Re: [PATCH] rust: implement ForeignOwnable for Pin<Box<T>>

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

On 30.07.24 15:06, Alice Ryhl wrote:
> @@ -89,6 +90,32 @@ unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
>      }
>  }
> 
> +impl<T: 'static> ForeignOwnable for Pin<Box<T>> {
> +    type Borrowed<'a> = Pin<&'a T>;
> +
> +    fn into_foreign(self) -> *const core::ffi::c_void {
> +        // SAFETY: We are still treating the box as pinned.

I don't think that we have the guarantee that the pointee at the pointer
that is returned by `into_foreign` is not moved.
AFAIU `ForeignOwnable` is used to store these pointers in C structures
and never to actually access the value behind the returned pointer. So
we could add the requirement to `into_foreign` (thus making it `unsafe`)
that the pointer should not be dereferenced/used aside from `borrow` and
`from_foreign`. Otherwise I don't see how the call below can be OK.
What do you think?

---
Cheers,
Benno

> +        Box::into_raw(unsafe { Pin::into_inner_unchecked(self) }) as _
> +    }