derive macros: auto-implementing common traits

Manually implementing Debug, Clone, PartialEq and similar traits for every struct is boilerplate-heavy and error-prone.

Use #[derive(...)] to auto-implement standard and library traits:

use serde::{Deserialize, Serialize};

#[derive(
    Debug,       // enables {:?} formatting
    Clone,       // enables .clone()
    PartialEq,   // enables == and !=
    Eq,          // total equality (requires PartialEq)
    Hash,        // enables use as HashMap key
    Serialize,   // serde JSON serialization
    Deserialize, // serde JSON deserialization
)]
struct User {
    id: u64,
    username: String,
    email: String,
}

let u1 = User { id: 1, username: String::from("alice"), email: String::from("a@b.com") };
let u2 = u1.clone();
println!("{:?}", u1);
assert_eq!(u1, u2);

Derive macros expand at compile time, generating correct boilerplate implementations based on the struct's fields. They are zero-cost — the generated code is identical to what you would write manually.