facet-default

Custom defaults

facet-default derives Default through the Facet plugin chain, with per-field values, function calls, and explicit enum default variants. Reach for it when plain #[derive(Default)] is close, but your config or domain type needs defaults written next to the fields they affect.

Install

Add facet and facet-default to your crate.

Minimal example

rust
use facet::Facet;

#[derive(Facet, Debug, PartialEq)]
#[facet(derive(Default))]
struct Config {
    #[facet(default = "localhost")]
    host: String,

    #[facet(default = 8080u16)]
    port: u16,

    #[facet(default = default_timeout())]
    timeout_secs: u64,

    debug: bool,
}

fn default_timeout() -> u64 {
    30
}

let config = Config::default();
assert_eq!(config.host, "localhost");
assert_eq!(config.port, 8080);
assert_eq!(config.timeout_secs, 30);
assert!(!config.debug);

The field defaults are real Rust expressions. String literals are converted for String fields, and omitted fields fall back to that field type's own Default::default().

Field defaults

Use #[facet(default = ...)] for a literal or expression, and #[facet(default)] when you want to be explicit about using Default::default() for that field. Type suffixes such as 8080u16 are useful when the expression would otherwise be ambiguous.

Enum defaults

For enums, mark the variant that Default::default() should return:

rust
use facet::Facet;

#[derive(Facet, Debug, PartialEq)]
#[facet(derive(Default))]
#[repr(u8)]
enum Status {
    #[facet(default::variant)]
    Pending,
    Active,
    Done,
}

assert_eq!(Status::default(), Status::Pending);

Default variants can also have fields; those fields use their own facet default attributes or their type's Default implementation.

  • figue — layer CLI, env, files, and defaults into one config type
  • facet-json — let missing JSON fields use facet defaults
  • facet-validate — pair defaults with constraints
  • Ecosystem — other facet derive plugins and format crates