Specialization should allow for delegation to default trait method implementation

[jazzfool]

Given the following trait:

trait Trait {
    fn test(&self) { println!("default implementation"); }
}

To provide a default specialization on Trait::test, test must be redefined;

impl<T> Trait for T {
    default fn test(&self) { println!("default implementation"); }
}

However this violates the DRY principle. The solution here would be to provide a way to delegate the implementation of test to the already existing default implementation found in the trait definition.

Proposed syntax to allow for this delegation:

impl<T> Trait for T {
    default fn test(&self);
}

[jazzfool]

cc @Centril

[Centril]

I believe this is intentional fallout from #64564 (cc @jonas-schievink).

Having some sort of forwarding syntax to recover DRY seems reasonable and default fn test(&self); isn't a bad one.

[jonas-schievink]

Yup, sounds reasonable to me. I noticed the amount of repetition when working on that PR too:

One annoyance with doing this is that it sometimes requires users to copy-paste a provided trait method into an impl just to mark it default (ie. there is no syntax to forward this impl method to the provided trait method).

[Centril]

Syntactic support for default fn test(&self); in an impl should already be there after #67131. To support it semantically we would need changes in lowering and beyond.