CODE EXAMPLE FOR RUST

armanriazi•rust•trait•objectsafe•vs•nonobjectsafe

Here is an example trait that is not object safe:

trait SomeTrait {
    fn foo(&self) -> int { ... }
    
    // Object-safe methods may not return `Self`:
    fn new() -> Self;
}
Splitting a trait:
One option is to split a trait into object-safe and non-object-safe parts. We hope that this will lead to better design. We are not sure how much code this will affect, it would be good to have data about this.

trait SomeTrait {
    fn foo(&self) -> int { ... }
}

trait SomeTraitCtor : SomeTrait {
    fn new() -> Self;
}
Adding a where-clause:
Sometimes adding a second trait feels like overkill. In that case, it is often an option to simply add a where Self:Sized clause to the methods of the trait that would otherwise violate the object safety rule.

trait SomeTrait {
    fn foo(&self) -> int { ... }
    
    fn new() -> Self
        where Self : Sized; // this condition is new
}