Date:   Fri, 27 Jan 2023 14:03:19 +0000
From:   Gary Guo <gary@garyguo.net>
To:     Wedson Almeida Filho <wedsonaf@gmail.com>
Cc:     rust-for-linux@vger.kernel.org, Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        =?UTF-8?B?QmrDtnJu?= Roy Baron <bjorn3_gh@protonmail.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/5] rust: types: implement `ForeignOwnable` for the
 unit type
Message-ID: <20230127140319.22af6060.gary@garyguo.net>
In-Reply-To: <20230119174036.64046-4-wedsonaf@gmail.com>
References: <20230119174036.64046-1-wedsonaf@gmail.com>
        <20230119174036.64046-4-wedsonaf@gmail.com>
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On Thu, 19 Jan 2023 14:40:35 -0300
Wedson Almeida Filho <wedsonaf@gmail.com> wrote:

> This allows us to use the unit type `()` when we have no object whose
> ownership must be managed but one implementing the `ForeignOwnable`
> trait is needed.
> 
> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> ---
>  rust/kernel/types.rs | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index e037c262f23e..8f80cffbff59 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -85,6 +85,19 @@ impl<T: 'static> ForeignOwnable for Box<T> {
>      }
>  }
>  
> +impl ForeignOwnable for () {
> +    type Borrowed<'a> = ();
> +
> +    fn into_foreign(self) -> *const core::ffi::c_void {
> +        // We use 1 to be different from a null pointer.
> +        1usize as _

this should really be `core::ptr::invalid(1)`. That's currently
unstable, but can be equivalently written as
`NonNull::<()>::dangling().as_ptr()`.

This has a different semantic meaning from `as` since it explicitly
suggests an invalid provenance and thus will not alias with other
pointers. (Although I don't think compiler currently can take advantage
of this fact yet)

> +    }
> +
> +    unsafe fn borrow<'a>(_: *const core::ffi::c_void) -> Self::Borrowed<'a> {}
> +
> +    unsafe fn from_foreign(_: *const core::ffi::c_void) -> Self {}
> +}
> +
>  /// Runs a cleanup function/closure when dropped.
>  ///
>  /// The [`ScopeGuard::dismiss`] function prevents the cleanup function from running.