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

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Date:   Mon, 30 Jan 2023 02:55:31 -0300
From:   Wedson Almeida Filho <wedsonaf@...il.com>
To:     Gary Guo <gary@...yguo.net>
Cc:     rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] rust: types: implement `ForeignOwnable` for the unit type

On Fri, 27 Jan 2023 at 11:03, Gary Guo <gary@...yguo.net> wrote:
>
> On Thu, 19 Jan 2023 14:40:35 -0300
> Wedson Almeida Filho <wedsonaf@...il.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@...il.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 the wrong type, but I agree with the spirit of the
suggestion. I'll add it to V2.

> 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.
>