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References: <20240309155243.482334-1-benno.lossin@proton.me>
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From: Alice Ryhl <aliceryhl@google.com>
Date: Mon, 11 Mar 2024 10:06:29 +0100
Message-ID: <CAH5fLgiEsYF6AanS4mT_8sPHL6YM6SAsdg1LNMB-FE9yWvSh8w@mail.gmail.com>
Subject: Re: [PATCH v3 1/3] rust: macros: add `decl_generics` to `parse_generics()`
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>, 
	Asahi Lina <lina@asahilina.net>, rust-for-linux@vger.kernel.org, 
	linux-kernel@vger.kernel.org
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On Sat, Mar 9, 2024 at 4:53=E2=80=AFPM Benno Lossin <benno.lossin@proton.me=
> 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 =3D 0>
>
> The `impl_generics` will be set to `T: Clone, const N: usize =3D 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 =3D 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<$ty_generics>`.
>
> Here is an example that uses all three different types of generics:
>
>     let (Generics { decl_generics, impl_generics, ty_generics }, rest) =
=3D parse_generics(input);
>     quote! {
>         struct Foo<$($decl_generics)*> {
>             // ...
>         }
>
>         impl<$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@proton.me>

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