Dyn upcast Charter


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.


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;


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