Deref 胁迫

鉴于 T 和 U 两种类型，当且仅当 T 实现 Deref<Target=U> 时，&T 将强制（隐式转换）为 &U

这允许我们做这样的事情：

fn foo(a: &[i32]) {
    // code
}

fn bar(s: &str) {
    // code
}

let v = vec![1, 2, 3];
foo(&v); // &Vec<i32> coerces into &[i32] because Vec<T> impls Deref<Target=[T]>

let s = "Hello world".to_string();
let rc = Rc::new(s);
// This works because Rc<T> impls Deref<Target=T> ∴ &Rc<String> coerces into 
// &String which coerces into &str. This happens as much as needed at compile time.
bar(&rc);