linux-kernel - Re: [PATCH v2 4/5] rust: types: implement `ForeignOwnable` for the unit type

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Date:   Wed, 01 Feb 2023 10:58:27 +0100
From:   Andreas Hindborg <nmi@...aspace.dk>
To:     Wedson Almeida Filho <wedsonaf@...il.com>
Cc:     rust-for-linux@...r.kernel.org, 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>,
        linux-kernel@...r.kernel.org,
        Vincenzo Palazzo <vincenzopalazzodev@...il.com>,
        Andreas Hindborg <a.hindborg@...sung.com>
Subject: Re: [PATCH v2 4/5] rust: types: implement `ForeignOwnable` for the
 unit type


Wedson Almeida Filho <wedsonaf@...il.com> writes:

> 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@...il.com>
> Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@...il.com>
> ---

Reviewed-by: Andreas Hindborg <a.hindborg@...sung.com>

> v1 -> v2: use `NonNull::dangling` to generate pointer
>
>  rust/kernel/types.rs | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 411655eca3e9..9d0fdbc55843 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -85,6 +85,18 @@ impl<T: 'static> ForeignOwnable for Box<T> {
>      }
>  }
>  
> +impl ForeignOwnable for () {
> +    type Borrowed<'a> = ();
> +
> +    fn into_foreign(self) -> *const core::ffi::c_void {
> +        core::ptr::NonNull::dangling().as_ptr()
> +    }
> +
> +    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.