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References: <20231213220447.3613500-1-benno.lossin@proton.me> <20231213220447.3613500-2-benno.lossin@proton.me>
In-Reply-To: <20231213220447.3613500-2-benno.lossin@proton.me>
From: Alice Ryhl <aliceryhl@google.com>
Date: Thu, 4 Jan 2024 16:18:38 +0100
Message-ID: <CAH5fLgiKJpXF_jPnPj668P4DGjDwtfXs7dGV26i5kyLXCXLQ4Q@mail.gmail.com>
Subject: Re: [PATCH v2 2/3] rust: macros: allow generic parameter default
 values in `#[pin_data]`
To: Benno Lossin <benno.lossin@proton.me>
Cc: Miguel Ojeda <ojeda@kernel.org>, Alex Gaynor <alex.gaynor@gmail.com>, 
	Wedson Almeida Filho <wedsonaf@gmail.com>, Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>, 
	=?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, 
	Andreas Hindborg <a.hindborg@samsung.com>, Martin Rodriguez Reboredo <yakoyoku@gmail.com>, 
	rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org
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On Wed, Dec 13, 2023 at 11:09=E2=80=AFPM Benno Lossin <benno.lossin@proton.=
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 =3D 0> {
>         // ...
>     }
>
> because it would be expanded to this:
>
>     struct Foo<const N: usize =3D 0> {
>         // ...
>     }
>
>     const _: () =3D {
>         struct __ThePinData<const N: usize =3D 0> {
>             __phantom: ::core::marker::PhantomData<fn(Foo<N>) -> Foo<N>>,
>         }
>         impl<const N: usize =3D 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 =3D 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@proton.me>

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