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

[Gary Guo <gary@garyguo.net>]

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.