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

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Date:   Wed, 25 Oct 2023 20:14:45 +0100
From:   Gary Guo <gary@...yguo.net>
To:     Benno Lossin <benno.lossin@...ton.me>
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 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)]`...

> 
> > 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.

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.