Crate serde_const_default 

Serde defaults backed by const expressions.

Serde’s #[serde(default = "...")] field attribute expects a path to a zero-argument function. This crate lets fields name the expression directly and rewrites that expression into the helper function Serde already knows how to call.

Use serde_const_default on the struct, then mark fields with either #[const_default = EXPR] or #[const_default_from(EXPR)].

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,
}

let value: Config = serde_json::from_str("{}").unwrap();
assert_eq!(value.count, 7);
assert_eq!(value.name, "anonymous");

Attribute Macros

serde_const_default

Enables const-expression defaults for fields on a named struct.