Re: [PATCH v7 03/16] rust: implement `IdArray`, `IdTable` and `RawDeviceId`

[Danilo Krummrich]

On Sat, Mar 15, 2025 at 12:52:27PM -0400, Tamir Duberstein wrote:
> On Thu, Dec 19, 2024 at 12:08 PM Danilo Krummrich <dakr@xxxxxxxxxx> wrote:
> >
> > +/// Marker trait to indicate a Rust device ID type represents a corresponding C device ID type.
> > +///
> > +/// This is meant to be implemented by buses/subsystems so that they can use [`IdTable`] to
> > +/// guarantee (at compile-time) zero-termination of device id tables provided by drivers.
> > +///
> > +/// # Safety
> > +///
> > +/// Implementers must ensure that:
> > +///   - `Self` is layout-compatible with [`RawDeviceId::RawType`]; i.e. it's safe to transmute to
> > +///     `RawDeviceId`.
> > +///
> > +///     This requirement is needed so `IdArray::new` can convert `Self` to `RawType` when building
> > +///     the ID table.
> > +///
> > +///     Ideally, this should be achieved using a const function that does conversion instead of
> > +///     transmute; however, const trait functions relies on `const_trait_impl` unstable feature,
> > +///     which is broken/gone in Rust 1.73.
> > +///
> > +///   - `DRIVER_DATA_OFFSET` is the offset of context/data field of the device ID (usually named
> > +///     `driver_data`) of the device ID, the field is suitable sized to write a `usize` value.
> > +///
> > +///     Similar to the previous requirement, the data should ideally be added during `Self` to
> > +///     `RawType` conversion, but there's currently no way to do it when using traits in const.
> > +pub unsafe trait RawDeviceId {
> > +    /// The raw type that holds the device id.
> > +    ///
> > +    /// Id tables created from [`Self`] are going to hold this type in its zero-terminated array.
> > +    type RawType: Copy;
> > +
> > +    /// The offset to the context/data field.
> > +    const DRIVER_DATA_OFFSET: usize;
> > +
> > +    /// The index stored at `DRIVER_DATA_OFFSET` of the implementor of the [`RawDeviceId`] trait.
> > +    fn index(&self) -> usize;
> > +}
> 
> Very late to the game here, but have a question about the use of
> OFFSET here. Why is this preferred to a method that returns a pointer
> to the field?

We need it from const context, trait methods can't be const (yet).