Re: [PATCH 3/8] rust: pin-init: Implement `Wrapper` for `UnsafePinned` behind feature flag.

[Christian Schrefl <chrisi.schrefl@gmail.com>]

On 22.04.25 12:18 AM, Benno Lossin wrote:
> From: Christian Schrefl <chrisi.schrefl@gmail.com>
> 
> Add the `unsafe-pinned` feature which gates the `Wrapper`
> implementation of the `core::pin::UnsafePinned` struct.
> 
> For now this is just a cargo feature, but once `core::pin::UnsafePinned`
> is stable a config flag can be added to allow the usage of this
> implementation in the linux kernel.
> 
> Signed-off-by: Christian Schrefl <chrisi.schrefl@gmail.com>
> Link: https://github.com/Rust-for-Linux/pin-init/pull/37/commits/99cb1934425357e780ea5b0628f66633123847b8
> [ Fixed commit authorship. - Benno ]
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>
> ---
>  rust/pin-init/src/lib.rs | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/rust/pin-init/src/lib.rs b/rust/pin-init/src/lib.rs
> index 467ccc8bd616..745cf534d239 100644
> --- a/rust/pin-init/src/lib.rs
> +++ b/rust/pin-init/src/lib.rs
> @@ -269,6 +269,10 @@
>  #![forbid(missing_docs, unsafe_op_in_unsafe_fn)]
>  #![cfg_attr(not(feature = "std"), no_std)]
>  #![cfg_attr(feature = "alloc", feature(allocator_api))]
> +#![cfg_attr(
> +    all(feature = "unsafe-pinned", CONFIG_RUSTC_HAS_UNSAFE_PINNED),
> +    feature(unsafe_pinned)
> +)]
>  
>  use core::{
>      cell::UnsafeCell,
> @@ -1557,3 +1561,11 @@ fn pin_init<E>(value_init: impl PinInit<T, E>) -> impl PinInit<Self, E> {
>          unsafe { cast_pin_init(value_init) }
>      }
>  }
> +
> +#[cfg(all(feature = "unsafe-pinned", CONFIG_RUSTC_HAS_UNSAFE_PINNED))]
> +impl<T> Wrapper<T> for core::pin::UnsafePinned<T> {
> +    fn pin_init<E>(init: impl PinInit<T, E>) -> impl PinInit<Self, E> {
> +        // SAFETY: `UnsafePinned<T>` has a compatible layout to `T`.
> +        unsafe { cast_pin_init(init) }
> +    }
> +}

I've realized that for us to use `UnsafePinned` in `Opaque` internally 
we need to have a `T: ?Sized` for this implementation. `cast_pin_init`
won't work for that since we can't cast pointers between different DSTs.
We could add something similar that uses a closure to convert a
`*mut T` to `*mut U`.

That won't really be relevant until we actually use 
`core::pin::UnsafePinned` however.

Should I do another PR to extend this to `T: ?Sized`?

Cheers
Christian