// 一个 `Borrowed` 类型，含有一个指向 `i32` 类型的引用。
// 指向 `i32` 的引用必须比 `Borrowed` 寿命更长。
// （原望：A type `Borrowed` which houses a reference to an
// `i32`. The reference to `i32` must outlive `Borrowed`.）
#[derive(Debug)]
struct Borrowed<'a>(&'a i32);

// 和前面类似，这里的两个引用都必须比这个结构体长寿。
#[derive(Debug)]
struct NamedBorrowed<'a> {
    x: &'a i32,
    y: &'a i32,
}

// 一个枚举类型，不是 `i32` 类型就是一个指向某个量的引用。
//（原文： An enum which is either an `i32` or a reference to one.）
#[derive(Debug)]
enum Either<'a> {
    Num(i32),
    Ref(&'a i32),
}

fn main() {
    let x = 18;
    let y = 15;

    let single = Borrowed(&x);
    let double = NamedBorrowed { x: &x, y: &y };
    let reference = Either::Ref(&x);
    let number    = Either
::Num(y);

    println!("x is borrowed in {:?}", single);
    println!("x and y are borrowed in {:?}", double);
    println!("x is borrowed in {:?}", reference);
    println!("y is *not* borrowed in {:?}", number);
}
