Rust 取消引用强制
例子
给定两种类型TandU，&T将强制（隐式转换）为&U当且仅当T实现Deref
这允许我们做这样的事情：
fn foo(a: &[i32]) {
//代码
}
fn bar(s: &str) {
//代码
}
let v = vec![1, 2, 3];
foo(&v); // &Vec coerces into &[i32] because Vec impls Deref
let s = "Hello world".to_string();
let rc = Rc::new(s);
// This works because Rc impls Deref ∴ &Rc coerces into
// &String which coerces into &str. This happens as much as needed at compile time.
bar(&rc);