Re: [PATCH 1/8] rust: pin-init: add `cast_[pin_]init` functions to change the initialized type

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

On 22.04.25 12:17 AM, Benno Lossin wrote:
> These functions cast the given pointer from one type to another. They
> are particularly useful when initializing transparent wrapper types.
> 
> Link: https://github.com/Rust-for-Linux/pin-init/pull/39/commits/80c03ddee41b154f1099fd8cc7c2bbd8c80af0ad
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>
> ---

Reviewed-by: Christian Schrefl <chrisi.schrefl@gmail.com>

>  rust/pin-init/src/lib.rs | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/rust/pin-init/src/lib.rs b/rust/pin-init/src/lib.rs
> index 0806c689f693..a880c21d3f09 100644
> --- a/rust/pin-init/src/lib.rs
> +++ b/rust/pin-init/src/lib.rs
> @@ -1216,6 +1216,38 @@ unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), E> {
>      __internal::InitClosure(f, PhantomData)
>  }
>  
> +/// Changes the to be initialized type.
> +///
> +/// # Safety
> +///
> +/// - `*mut U` must be castable to `*mut T` and any value of type `T` written through such a
> +///   pointer must result in a valid `U`.
> +#[expect(clippy::let_and_return)]
> +pub const unsafe fn cast_pin_init<T, U, E>(init: impl PinInit<T, E>) -> impl PinInit<U, E> {
> +    // SAFETY: initialization delegated to a valid initializer. Cast is valid by function safety
> +    // requirements.
> +    let res = unsafe { pin_init_from_closure(|ptr: *mut U| init.__pinned_init(ptr.cast::<T>())) };
> +    // FIXME: remove the let statement once the nightly-MSRV allows it (1.78 otherwise encounters a
> +    // cycle when computing the type returned by this function)
> +    res
> +}
> +
> +/// Changes the to be initialized type.
> +///
> +/// # Safety
> +///
> +/// - `*mut U` must be castable to `*mut T` and any value of type `T` written through such a
> +///   pointer must result in a valid `U`.
> +#[expect(clippy::let_and_return)]
> +pub const unsafe fn cast_init<T, U, E>(init: impl Init<T, E>) -> impl Init<U, E> {
> +    // SAFETY: initialization delegated to a valid initializer. Cast is valid by function safety
> +    // requirements.
> +    let res = unsafe { init_from_closure(|ptr: *mut U| init.__init(ptr.cast::<T>())) };
> +    // FIXME: remove the let statement once the nightly-MSRV allows it (1.78 otherwise encounters a
> +    // cycle when computing the type returned by this function)
> +    res
> +}
> +>  /// An initializer that leaves the memory uninitialized.
>  ///
>  /// The initializer is a no-op. The `slot` memory is not changed.