linux-kernel - Re: [PATCH] rust: make `UnsafeCell` the outer type in `Opaque`

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Message-ID: <-MHKDYSM29ri6c9QKvSWSeFjWwM8yj6uNgAZ-2hMzY_vEokxnrcETWGTZtcNCmgmMwH4VLswDOtRWhjfVboo360PSTXxYjhiffD5B7CkkH0=@proton.me>
Date:   Wed, 14 Jun 2023 19:04:52 +0000
From:   Benno Lossin <benno.lossin@...ton.me>
To:     Gary Guo <gary@...yguo.net>
Cc:     Alice Ryhl <aliceryhl@...gle.com>, rust-for-linux@...r.kernel.org,
        Miguel Ojeda <ojeda@...nel.org>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH] rust: make `UnsafeCell` the outer type in `Opaque`

> > /// Gets the value behind `this`.
> > @@ -266,7 +266,7 @@ pub fn get(&self) -> *mut T {
> > /// This function is useful to get access to the value without creating intermediate
> > /// references.
> > pub const fn raw_get(this: *const Self) -> *mut T {
> > - UnsafeCell::raw_get(this.cast::<UnsafeCell<T>>())
> > + UnsafeCell::raw_get(this.cast::<UnsafeCell<MaybeUninit<T>>>()).cast::<T>()
> 
> 
> This can just be `this.cast_mut().cast()` since all types involved are
> transparent.

I would advise against that, see [1]. It would be bad for people to
assume that one is always allowed to do that. I also like it explicit here.

[1]: https://github.com/rust-lang/unsafe-code-guidelines/issues/281

--
Cheers,
Benno