https://doc.rust-lang.org/book/ch17-02-trait-objects.html
Here's a random example: if you want a trait object with more than one trait, that only works today if one of the two is an auto trait: https://play.rust-lang.org/?version=stable&mode=debug&editio...
https://doc.rust-lang.org/book/ch17-02-trait-objects.html