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

[Benno Lossin <benno.lossin@proton.me>]

On Tue Apr 22, 2025 at 11:42 AM CEST, Christian Schrefl wrote:
> On 22.04.25 12:18 AM, Benno Lossin wrote:
>> @@ -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`.

I don't follow, which types do you need to wrap in `Opaque` that are not
sized?

Since `Opaque` uses `MaybeUninit` under the hood, it cannot support
`!Sized` types anyways, so I'm a bit confused.

---
Cheers,
Benno