dyn compatible

A trait simple enough to be used as a trait object.

A trait is dyn compatible (formerly object safe) when every method can be called through a vtable: no generic methods, no Self-returning methods without where Self: Sized, no Sized supertrait.

Examples

Adding fn duplicate(&self) -> Self; to Notifier makes Box<dyn Notifier> fail with E0038; adding where Self: Sized to that method makes it compile again.
Vec<Box<dyn Clone>> fails with E0038, because clone returns Self.