使用关联常量

// Must enable the feature to use associated constants
#![feature(associated_consts)]

use std::mem;

// Associated constants can be used to add constant attributes to types
trait Foo {
    const ID: i32;
}

// All implementations of Foo must define associated constants
// unless a default value is supplied in the definition.
impl Foo for i32 {
    const ID: i32 = 1;
}

struct Bar;

// Associated constants don't have to be bound to a trait to be defined
impl Bar {
    const BAZ: u32 = 5;
}

fn main() {
    assert_eq!(1, i32::ID);

    // The defined constant value is only stored once, so the size of
    // instances of the defined types doesn't include the constants.
    assert_eq!(4, mem::size_of::<i32>());
    assert_eq!(0, mem::size_of::<Bar>());
}