rust: macros: allow generic parameter default values in `#[pin_data]`

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 <[email protected]>
Reviewed-by: Martin Rodriguez Reboredo <[email protected]>
Reviewed-by: Gary Guo <[email protected]>
Reviewed-by: Alice Ryhl <[email protected]>
Tested-by: Alice Ryhl <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

1 files changed, 0 insertions, 1 deletions

diff --git a/rust/macros/helpers.rs b/rust/macros/helpers.rs
index 2497d6519f2f..563dcd2b7ace 100644
--- a/rust/macros/helpers.rs
+++ b/rust/macros/helpers.rs
@@ -95,7 +95,6 @@ pub(crate) struct Generics {
     /// The generics with bounds and default values (e.g. `T: Clone, const N: usize = 0`).
     ///
     /// Use this on type definitions e.g. `struct Foo<$decl_generics> ...` (or `union`/`enum`).
-    #[allow(dead_code)]
     pub(crate) decl_generics: Vec<TokenTree>,
     /// The generics with bounds (e.g. `T: Clone, const N: usize`).
     ///