Macro unsize_box

macro_rules! unsize_box {
    ( $boxed:expr $(,)? ) => { ... };
}

Allows turning a Box<T: Sized, A> into a Box<U: ?Sized, A> where T can be unsizing-coerced into a U.

This is the only way to create an allocator_api2::boxed::Box of an unsized type on stable.

With the standard library’s alloc::boxed::Box, this is done automatically using the unstable unsize traits, but this crate’s Box can’t take advantage of that machinery on stable. So, we need to use type inference and the fact that you can still coerce the inner pointer of a box to get the compiler to help us unsize it using this macro.

Example

use allocator_api2::unsize_box;
use allocator_api2::boxed::Box;
use core::any::Any;

let sized_box: Box<u64> = Box::new(0);
let unsized_box: Box<dyn Any> = unsize_box!(sized_box);