Trait upcasting coercion

[crlf0710]

Proposal

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.

#![feature(trait_upcasting)]

trait Foo {}

trait Bar: Foo {}

impl Foo for i32 {}

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

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

Prioritization

• This belongs to the "Trait and type system extensions" part of the lang team priorities.

Links and related work

• Previously established tracking issue: Tracking issue for dyn upcasting coercion rust#65991

Initial people involved

• @crlf0710
• @nikomatsakis

Also cc @alexreg and @eddyb in case they're interested.

What happens now?

This issue is part of the experimental MCP process described in RFC 2936. Once this issue is filed, a Zulip topic will be opened for discussion, and the lang-team will review open MCPs in its weekly triage meetings. You should receive feedback within a week or two.

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[joshtriplett]

@rustbot second

[nikomatsakis]

This has been seconded and the https://rust-lang.github.io/dyn-upcasting-coercion-initiative/ repository created to track this work!

[JeremyRubin]

the example seems backwards? should it not be:

#![feature(trait_upcasting)]

trait Foo {}

trait Bar: Foo {}

impl Foo for i32 {}

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

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

where Bar is able to cast up to Foo, as described "trait object of type dyn Bar to be cast to a trait object of type dyn Foo
so long as Bar: Foo.".

Of course, both directions could be possible, but it seems reversed here.

[iddm]

Is there a workaround until this becomes available?

[JeremyRubin]

yes, i think casting through a struct should work?

struct CastFoo<'a>(&'a dyn Bar);

impl<'a> Foo for CastFoo<'a> {
  fn fo_every_foo_method(&self, a:Param) -> Output {
    self.0.that_method(a)
  }
}

let bar: &dyn Bar = &123;
let tmp = CastFoo(bar);
let foo: &dyn Foo = &tmp;