serde: serializing and deserializing Rust types to JSON

Converting Rust structs to and from JSON requires significant boilerplate without a library, and getting field naming conventions right is tricky.

Add serde and serde_json to Cargo.toml, derive Serialize/Deserialize, and use serde attributes for customization:

# Cargo.toml
[dependencies]
serde = { version = "1", features = ["derive"] }
serde_json = "1"

use serde::{Deserialize, Serialize};

#[derive(Debug, Serialize, Deserialize)]
#[serde(rename_all = "camelCase")]  // snake_case fields -> camelCase JSON
struct ApiResponse {
    user_id: u64,
    display_name: String,
    #[serde(skip_serializing_if = "Option::is_none")]
    avatar_url: Option<String>,
    #[serde(default)]
    is_active: bool,
}

let json = r#"{"userId":42,"displayName":"Alice"}"#;
let resp: ApiResponse = serde_json::from_str(json).unwrap();
println!("{:?}", resp);

let serialized = serde_json::to_string_pretty(&resp).unwrap();
println!("{}", serialized);

Serde is a zero-overhead serialization framework — the derive macros generate specialized code at compile time rather than using runtime reflection, making it faster than most alternatives in other languages.