Re: [PATCH v6 6/9] rust: sync: atomic: Add the framework of arithmetic operations

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

On Fri, Jul 11, 2025 at 02:22:13PM -0700, Boqun Feng wrote:
[...]
> > >     pub unsafe trait AtomicAdd<Rhs>: AllowAtomic {
> > >         type Delta = Self::Repr;
> > >         fn rhs_into_delta(rhs: Rhs) -> Delta;
> > >     }
> > >
> > > Note that I have to provide a `Delta` (or better named as `ReprDelta`?)
> > > because of when pointer support is added, atomic addition is between
> > > a `*mut ()` and a `isize`, not two `*mut()`.
> > 
> > Makes sense, but we don't have default associated types yet :(
> > 
> 
> Oops, we are lucky enough to only have a few types to begin with ;-)
> Maybe we can use `#[derive(AtomicAdd)] to select the default delta type
> later.
> 

Turn out I could give `AtomcImpl` an associate type:

    pub trait AtomicImpl: Sealed {
    	type Delta;
    }

and then

    pub unsafe trait AllowAtomicAdd<Rhs = Self>: AllowAtomic {
        /// Converts `Rhs` into the `Delta` type of the atomic implementation.
        fn rhs_into_delta(rhs: Rhs) -> <Self::Repr as AtomicImpl>::Delta;
    }

(Yeah, I named it `AllowAtomicAdd` ;-))

and I only need to provide `Delta` for three types (i32, i64 and *mut
()).

BTW, since atomic arithmetic is wrapping, do we want things like `impl
AllowAtomicAdd<u32> for i32`, similar to the concept of
`i32::wrapping_add_unsigned()` ;-)

Regards,
Boqun

> Regards,
> Boqun