linux-kernel - Re: [PATCH v2] rust: macros: improve `#[vtable]` documentation

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Date:   Wed, 25 Oct 2023 21:34:02 +0000
From:   Benno Lossin <benno.lossin@...ton.me>
To:     Gary Guo <gary@...yguo.net>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Alice Ryhl <aliceryhl@...gle.com>,
        Andreas Hindborg <nmi@...aspace.dk>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rust: macros: improve `#[vtable]` documentation

On 25.10.23 21:14, Gary Guo wrote:
> On Tue, 24 Oct 2023 14:43:30 +0000
> Benno Lossin <benno.lossin@...ton.me> wrote:
> 
>> On 24.10.23 13:24, Gary Guo wrote:
>>> On Thu, 19 Oct 2023 17:15:53 +0000
>>> Benno Lossin <benno.lossin@...ton.me> wrote:
>>
>> [...]
>>
>>>> -/// 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 function to be optional, it must have a default implementation. But this default
>>>> +/// implementation will never be executed, since these functions are exclusively called from
>>>> +/// callbacks from the C side. This is because the vtable will have a `NULL` entry and the C side
>>>> +/// will execute the default behavior. Since it is not maintainable to replicate the default
>>>> +/// behavior in Rust, the default implementation should be:
>>>> +///
>>>> +/// ```compile_fail
>>>> +/// # use kernel::error::VTABLE_DEFAULT_ERROR;
>>>> +/// kernel::build_error(VTABLE_DEFAULT_ERROR)
>>>
>>> Note that `build_error` function is considered impl detail and is
>>> hidden.
>>
>> I see, we should mention that in the docs on `build_error`.
> 
> Well, it's marked as `#[doc(hidden)]`...

Yes, but I did not even build the docs, but read it directly
inside of the `build_error` crate and thus I did not see the
`#[doc(hidden)]`.

>>> This should use the macro version instead:
>>>
>>> kernel::build_error!(VTABLE_DEFAULT_ERROR)
>>
>> Is there a reason that it is a macro? Why is it re-exported in the
>> kernel crate? The macro could just use `::bulid_error::build_error()`.
> 
> The original intention is to allow format strings, but Rust const
> panic is not powerful enough to support it at the moment. Macro
> allows higher flexibility.

That is what I thought. But should we then not always require a
string literal? So

     kernel::build_error!("{}", VTABLE_DEFAULT_ERROR)

> It's re-exported so the macro can reference them (note that downstream
> crates can't reference build_error directly). Perhaps I should put it
> inside __private_reexport or something instead.

I see, I did not know that they cannot reference build error directly.
Is that some limitation of the build system? If it is possible to not
re-export it, I think that would be better, otherwise just put it in
`__private`.

-- 
Cheers,
Benno