Re: [PATCH] rust: alloc: allow different error types in `KBox::pin_slice`

[Danilo Krummrich]

On Sat Feb 14, 2026 at 2:28 PM CET, Andreas Hindborg wrote:
> Previously, `KBox::pin_slice` required the initializer error type to match
> the return error type via `E: From<AllocError>`. This prevented using
> infallible initializers like `new_mutex!` inside `pin_slice`, because
> `Infallible` does not implement `From<AllocError>`.
>
> Introduce a separate type parameter `E2` for the initializer error type and
> require `AllocError: Into<E>` and `E2: Into<E>` instead. This allows the
> initializer to return a different error type that can be converted into the
> final error type, enabling use of infallible pin initializers in fallible
> allocation contexts.
>
> Signed-off-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>

I assume you have a user? I.e. do you need this patch in another tree?

> @@ -333,24 +333,31 @@ pub fn pin(x: T, flags: Flags) -> Result<Pin<Box<T, A>>, AllocError>
>      /// assert_eq!(s[3].d.lock().a, 20);
>      /// # Ok::<(), Error>(())
>      /// ```
> -    pub fn pin_slice<Func, Item, E>(
> +    pub fn pin_slice<Func, Item, E, E2>(
>          mut init: Func,
>          len: usize,
>          flags: Flags,
>      ) -> Result<Pin<Box<[T], A>>, E>
>      where
>          Func: FnMut(usize) -> Item,
> -        Item: PinInit<T, E>,
> -        E: From<AllocError>,

I think we should keep this bound and just add:

	E: From<E2>,

> +        Item: PinInit<T, E2>,
> +        AllocError: Into<E>,
> +        E2: Into<E>,
>      {
> -        let mut buffer = super::Vec::<T, A>::with_capacity(len, flags)?;
> +        let mut buffer = match super::Vec::<T, A>::with_capacity(len, flags) {
> +            Ok(buffer) => buffer,
> +            Err(err) => return Err(err.into()),
> +        };

This...

>          for i in 0..len {
>              let ptr = buffer.spare_capacity_mut().as_mut_ptr().cast();
>              // SAFETY:
>              // - `ptr` is a valid pointer to uninitialized memory.
>              // - `ptr` is not used if an error is returned.
>              // - `ptr` won't be moved until it is dropped, i.e. it is pinned.
> -            unsafe { init(i).__pinned_init(ptr)? };
> +            match unsafe { init(i).__pinned_init(ptr) } {
> +                Ok(()) => (),
> +                Err(err) => return Err(err.into()),
> +            }

...and this match becomes unnecessary then.