96WuhqVrWyJ-bBhXqFr9Caz6FA@mail.gmail.com>
Subject: Re: [PATCH] rust: stop using ptr_metadata feature
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>, =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, 
	Benno Lossin <benno.lossin@proton.me>, rust-for-linux@vger.kernel.org, 
	linux-kernel@vger.kernel.org
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On Mon, Feb 5, 2024 at 10:02=E2=80=AFPM Trevor Gross <tmgross@umich.edu> wr=
ote:
>
> On Mon, Feb 5, 2024 at 3:19=E2=80=AFAM Alice Ryhl <aliceryhl@google.com> =
wrote:
> >          // SAFETY: The metadata of `T` and `ArcInner<T>` is the same b=
ecause `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 =3D
> > -            unsafe { core::mem::transmute_copy(&metadata) };
> > +        let ptr =3D 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