Macro serde_with::serde_conv

macro_rules! serde_conv {
    ($m:ident, $t:ty, $ser:expr, $de:expr) => { ... };
    ($vis:vis $m:ident, $t:ty, $ser:expr, $de:expr) => { ... };
}

Create new conversion adapters from functions

The macro lets you create a new converter, which is usable for serde’s with-attribute and #[serde_as]. Its main use case is to write simple converters for types, which are not serializable. Another use-case is to change the serialization behavior if the implemented Serialize/Deserialize trait is insufficient.

The macro takes four arguments:

1. The name of the converter type. The type can be prefixed with a visibility modifies like pub or pub(crate). By default, the type is not marked as public (pub(self)).
2. The type T we want to extend with custom behavior.
3. A function or macro taking a &T and returning a serializable type.
4. A function or macro taking a deserializable type and returning a Result<T, E>. The error type E must implement Display.

Example

In this example, we write custom serialization behavior for a Rgb type. We want to serialize it as a [u8; 3].

#[derive(Clone, Copy, Debug, PartialEq)]
struct Rgb {
    red: u8,
    green: u8,
    blue: u8,
}

serde_with::serde_conv!(
    RgbAsArray,
    Rgb,
    |rgb: &Rgb| [rgb.red, rgb.green, rgb.blue],
    |value: [u8; 3]| -> Result<_, std::convert::Infallible> {
        Ok(Rgb {
            red: value[0],
            green: value[1],
            blue: value[2],
        })
    }
);

//////////////////////////////////////////////////

// We define some colors to be used later

let green = Rgb {red: 0, green: 255, blue: 0};
let orange = Rgb {red: 255, green: 128, blue: 0};
let pink = Rgb {red: 255, green: 0, blue: 255};

//////////////////////////////////////////////////

// We can now use the `RgbAsArray` adapter with `serde_as`.

#[serde_with::serde_as]
#[derive(Debug, PartialEq, Serialize, Deserialize)]
struct Colors {
    #[serde_as(as = "RgbAsArray")]
    one_rgb: Rgb,
    #[serde_as(as = "Vec<RgbAsArray>")]
    rgbs_in_vec: Vec<Rgb>,
}

let data = Colors {
    one_rgb: orange,
    rgbs_in_vec: vec![green, pink],
};
let json = serde_json::json!({
    "one_rgb": [255, 128, 0],
    "rgbs_in_vec": [
        [0, 255, 0],
        [255, 0, 255]
    ]
});

assert_eq!(json, serde_json::to_value(&data).unwrap());
assert_eq!(data, serde_json::from_value(json).unwrap());

//////////////////////////////////////////////////

// The types generated by `serde_conv` is also compatible with serde's with attribute

#[derive(Debug, PartialEq, Serialize, Deserialize)]
struct ColorsWith {
    #[serde(with = "RgbAsArray")]
    rgb_with: Rgb,
}

let data = ColorsWith {
    rgb_with: pink,
};
let json = serde_json::json!({
    "rgb_with": [255, 0, 255]
});

assert_eq!(json, serde_json::to_value(&data).unwrap());
assert_eq!(data, serde_json::from_value(json).unwrap());