Re: [PATCH v3] rust: macros: improve `#[vtable]` documentation

[Finn Behrens <me@kloenk.dev>]

On 26 Oct 2023, at 22:19, Benno Lossin wrote:

> Traits marked with `#[vtable]` need to provide default implementations
> for optional functions. The C side represents these with `NULL` in the
> vtable, so the default functions are never actually called. We do not
> want to replicate the default behavior from C in Rust, because that is
> not maintainable. Therefore we should use `build_error` in those default
> implementations. The error message for that is provided at
> `kernel::error::VTABLE_DEFAULT_ERROR`.
>
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>
> ---
> diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
> index c42105c2ff96..917a51183c23 100644
> --- a/rust/macros/lib.rs
> +++ b/rust/macros/lib.rs
> @@ -87,27 +87,48 @@ pub fn module(ts: TokenStream) -> TokenStream {
>  /// implementation could just return `Error::EINVAL`); Linux typically use C
>  /// `NULL` pointers to represent these functions.
>  ///
> -/// This attribute is intended to close the gap. Traits can be declared and
> -/// implemented with the `#[vtable]` attribute, and a `HAS_*` associated constant
> -/// will be generated for each method in the trait, indicating if the implementor
> -/// has overridden a method.
> +/// This attribute closes that gap. A trait can be annotated with the `#[vtable]` attribute.
> +/// Implementers of the trait will then also have to annotate the trait with `#[vtable]`. This
> +/// attribute generates a `HAS_*` associated constant bool for each method in the trait that is set
> +/// to true if the implementer has overridden the associated method.
> +///
> +/// For a trait method to be optional, it must have a default implementation.
> +/// This is also the case for traits annotated with `#[vtable]`, but in this
> +/// case the default implementation will never be executed. The reason for this
> +/// is that the functions will be called through function pointers installed in
> +/// C side vtables. When an optional method is not implemented on a `#[vtable]`
> +/// trait, a NULL entry is installed in the vtable. Thus the default
> +/// implementation is never called. Since these traits are not designed to be
> +/// used on the Rust side, it should not be possible to call the default
> +/// implementation. This is done to ensure that we call the vtable methods
> +/// through the C vtable, and not through the Rust vtable. Therefore, the
> +/// default implementation should call `kernel::build_error`, which prevents
> +/// calls to this function at compile time:
In the future it would be nice to have something like `#[default]` or `#[optional]` to automatically derive the implementation.
> +///
> +/// ```compile_fail
> +/// # use kernel::error::VTABLE_DEFAULT_ERROR;
> +/// kernel::build_error(VTABLE_DEFAULT_ERROR)
> +/// ```
> +///
> +/// note that you might need to import [`kernel::error::VTABLE_DEFAULT_ERROR`].
>  ///
> -/// This attribute is not needed if all methods are required.
> +/// This macro should not be used when all functions are required.
Reviewed-by: Finn Behrens <me@kloenk.dev>