Re: [PATCH 2/2] rust: pin-init: add pin projections to `#[pin_data]`

[Alice Ryhl <aliceryhl@google.com>]

On Fri, Sep 5, 2025 at 7:12 PM Benno Lossin <lossin@kernel.org> wrote:
>
> Make the `#[pin_data]` macro generate a `*Projection` struct that holds
> either `Pin<&mut Field>` or `&mut Field` for every field of the original
> struct. Which version is chosen depends on weather there is a `#[pin]`
> or not respectively. Access to this projected version is enabled through
> generating `fn project(self: Pin<&mut Self>) -> SelfProjection<'_>`.
>
> Link: https://github.com/Rust-for-Linux/pin-init/pull/75/commits/2d698367d646c7ede90e01aa22842c1002d017b3
> [ Adapt workqueue to use the new projection instead of its own, custom
>   one - Benno ]
> Signed-off-by: Benno Lossin <lossin@kernel.org>
> ---
>  rust/kernel/workqueue.rs    | 10 ++-----
>  rust/pin-init/src/macros.rs | 60 +++++++++++++++++++++++++++++++++++++
>  2 files changed, 62 insertions(+), 8 deletions(-)
>
> diff --git a/rust/kernel/workqueue.rs b/rust/kernel/workqueue.rs
> index b9343d5bc00f..6ca14c629643 100644
> --- a/rust/kernel/workqueue.rs
> +++ b/rust/kernel/workqueue.rs
> @@ -356,18 +356,12 @@ struct ClosureWork<T> {
>      func: Option<T>,
>  }
>
> -impl<T> ClosureWork<T> {
> -    fn project(self: Pin<&mut Self>) -> &mut Option<T> {
> -        // SAFETY: The `func` field is not structurally pinned.
> -        unsafe { &mut self.get_unchecked_mut().func }
> -    }
> -}
> -
>  impl<T: FnOnce()> WorkItem for ClosureWork<T> {
>      type Pointer = Pin<KBox<Self>>;
>
>      fn run(mut this: Pin<KBox<Self>>) {
> -        if let Some(func) = this.as_mut().project().take() {
> +        if let Some(func) = this.as_mut().project().func.take() {
> +            // if let Some(func) = this.as_mut().project_func().take() {
>              (func)()
>          }
>      }
> diff --git a/rust/pin-init/src/macros.rs b/rust/pin-init/src/macros.rs
> index 9ced630737b8..d225cc144904 100644
> --- a/rust/pin-init/src/macros.rs
> +++ b/rust/pin-init/src/macros.rs
> @@ -831,6 +831,17 @@ macro_rules! __pin_data {
>              $($fields)*
>          }
>
> +        $crate::__pin_data!(make_pin_projections:
> +            @vis($vis),
> +            @name($name),
> +            @impl_generics($($impl_generics)*),
> +            @ty_generics($($ty_generics)*),
> +            @decl_generics($($decl_generics)*),
> +            @where($($whr)*),
> +            @pinned($($pinned)*),
> +            @not_pinned($($not_pinned)*),
> +        );
> +
>          // We put the rest into this const item, because it then will not be accessible to anything
>          // outside.
>          const _: () = {
> @@ -980,6 +991,55 @@ fn drop(&mut self) {
>              stringify!($($rest)*),
>          );
>      };
> +    (make_pin_projections:
> +        @vis($vis:vis),
> +        @name($name:ident),
> +        @impl_generics($($impl_generics:tt)*),
> +        @ty_generics($($ty_generics:tt)*),
> +        @decl_generics($($decl_generics:tt)*),
> +        @where($($whr:tt)*),
> +        @pinned($($(#[$($p_attr:tt)*])* $pvis:vis $p_field:ident : $p_type:ty),* $(,)?),
> +        @not_pinned($($(#[$($attr:tt)*])* $fvis:vis $field:ident : $type:ty),* $(,)?),
> +    ) => {
> +        $crate::macros::paste! {
> +            #[doc(hidden)]
> +            $vis struct [< $name Projection >] <'__pin, $($decl_generics)*> {

I'm not sure we want $vis here. That's the visibility of the original
struct, but I don't think we want it to be pub just because the struct
is.

Otherwise looks reasonable.

Alice