The one thing I don't love about Swift's Codable is the lack of customizability in the "magic" part: the part where the compiler generates the Encodable/Decodable implementations. Most notably, the compiler can't generate implementations for enums. The only thing that Swift supports customizing without fully implementing the methods for Encodable and Decodable is the name of the keys, using a custom CodingKeys type.

Serde, an equivalent third-party crate in Rust, supports a lot of customization which I find invaluable. It can (de)serialize values like this with ease:

    {"type": "location", "value": {"latitude": 0, "longitude": 0}}

in a very small amount of code:

    #[derive(Serialize, Deserialize)]
    #[serde(tag = "type", content = "value")]
    #[serde(rename_all = "lowercase")]
    enum Value {
        Location { latitude: f64, longitude: f64 },
        String(String),
        ...
    }

Serde also supports customizing serialization on a per-field basis without having to implement the entire protocol, which is nice:

    #[derive(Deserialize, Serialize)]
    struct Record {
        #[serde(with = "chrono::serde::ts_seconds")]
        updated: DateTime<Utc>
    }

I really hope that Swift has better ways (like the above) to customize Codable in the future. I find myself implementing the protocol myself in 90% of cases, whereas I very rarely have to do that for Serde.