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

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Date:   Fri, 27 Oct 2023 09:32:19 +0000
From:   Benno Lossin <benno.lossin@...ton.me>
To:     "Ariel Miculas (amiculas)" <amiculas@...co.com>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Andreas Hindborg <a.hindborg@...sung.com>,
        Alice Ryhl <aliceryhl@...gle.com>,
        Martin Rodriguez Reboredo <yakoyoku@...il.com>,
        Asahi Lina <lina@...hilina.net>,
        Sven Van Asbroeck <thesven73@...il.com>,
        Viktor Garske <viktor@...ar.de>, Finn Behrens <me@...enk.dev>,
        "rust-for-linux@...r.kernel.org" <rust-for-linux@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] rust: macros: improve `#[vtable]` documentation

On 10/26/23 23:12, Ariel Miculas (amiculas) wrote:
> On 23/10/26 08:19PM, 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@...ton.me>
>> ---
>> v2 -> v3:
>> - don't hide the import of the constant in the example
>> - fixed "function" typo
>> - improve paragraph about optional functions
>> - do not remove the `Send + Sync + Sized` bounds on the example
>>
>> v1 -> v2:
>> - removed imperative mode in the paragraph describing optional
>>   functions.
>>
>>  rust/kernel/error.rs |  4 ++++
>>  rust/macros/lib.rs   | 37 ++++++++++++++++++++++++++++++-------
>>  2 files changed, 34 insertions(+), 7 deletions(-)
>>
>> diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
>> index 05fcab6abfe6..1373cde025ef 100644
>> --- a/rust/kernel/error.rs
>> +++ b/rust/kernel/error.rs
>> @@ -335,3 +335,7 @@ pub(crate) fn from_result<T, F>(f: F) -> T
>>          Err(e) => T::from(e.to_errno() as i16),
>>      }
>>  }
>> +
>> +/// Error message for calling a default function of a [`#[vtable]`](macros::vtable) trait.
>> +pub const VTABLE_DEFAULT_ERROR: &str =
>> +    "This function must not be called, see the #[vtable] documentation.";
>> 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.
> 
> The above paragraph seems to be wrapped at 100 characters while the
> paragraph below seems to be wrapped at 80 characters.

Oh I forgot about that. Miguel, would it be reasonable for you to fix
this when picking the patch?

-- 
Cheers,
Benno