dyn Upcasting for unsized generics

[dhardy]

This fails:

#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;
use std::any::Any;

fn box_to_any<T: Any + ?Sized>(x: Box<T>) -> Box<dyn Any>
where
    T: CoerceUnsized<dyn Any>,
{
    x as Box<dyn Any>
}

with:

error[E0277]: the size for values of type T cannot be known at compilation time

note: required for the cast from Box<T> to Box<(dyn Any + 'static)>

Dyn upcasting adds support for upcasting trait objects dyn A where A: Any to dyn Any, but not just any type (e.g. str cannot be cast to dyn Any).

For whatever reason, the bound T: Any + ?Sized is not enough to support the cast: [i32] implements Any (strange) yet cannot be cast to dyn Any (if it could this would break memory safety since the impl of Any returns a single TypeId for all [i32] regardless of length).

The additional T: CoerceUnsized<dyn Any> looks like it should be enough to satisfy the coercion in the method, yet it is not. It should be?

I guess this should be categorised as a bug in #18598, but it could be a bug in #65991 or even a feature request.

[dhardy]

Motivation

Enable Box::downcast to be generic over all types which can support it: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=863a1a05e024c3f2d73cba48690ae551. (This would of course be breaking and a separate issue/RFC. Just wanted to add for context.)

[compiler-errors]

The correct bound is T: Unsize<dyn Any>. T: CoerceUnsized<U> does not imply Box<T>: CoerceUnsized<Box<U>>, and Box<T>: CoerceUnsized<Box<U>> is implemented only if T: Unsize<U>.

[dhardy]

Thanks