ManuallyDrop<T> where T: ?Sized

[mikeyhew]

ManuallyDrop<T> currently requires T: Sized. It would be nice if we could remove this restriction. I'm not sure how to do it with the current implementation using union though.

[jonas-schievink]

Doesn't seem possible until we get by-value DSTs

[mikeyhew]

@jonas-schievink care to explain why?

[jonas-schievink]

Ah, nevermind. It would just need to make ManuallyDrop unsized, too.

[mikeyhew]

The new and into_inner methods can keep could keep their Sized bounds, we just need to relax the Sized bound on the struct itself, and make sure ManuallyDrop<T>: Unsize<ManuallyDrop<U>> where T: Unsize<U>, U: ?Sized. Then the following code would be possible:

struct Foo<T>(ManuallyDrop<T>);

let foo = Box::new(Foo(ManuallyDrop::new(5i32))) as Box<Foo<::std::any::Any>;

[mikeyhew]

I guess this would be possible by allowing union types to be ?Sized when there is only one variant. I was worried that would be unsound, but then I realized, the only way to create a DST union type would be from an unsize coercion, and then way you know what the erased Sized type is and can get the vtable/slice length then, just like any other unsize coercion.

[RalfJung]

See https://internals.rust-lang.org/t/pre-rfc-unions-drop-types-and-manuallydrop/8025