Re: [PATCH v4 03/10] rust: sync: atomic: Add ordering annotation types

[Boqun Feng <boqun.feng@gmail.com>]

On Tue, Jun 10, 2025 at 10:30:55AM -0700, Boqun Feng wrote:
[...]
> > > +/// Describes the exact memory ordering of an `impl` [`All`].
> > > +pub enum OrderingDesc {
> > 
> > Why not name this `Ordering`?
> > 
> 
> I was trying to avoid having an `Ordering` enum in a `ordering` mod.
> Also I want to save the name "Ordering" for the generic type parameter
> of an atomic operation, e.g.
> 
>     pub fn xchg<Ordering: ALL>(..)
> 
> this enum is more of an internal implementation detail, and users should
> not use this enum directly, so I would like to avoid potential
> confusion.
> 
> I have played a few sealed trait tricks on my end, but seems I cannot
> achieve:
> 
> 1) `OrderingDesc` is only accessible in the atomic mod.
> 2) `All` is only impl-able in the atomic mod, while it can be used as a
> trait bound outside kernel crate.
> 
> Maybe there is a trick I'm missing?
> 

Something like this seems to work:

    pub(super) mod private {
        /// Describes the exact memory ordering of an `impl` [`All`].
        pub enum Ordering {
            /// Relaxed ordering.
            Relaxed,
            /// Acquire ordering.
            Acquire,
            /// Release ordering.
            Release,
            /// Fully-ordered.
            Full,
        }
    
        pub trait HasOrderingDesc {
            /// Describes the exact memory ordering.
            const ORDERING: Ordering;
        }
    }

    /// The trait bound for annotating operations that should support all orderings.
    pub trait All: private::HasOrderingDesc { }

    impl private::HasOrderingDesc for Relaxed {
        const ORDERING: private::Ordering = private::Ordering::Relaxed;
    }

the trick is to seal the enum and the trait together.

Regards,
Boqun

> > > +    /// Relaxed ordering.
> > > +    Relaxed,
> > > +    /// Acquire ordering.
> > > +    Acquire,
> > > +    /// Release ordering.
> > > +    Release,
> > > +    /// Fully-ordered.
> > > +    Full,
> > > +}
> > > +
> > > +/// The trait bound for annotating operations that should support all orderings.
> > > +pub trait All {
> > > +    /// Describes the exact memory ordering.
> > > +    const ORDER: OrderingDesc;
> > 
> > And then here: `ORDERING`.
> 
[..]