This is a general principal in the Rust standard library, but it is confusing at first. Similarly you can construct a HashMap without restriction but calling `.get` on it requires the key to implement Hash.
I'll let someone else clarify the reasoning because I'm not confident I remember it correctly. I though this was in the Rust API guidelines but I can't find it after a quick skin.
I couldn't find it in the API guidelines either. From what I understand, the idea is that any trait bounds, which includes generic type parameter bounds and lifetime bound on a type (struct or enum) would be repeated back in the impl block
