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

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