Can DynMetadata be made constructable?

[Rua]

Related tracking issue: #81513

As of right now, the only way to get a DynMetadata is to call ptr::metadata on an existing pointer, which can create a chicken-and-egg problem. It could be useful to be able to get the metadata for a hypothetical trait object Foo as Trait as well, without first having to construct such a trait object. However, I don't know if this would be possible, and whether it would lead to soundness issues or have other undesired consequences.

[bjorn3]

I think std::ptr::metadata(std::ptr::null::<Foo>() as *const dyn Trait) would work.