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Date:   Tue, 24 Oct 2023 14:43:30 +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 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`.

> 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()`.

> Actually, the string here provides little use other than documentation,

Sure, but that is the whole purpose of this patch.

> since the string provided to build_error is only visible in const eval,
> so this you might just omit that and write
> 
> kernel::build_error!()

Note that it is also useful for people who read the code, as they
can search for the constant and understand why it is a build error.

-- 
Cheers,
Benno


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