Re: [PATCH v2 2/3] rust: macros: allow generic parameter default values in `#[pin_data]`

[Alice Ryhl]

On Wed, Dec 13, 2023 at 11:09 PM Benno Lossin <benno.lossin@xxxxxxxxx> 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@xxxxxxxxx>

Tested-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>