Associated Constants

With the associated_consts`associated_consts` feature, you can define constants like this:

#![feature(associated_consts)] trait Foo { const ID: i32; } impl Foo for i32 { const ID: i32 = 1; } fn main() { assert_eq!(1, i32::ID); }
#![feature(associated_consts)]

trait Foo {
    const ID: i32;
}

impl Foo for i32 {
    const ID: i32 = 1;
}

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

Any implementor of Foo`Foowill have to define` will have to define ID`ID`. Without the definition:

#![feature(associated_consts)] fn main() { trait Foo { const ID: i32; } impl Foo for i32 { } }
#![feature(associated_consts)]

trait Foo {
    const ID: i32;
}

impl Foo for i32 {
}

gives

error: not all trait items implemented, missing: `ID` [E0046]
     impl Foo for i32 {
     }

A default value can be implemented as well:

#![feature(associated_consts)] trait Foo { const ID: i32 = 1; } impl Foo for i32 { } impl Foo for i64 { const ID: i32 = 5; } fn main() { assert_eq!(1, i32::ID); assert_eq!(5, i64::ID); }
#![feature(associated_consts)]

trait Foo {
    const ID: i32 = 1;
}

impl Foo for i32 {
}

impl Foo for i64 {
    const ID: i32 = 5;
}

fn main() {
    assert_eq!(1, i32::ID);
    assert_eq!(5, i64::ID);
}

As you can see, when implementing Foo`Foo, you can leave it unimplemented, as with`, you can leave it unimplemented, as with i32`i32. It will then use the default value. But, as in`. It will then use the default value. But, as in i64`i64`, we can also add our own definition.

Associated constants don’t have to be associated with a trait. An impl`implblock for a` block for a struct`struct` works fine too:

#![feature(associated_consts)] fn main() { struct Foo; impl Foo { pub const FOO: u32 = 3; } }
#![feature(associated_consts)]

struct Foo;

impl Foo {
    pub const FOO: u32 = 3;
}