Re: [PATCH v3 1/3] rust: macros: add `decl_generics` to `parse_generics()`

[Alice Ryhl]

On Sat, Mar 9, 2024 at 4:53 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
>
> The generic parameters on a type definition can specify default values.
> Currently `parse_generics()` cannot handle this though. For example when
> parsing the following generics:
>
>     <T: Clone, const N: usize = 0>
>
> The `impl_generics` will be set to `T: Clone, const N: usize = 0` and
> `ty_generics` will be set to `T, N`. Now using the `impl_generics` on an
> impl block:
>
>     impl<$($impl_generics)*> Foo {}
>
> will result in invalid Rust code, because default values are only
> available on type definitions.
>
> Therefore add parsing support for generic parameter default values using
> a new kind of generics called `decl_generics` and change the old
> behavior of `impl_generics` to not contain the generic parameter default
> values.
>
> Now `Generics` has three fields:
> - `impl_generics`: the generics with bounds
>   (e.g. `T: Clone, const N: usize`)
> - `decl_generics`: the generics with bounds and default values
>   (e.g. `T: Clone, const N: usize = 0`)
> - `ty_generics`:  contains the generics without bounds and without
>   default values (e.g. `T, N`)
>
> `impl_generics` is designed to be used on `impl<$impl_generics>`,
> `decl_generics` for the type definition, so `struct Foo<$decl_generics>`
> and `ty_generics` whenever you use the type, so `Foo<$ty_generics>`.
>
> Here is an example that uses all three different types of generics:
>
>     let (Generics { decl_generics, impl_generics, ty_generics }, rest) = parse_generics(input);
>     quote! {
>         struct Foo<$($decl_generics)*> {
>             // ...
>         }
>
>         impl<$impl_generics> Foo<$ty_generics> {
>             fn foo() {
>                 // ...
>             }
>         }
>     }
>
> The next commit contains a fix to the `#[pin_data]` macro making it
> compatible with generic parameter default values by relying on this new
> behavior.
>
> Signed-off-by: Benno Lossin <benno.lossin@xxxxxxxxx>

Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>