Composition over Inheritance

# // Copyright 2025 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
pub struct Uuid([u8; 16]);

pub struct Address {
    street: String,
    city_or_province: String,
    code: String,
    country: String,
}

pub struct User {
    id: Uuid,
    address: Address,
}

• Rather than mixins or inheritance, we compose types by creating fields of different types.

  This has downsides, largely in ergonomics of field access, but gives developers a lot of control and clarity over what a type does and it has access to.

• When deriving traits, make sure all the field types of a struct or variant types of an enum implement that trait. Derive macros often assume all types that compose a new type implement that trait already.