使用 type 关键字定义某个类型的别名。类型名称必须采用骆驼命名法（即所有单词首字母大写），否则编译器将警告。只有基础类型（uintf32等）例外。
// `NanoSecond` is a new name for `u64`
type NanoSecond = u64;
type Inch = u64;
// Use an attribute to silence warning
#[allow(non_camel_case_types)]
type uint64_t = u64;
// TODO ^ Try removing the attribute
fn main() {
// `NanoSecond` = `Inch` = `uint64_t` = `u64`
let nanoseconds: NanoSecond = 5 as uint64_t;
let inches: Inch = 2 as uint64_t;
// Note that type aliases *don't* provide any extra type safety, because
// aliases are *not* new types
println!("{} nanoseconds + {} inches = {} unit?",
nanoseconds,
inches,
nanoseconds + inches);
}