linux-kernel - Re: [PATCH v2 2/3] rust: macros: allow generic parameter default values in `#[pin

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Date:   Thu, 14 Dec 2023 00:11:19 -0300
From:   Martin Rodriguez Reboredo <yakoyoku@...il.com>
To:     Benno Lossin <benno.lossin@...ton.me>,
        Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...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>
Cc:     rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] rust: macros: allow generic parameter default
 values in `#[pin_data]`

On 12/13/23 19:08, Benno Lossin wrote:
> Add support for generic parameters defaults in `#[pin_data]` by using
> the newly introduced `decl_generics` instead of the `impl_generics`.
> 
> Before this would not compile:
> 
>      #[pin_data]
>      struct Foo<const N: usize = 0> {
>          // ...
>      }
> 
> because it would be expanded to this:
> 
>      struct Foo<const N: usize = 0> {
>          // ...
>      }
> 
>      const _: () = {
>          struct __ThePinData<const N: usize = 0> {
>              __phantom: ::core::marker::PhantomData<fn(Foo<N>) -> Foo<N>>,
>          }
>          impl<const N: usize = 0> ::core::clone::Clone for __ThePinData<N> {
>              fn clone(&self) -> Self {
>                  *self
>              }
>          }
> 
>          // [...] rest of expansion omitted
>      };
> 
> The problem is with the `impl<const N: usize = 0>`, since that is
> invalid Rust syntax. It should not mention the default value at all,
> since default values only make sense on type definitions.
> 
> The new `impl_generics` do not contain the default values, thus
> generating correct Rust code.
> 
> This is used by the next commit that puts `#[pin_data]` on
> `kernel::workqueue::Work`.
> 
> Signed-off-by: Benno Lossin <benno.lossin@...ton.me>
> ---
> [...]
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@...il.com>