We can't use all traits to create a trait object. Take the following:

let my_vec = vec![1,3,5,7,9]; 
let dupe = &my_vec as &Clone;

This will not compile as Clone is not object-safe as Clone contains Self: Sized, which a trait cannot have.

If the trait doesn't require Self: Sized and all of the methods are object-safe, it is an object-safe trait. For a method to be object-safe, it must require Self: Sized. If the method doesn't require Self: Sized, it can still be object-safe if the method doesn't require any parameters and doesn't use Self.