I think for struct base languages (e.g. Rust) this...

# language-proposals

[Ruckus]

I think for struct base languages (e.g. Rust) this is a good idea, since the base struct cannot implement anything itself, but for Kotlin it's a bad fit, since it could potentially have unintended side effects, such as changing how filterIsInstance and is checks work in existing code.
(But I've never been a huge fan of implicit interfaces, and this feels pretty close to me, so there is plenty of personal bias there)