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 <benno.lossin@proton.me>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240309155243.482334-2-benno.lossin@proton.me
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/init/macros.rs

@@ -538,6 +538,7 @@ macro_rules! __pin_data {
         ),
         @impl_generics($($impl_generics:tt)*),
         @ty_generics($($ty_generics:tt)*),
+        @decl_generics($($decl_generics:tt)*),
         @body({ $($fields:tt)* }),
     ) => {
         // We now use token munching to iterate through all of the fields. While doing this we
@@ -560,6 +561,9 @@ macro_rules! __pin_data {
             @impl_generics($($impl_generics)*),
             // The 'ty generics', the generics that will need to be specified on the impl blocks.
             @ty_generics($($ty_generics)*),
+            // The 'decl generics', the generics that need to be specified on the struct
+            // definition.
+            @decl_generics($($decl_generics)*),
             // The where clause of any impl block and the declaration.
             @where($($($whr)*)?),
             // The remaining fields tokens that need to be processed.
@@ -585,6 +589,7 @@ macro_rules! __pin_data {
         @name($name:ident),
         @impl_generics($($impl_generics:tt)*),
         @ty_generics($($ty_generics:tt)*),
+        @decl_generics($($decl_generics:tt)*),
         @where($($whr:tt)*),
         // We found a PhantomPinned field, this should generally be pinned!
         @fields_munch($field:ident : $($($(::)?core::)?marker::)?PhantomPinned, $($rest:tt)*),
@@ -607,6 +612,7 @@ macro_rules! __pin_data {
             @name($name),
             @impl_generics($($impl_generics)*),
             @ty_generics($($ty_generics)*),
+            @decl_generics($($decl_generics)*),
             @where($($whr)*),
             @fields_munch($($rest)*),
             @pinned($($pinned)* $($accum)* $field: ::core::marker::PhantomPinned,),
@@ -623,6 +629,7 @@ macro_rules! __pin_data {
         @name($name:ident),
         @impl_generics($($impl_generics:tt)*),
         @ty_generics($($ty_generics:tt)*),
+        @decl_generics($($decl_generics:tt)*),
         @where($($whr:tt)*),
         // We reached the field declaration.
         @fields_munch($field:ident : $type:ty, $($rest:tt)*),
@@ -640,6 +647,7 @@ macro_rules! __pin_data {
             @name($name),
             @impl_generics($($impl_generics)*),
             @ty_generics($($ty_generics)*),
+            @decl_generics($($decl_generics)*),
             @where($($whr)*),
             @fields_munch($($rest)*),
             @pinned($($pinned)* $($accum)* $field: $type,),
@@ -656,6 +664,7 @@ macro_rules! __pin_data {
         @name($name:ident),
         @impl_generics($($impl_generics:tt)*),
         @ty_generics($($ty_generics:tt)*),
+        @decl_generics($($decl_generics:tt)*),
         @where($($whr:tt)*),
         // We reached the field declaration.
         @fields_munch($field:ident : $type:ty, $($rest:tt)*),
@@ -673,6 +682,7 @@ macro_rules! __pin_data {
             @name($name),
             @impl_generics($($impl_generics)*),
             @ty_generics($($ty_generics)*),
+            @decl_generics($($decl_generics)*),
             @where($($whr)*),
             @fields_munch($($rest)*),
             @pinned($($pinned)*),
@@ -689,6 +699,7 @@ macro_rules! __pin_data {
         @name($name:ident),
         @impl_generics($($impl_generics:tt)*),
         @ty_generics($($ty_generics:tt)*),
+        @decl_generics($($decl_generics:tt)*),
         @where($($whr:tt)*),
         // We found the `#[pin]` attr.
         @fields_munch(#[pin] $($rest:tt)*),
@@ -705,6 +716,7 @@ macro_rules! __pin_data {
             @name($name),
             @impl_generics($($impl_generics)*),
             @ty_generics($($ty_generics)*),
+            @decl_generics($($decl_generics)*),
             @where($($whr)*),
             @fields_munch($($rest)*),
             // We do not include `#[pin]` in the list of attributes, since it is not actually an
@@ -724,6 +736,7 @@ macro_rules! __pin_data {
         @name($name:ident),
         @impl_generics($($impl_generics:tt)*),
         @ty_generics($($ty_generics:tt)*),
+        @decl_generics($($decl_generics:tt)*),
         @where($($whr:tt)*),
         // We reached the field declaration with visibility, for simplicity we only munch the
         // visibility and put it into `$accum`.
@@ -741,6 +754,7 @@ macro_rules! __pin_data {
             @name($name),
             @impl_generics($($impl_generics)*),
             @ty_generics($($ty_generics)*),
+            @decl_generics($($decl_generics)*),
             @where($($whr)*),
             @fields_munch($field $($rest)*),
             @pinned($($pinned)*),
@@ -757,6 +771,7 @@ macro_rules! __pin_data {
         @name($name:ident),
         @impl_generics($($impl_generics:tt)*),
         @ty_generics($($ty_generics:tt)*),
+        @decl_generics($($decl_generics:tt)*),
         @where($($whr:tt)*),
         // Some other attribute, just put it into `$accum`.
         @fields_munch(#[$($attr:tt)*] $($rest:tt)*),
@@ -773,6 +788,7 @@ macro_rules! __pin_data {
             @name($name),
             @impl_generics($($impl_generics)*),
             @ty_generics($($ty_generics)*),
+            @decl_generics($($decl_generics)*),
             @where($($whr)*),
             @fields_munch($($rest)*),
             @pinned($($pinned)*),
@@ -789,6 +805,7 @@ macro_rules! __pin_data {
         @name($name:ident),
         @impl_generics($($impl_generics:tt)*),
         @ty_generics($($ty_generics:tt)*),
+        @decl_generics($($decl_generics:tt)*),
         @where($($whr:tt)*),
         // We reached the end of the fields, plus an optional additional comma, since we added one
         // before and the user is also allowed to put a trailing comma.
@@ -802,7 +819,7 @@ macro_rules! __pin_data {
     ) => {
         // Declare the struct with all fields in the correct order.
         $($struct_attrs)*
-        $vis struct $name <$($impl_generics)*>
+        $vis struct $name <$($decl_generics)*>
         where $($whr)*
         {
             $($fields)*

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`).
     ///

rust/macros/pin_data.rs

@@ -10,7 +10,7 @@ pub(crate) fn pin_data(args: TokenStream, input: TokenStream) -> TokenStream {
     let (
         Generics {
             impl_generics,
-            decl_generics: _,
+            decl_generics,
             ty_generics,
         },
         rest,
@@ -77,6 +77,7 @@ pub(crate) fn pin_data(args: TokenStream, input: TokenStream) -> TokenStream {
         @sig(#(#rest)*),
         @impl_generics(#(#impl_generics)*),
         @ty_generics(#(#ty_generics)*),
+        @decl_generics(#(#decl_generics)*),
         @body(#last),
     });
     quoted.extend(errs);