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

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Message-ID: <D9D4M8XF14X9.1YUVFH9EW0IMX@proton.me>
Date: Tue, 22 Apr 2025 11:21:25 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Christian Schrefl <chrisi.schrefl@...il.com>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, Fiona Behrens <me@...enk.dev>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/8] rust: pin-init: Implement `Wrapper` for `UnsafePinned` behind feature flag.

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