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

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Date: Sat, 23 Dec 2023 14:42:17 +0000
From: Gary Guo <gary@...yguo.net>
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>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...sung.com>,
 Alice Ryhl <aliceryhl@...gle.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, 13 Dec 2023 22:08:53 +0000
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>

Reviewed-by: Gary Guo <gary@...yguo.net>

> ---
> v1 -> v2:
> - clarify the change in the commit message
> - add motivation to the commit message
> 
>  rust/kernel/init/macros.rs | 19 ++++++++++++++++++-
>  rust/macros/pin_data.rs    |  3 ++-
>  2 files changed, 20 insertions(+), 2 deletions(-)