serde-const-default

Use const expressions as Serde defaults.

Install

cargo add serde-const-default

Or, add it to Cargo.toml:

[dependencies]
serde-const-default = "0.1"

Usage & Examples

use serde::Deserialize;
use serde_const_default::serde_const_default;

const SOME_VALUE: u8 = 7;
const DEFAULT_NAME: &str = "anonymous";

#[serde_const_default]
#[derive(Deserialize)]
struct Config {
    #[const_default = SOME_VALUE]
    count: u8,

    #[const_default_from(DEFAULT_NAME)]
    name: String,
}

#[const_default = EXPR] generates a Serde default const fn that returns EXPR directly:

const fn generated_default() -> FieldType {
    EXPR
}

Use this form when EXPR is valid inside a const fn.

#[const_default_from(EXPR)] generates a Serde default function that converts the expression into the field type with From::from:

fn generated_default() -> FieldType {
    ::core::convert::From::from(EXPR)
}

Use const_default_from when the expression is not already the field type, for example a &'static str default for a String field.

Lazy defaults should be accessed explicitly:

use serde::Deserialize;
use std::sync::LazyLock;
use serde_const_default::serde_const_default;

static DEFAULT_NAME: LazyLock<String> = LazyLock::new(|| "lazy".to_string());

#[serde_const_default]
#[derive(Deserialize)]
struct Config {
    #[const_default_from(DEFAULT_NAME.clone())]
    name: String,
}

The macro supports named structs. A field cannot use both const_default and Serde's own #[serde(default)] attribute.