Possible unsoundness in IntoOpaque to do with pointer provenance and Box/Arc

[zRedShift]

According to the alloc crate documentation, Arc::from_raw must be paired with a call to Arc::into_raw, otherwise pointer provenance may be violated (due to the ArcInner layout), and a slightly different issue with Box.

Regarding Arc, it could be solved by imitating Arc::from_raw and using Arc::as_ptr, as using the Deref::deref/AsRef::as_ref implementation might be problematic, but according to this comment, there's some extra ceremony required for Box due to the uniqueness requirement.

I'm proposing to switch the as_ptr method to into_ptr and take Self. With that, Box::into_raw and Arc::into_raw could be used directly. This requires a small extra step to avoid a partial move, and incompatible with !Sized, but the fat pointer metadata can't be stored in *mut c_void anyway, and would require double boxing anyway (e.g. Box<Box<dyn Trait>>).