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

[Benno Lossin]

On 10/27/23 10:02, Finn Behrens wrote:
> 
> 
> 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@xxxxxxxxx>
>> ---
>> 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.

I brought this up in the discussion on zulip [1]. But Wedson argued that
macros make it more magical and less easy to understand. So for the time
being, I just wanted to improve the docs.

[1]: https://rust-for-linux.zulipchat.com/#narrow/stream/288089-General/topic/.60bool.3A.3Athen.60.20helper/near/395659471

-- 
Cheers,
Benno