https://blog.rust-lang.org/2017/08/31/Rust-1.20.html "Rust 1.20 adds the ability to define “associated constants” as well: struct Struct; impl Struct { const ID: u32 = 0; } fn main() { println!("the ID of Struct is: {}", Struct::ID); } That is, the constant ID is associated with Struct. Like functions, associated constants work with traits and enums as well. Traits have an extra ability with associated constants that gives them some extra power. With a trait, you can use an associated constant in the same way you’d use an associated type: by declaring it, but not giving it a value. The implementor of the trait then declares its value upon implementation"