Registering constants

Constants can be used to share fixed values from Rust code to the Godot engine.

See also GDScript reference for constants.

Constant declaration

Constants are declared as const items in Rust, inside the inherent impl block of a class.
static declarations cannot be used.

The attribute #[constant] makes the constant available to Godot.

#![allow(unused)]
fn main() {
#[godot_api]
impl Monster {
    #[constant]
    const DEFAULT_HP: i32 = 100;

    #[func]
    fn from_name_hp(name: GString, hitpoints: i32) -> Gd<Self> { ... }
}
}

Usage in GDScript would look as follows:

var nom = Monster.from_name_hp("Nomster", Monster.DEFAULT_HP)
var orc = Monster.from_name_hp("Orc", 200)

(This particular example might be better suited for default parameters once they are implemented, but it illustrates the point.)

Limitations

Godot supports only integers to be registered as constants via GDExtension API.

You can work around this by registering a static function, called as Monster.DEFAULT_NAME() in GDScript.

#![allow(unused)]
fn main() {
#[godot_api]
impl Monster {
    #[func(rename = "DEFAULT_NAME")]
    fn default_name() -> GString {
        "Monster_001".into()
    }
}
}

While you could technically use read-only properties, this is problematic because:

• You need an existing instance of the class.
• Every object occupies space for the constant.¹

It's really just syntax, an extra () will not derail your game. If you have a heavier value that you don't want to recompute (e.g. array), you can always store it in a thread_local! in Rust.

Footnotes