Function deserialize_default_from_empty_object

pub fn deserialize_default_from_empty_object<'de, D, T>(
    deserializer: D,
) -> Result<T, D::Error>
where
    D: Deserializer<'de>,
    T: Deserialize<'de> + Default,

Deserializes default value from nullable value or empty object. If the original value is null or {}, Default::default() is used.

Example:

use serde_aux::prelude::*;

#[derive(serde::Serialize, serde::Deserialize, Debug)]
struct MyStruct {
    #[serde(deserialize_with = "deserialize_default_from_empty_object")]
    empty_as_default: Option<MyInnerStruct>,
}

#[derive(serde::Serialize, serde::Deserialize, Debug)]
struct MyInnerStruct {
    mandatory: u64,
}

let s = r#" { "empty_as_default": { "mandatory": 42 } } "#;
let a: MyStruct = serde_json::from_str(s).unwrap();
assert_eq!(a.empty_as_default.unwrap().mandatory, 42);

let s = r#" { "empty_as_default": null } "#;
let a: MyStruct = serde_json::from_str(s).unwrap();
assert!(a.empty_as_default.is_none());

let s = r#" { "empty_as_default": {} } "#;
let a: MyStruct = serde_json::from_str(s).unwrap();
assert!(a.empty_as_default.is_none());

let s = r#" { "empty_as_default": { "unknown": 42 } } "#;
assert!(serde_json::from_str::<MyStruct>(s).is_err());