Expressions

The default = <expr> attribute accepts any valid Rust expression — string literals, typed literals, constants, and method calls. The expression is passed through .into(), so it does not need to match the exact field type.

Literals

String literals and typed numeric literals work directly:

use moxy::Default;

#[derive(Default)]
struct Server {
    #[moxy(default = "0.0.0.0")]
    pub bind: String,
    #[moxy(default = 443u16)]
    pub port: u16,
    #[moxy(default = true)]
    pub tls: bool,
}

let s = Server::default();
assert_eq!(s.bind, "0.0.0.0");
assert_eq!(s.port, 443);
assert_eq!(s.tls, true);

impl Into<T> Coercion

The default expression is wrapped in .into(), so any type implementing Into<T> for the field type works:

use moxy::Default;

#[derive(Default)]
struct App {
    // &str → String via Into
    #[moxy(default = "my-app")]
    pub name: String,

    // method call returning String — identity Into
    #[moxy(default = "api".to_string())]
    pub prefix: String,
}

let app = App::default();
assert_eq!(app.name, "my-app");
assert_eq!(app.prefix, "api");

Constants

Named constants and static values work as expressions:

use moxy::Default;

const MAX_RETRIES: u32 = 3;

#[derive(Default)]
struct Client {
    #[moxy(default = MAX_RETRIES)]
    pub retries: u32,
}

let c = Client::default();
assert_eq!(c.retries, 3);

Complex Expressions

Any expression valid in value position works — function calls, constructor calls, and more:

use moxy::Default;

#[derive(Default)]
struct Collection {
    #[moxy(default = Vec::new())]
    pub items: Vec<String>,

    #[moxy(default = String::from("unnamed"))]
    pub label: String,
}

let c = Collection::default();
assert!(c.items.is_empty());
assert_eq!(c.label, "unnamed");

The expression is evaluated each time default() is called — there is no caching or static initialization.