Macro flattened_maybe

macro_rules! flattened_maybe {
    ($fn:ident, $field:tt) => { ... };
}

Support deserializing from flattened and non-flattened representation

When working with different serialization formats, sometimes it is more idiomatic to flatten fields, while other formats prefer nesting. Using #[serde(flatten)] only the flattened form is supported.

This helper creates a function, which support deserializing from either the flattened or the nested form. It gives an error, when both forms are provided. The flatten attribute is required on the field such that the helper works. The serialization format will always be flattened.

Examples

// Setup the types
#[derive(Deserialize, Debug)]
struct S {
    #[serde(flatten, deserialize_with = "deserialize_t")]
    t: T,
}

#[derive(Deserialize, Debug)]
struct T {
    i: i32,
}

// The macro creates custom deserialization code.
// You need to specify a function name and the field name of the flattened field.
serde_with::flattened_maybe!(deserialize_t, "t");

// Supports both flattened
let j = r#" {"i":1} "#;
assert!(serde_json::from_str::<S>(j).is_ok());

// and non-flattened versions.
let j = r#" {"t":{"i":1}} "#;
assert!(serde_json::from_str::<S>(j).is_ok());

// Ensure that the value is given
let j = r#" {} "#;
assert!(serde_json::from_str::<S>(j).is_err());

// and only occurs once, not multiple times.
let j = r#" {"i":1,"t":{"i":1}} "#;
assert!(serde_json::from_str::<S>(j).is_err());