Rust Project Goals

Current status: there is an RFC for auto impl supertraits that has received some discussion and updates (thank you, Ding Xiang Fei!).

The major open questions currently are:

Syntax

The current RFC proposes:

#![allow(unused)]
fn main() {
trait Subtrait: Supertrait {
    auto impl Supertrait {
        // Supertrait items defined in terms of Subtrait items, if any
    }
}
}

Additionally, there is an open question around the syntax of auto impl for unsafe supertraits. The current proposal is to require unsafe auto impl Supertrait.

Whether to require impls to opt-out of auto impls

The current RFC proposes that

#![allow(unused)]
fn main() {
impl Supertrait for MyType {}
impl Subtrait for MyType {
// Required in order to manually write Supertrait
}
 for MyType.
extern impl Supertrait;
}

This makes it explicit via opt-out whether an auto impl is being applied. However, this is in conflict with the goal of allowing auto impls to be added to existing trait hierarchies. The RFC proposes to resolve this via a temporary attribute which triggers a warning. See my comment here.

Note that properly resolving whether or not to apply an auto impl requires coherence-like analysis.