Crate defaulted 

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:

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:

Attribute	Behavior
(none)	Delegate to the field type’s Defaulted impl
#[defaulted(default = expr)]	Field is “default” when it equals expr
#[defaulted(with = func)]	Call func(&field) – true means “default”
#[defaulted(with = |v| ...)]	Inline closure predicate
#[defaulted(ignore)]	Always skip this field

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)]:

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.

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

Flag	Default	Description
std	yes	Enables impls for HashMap, Mutex, Path, etc. Implies alloc.
alloc	yes (via std)	Enables impls for String, Vec, Box, Arc, Cow, etc.
derive	no	Enables #[derive(Defaulted)]
serde	no	Enables #[skip_serializing_defaults] attribute macro
serde-json	no	Enables impls for serde_json::Value and serde_json::Map
serde-yaml	no	Enables impls for serde_yaml::Value and serde_yaml::Mapping
bytes	no	Enables impls for Bytes and BytesMut
indexmap	no	Enables impls for IndexMap and IndexSet
smallvec	no	Enables impls for SmallVec

Traits

Defaulted

Returns true if a value equals its “default” state.

Derive Macros

Defaulted

Derive macro for defaulted::Defaulted.