Dyn upcast Charter

Goals

Summary and problem statement

• Add the trait_upcasting feature to the language.

Motivation, use-cases, and solution sketches

• The trait_upcasting feature adds support for trait upcasting coercion. This allows a trait object of type dyn Bar to be cast to a trait object of type dyn Foo so long as Bar: Foo.

#![allow(unused)]
#![feature(trait_upcasting)]

fn main() {
trait Foo {}

trait Bar: Foo {}

impl Foo for i32 {}

impl<T: Foo + ?Sized> Bar for T {}

let bar: &dyn Bar = &123;
let foo: &dyn Foo = bar;
}

Non-goals

• To support dyn Trait1 + Trait2 for general traits (this syntax naturally works, and should continue to work, with auto traits).

Membership