defaulted 0.1.1

Trait and derive macro for testing whether a value equals its default state, with per-field customization and optional serde integration.

Core trait

[Defaulted] is a single-method trait:

pub trait Defaulted {
    fn is_defaulted(&self) -> bool;
}

Built-in impls cover all primitive types, String, Option<T>, Vec<T>, HashMap, HashSet, and (with feature flags) IndexMap/IndexSet and serde_json types.

Derive macro

Enable the derive feature and annotate your struct or enum:

# #[cfg(feature = "derive")] {
use defaulted::Defaulted;

#[derive(Default, Defaulted)]
struct Config {
    width: u32,
    height: u32,
    label: String,
}

assert!(Config::default().is_defaulted());
assert!(!Config { width: 1920, ..Config::default() }.is_defaulted());
# }

Per-field attributes

Each field can carry #[defaulted(...)] to control how it participates:

# #[cfg(feature = "derive")] {
use defaulted::Defaulted;

#[derive(Defaulted)]
struct Request {
    path: String,

    #[defaulted(default = 30)]
    timeout_secs: u64,

    #[defaulted(with = Vec::is_empty)]
    tags: Vec<String>,

    #[defaulted(ignore)]
    _request_id: u64,
}
# }

Struct-level attributes

#[defaulted(Default)] on a struct generates a Default impl whose per-field values match the #[defaulted(default = expr)] annotations, keeping the two in sync automatically.

Enums

Mark exactly one variant with #[defaulted(default)]:

# #[cfg(feature = "derive")] {
use defaulted::Defaulted;

#[derive(Defaulted)]
enum Status {
    #[defaulted(default)]
    Idle,
    Running,
    Failed(u32),
}

assert!(Status::Idle.is_defaulted());
assert!(!Status::Running.is_defaulted());
# }

Serde integration (serde feature)

#[skip_serializing_defaults] is an attribute macro that rewrites struct fields to add #[serde(skip_serializing_if = "...")] automatically, so fields at their default value are omitted from serialized output. #[defaulted(default = expr)] fields also get a matching #[serde(default = "...")] so they round-trip correctly.

# #[cfg(all(feature = "serde", feature = "serde-json", feature = "derive"))] {
use defaulted::Defaulted;
use serde::Serialize;

#[defaulted::skip_serializing_defaults]
#[derive(Serialize, Defaulted)]
#[defaulted(Default)]
struct ApiConfig {
    #[defaulted(default = "https://api.example.com")]
    base_url: String,

    #[defaulted(default = 60)]
    timeout_secs: u32,

    tags: Vec<String>,
}

let c = ApiConfig::default();
assert_eq!(serde_json::to_string(&c).unwrap(), "{}");
# }

Feature flags