VBox is a type erased Box of trait object that stores the vtable pointer.

VBox is just like a Box<dyn Trait> but erases type Trait so that to use it, there is no need to have Trait as one of its type parameters. Only the creator and the consumer needs to agree on the type parameters.

Internally, it stores the trait object’s data pointer in a Box<dyn Any + Send>, so that the Drop::drop() will be called when the wrapper is dropped. And it stores the vtable pointer in another usize to make sure it is Send.

// Pack a u64 into a `Debug` trait object and erase the type `Debug`.
let vbox: VBox = into_vbox!(dyn Debug, 10u64);

// Unpack to different trait object will panic:
// let _panic = from_vbox!(dyn Display, vbox);

// Unpack the `Debug` trait object.
let unpacked: Box<dyn Debug> = from_vbox!(dyn Debug, vbox);

assert_eq!("10", format!("{:?}", unpacked));