derivable traits and trait inheritance

• Rust book - Appendix C: Derivable Traits
• questions about deriving traits in stack overflow
• 뭐라고? trait가 trait를 상속할 수 있다고? stack overflow

trait A {}
trait B: A {}
struct S;
impl B for S {}
impl A for S {} // separately implement both B and A

let x: &B = &S;

• Rust is not an OOP, which cannot upcast to base type