Re: [PATCH v2 1/3] rust: add UnsafePinned type

[Alice Ryhl <aliceryhl@google.com>]

On Fri, May 02, 2025 at 02:08:13AM +0200, Christian Schrefl wrote:
> [cc Ralf]
> 
> On 02.05.25 12:51 AM, Benno Lossin wrote:
> > On Thu May 1, 2025 at 9:11 PM CEST, Christian Schrefl wrote:
> >> On 01.05.25 8:51 PM, Benno Lossin wrote:
> >>> On Wed Apr 30, 2025 at 7:30 PM CEST, Christian Schrefl wrote:
> >>>> On 30.04.25 11:45 AM, Benno Lossin wrote:
> >>>>> On Wed Apr 30, 2025 at 10:36 AM CEST, Christian Schrefl wrote:
> >>>>>> +/// This implementation works because of the "`!Unpin` hack" in rustc, which allows (some kinds of)
> >>>>>> +/// mutual aliasing of `!Unpin` types. This hack might be removed at some point, after which only
> >>>>>> +/// the `core::pin::UnsafePinned` type will allow this behavior. In order to simplify the migration
> >>>>>> +/// to future rust versions only this polyfill of this type should be used when this behavior is
> >>>>>> +/// required.
> >>>>>> +///
> >>>>>> +/// In order to disable niche optimizations this implementation uses [`UnsafeCell`] internally,
> >>>>>> +/// the upstream version however will not. So the fact that [`UnsafePinned`] contains an
> >>>>>> +/// [`UnsafeCell`] must not be relied on (Other than the niche blocking).
> >>>>>
> >>>>> I would make this last paragraph a normal comment, I don't think we
> >>>>> should expose it in the docs.
> >>>>
> >>>> I added this as docs since I wanted it to be a bit more visible,
> >>>> but I can replace the comment text (about `UnsafeCell`) with this paragraph
> >>>> and drop it from the docs if you want.
> >>>
> >>> I think we shouldn't talk about these implementation details in the
> >>> docs.
> >>
> >> Alright, what do you think of:
> >>
> >> // As opposed to the upstream Rust type this contains a `PhantomPinned`` and `UnsafeCell<T>`
> > 
> > There are two '`' after PhantomPinned.
> > 
> >> // - `PhantomPinned` to avoid needing a `impl<T> !Unpin for UnsafePinned<T>`
> > 
> > s/ a / an /
> > 
> > I find the phrasing 'avoid needing <negative impl>' a bit weird, I'd
> > just say "`PhantomPinned` to ensure the struct always is `!Unpin` and
> > thus enables the `!Unpin` hack".
> 
> Thanks I'll use that.
> 
> > 
> > If you have a link to somewhere that explains that hack, then I'd also
> > put it there. I forgot if it's written down somewhere.
> 
> I haven't found anything that describes the hack in detail.
> From what I understand its a combination of disabling `noalias`
> [0] (this PR enables it for `Unpin` types) and disabling 
> `dereferencable` [1] on `&mut !Unpin` types.
> Related rust issue about this [2].
> 
> Maybe Alice, Ralf or someone else form the rust side can provide
> a better reference?
> 
> [0]: https://github.com/rust-lang/rust/pull/82834
> [1]: https://github.com/rust-lang/rust/pull/106180
> [2]: https://github.com/rust-lang/rust/issues/63818

I wrote this a long time ago:
https://gist.github.com/Darksonn/1567538f56af1a8038ecc3c664a42462

But it doesn't really take the angle of explaining the !Unpin hack. It's
more of an early doc arguing for having an UnsafePinned type.

Alice