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

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Message-ID: <CAH5fLgiKJpXF_jPnPj668P4DGjDwtfXs7dGV26i5kyLXCXLQ4Q@mail.gmail.com>
Date: Thu, 4 Jan 2024 16:18:38 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: 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>, Martin Rodriguez Reboredo <yakoyoku@...il.com>, 
	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 Wed, Dec 13, 2023 at 11:09 PM Benno Lossin <benno.lossin@...ton.me> 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>

Tested-by: Alice Ryhl <aliceryhl@...gle.com>
Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>