Traits for Polymorphism users can extend

# // Copyright 2025 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
// Crate A

pub trait Trait {
    fn use_trait(&self) {}
}

// Crate B, depends on A

pub struct Data(u8);

impl Trait for Data {}

fn main() {
    let data = Data(7u8);
    data.use_trait();
}

• We've already covered normal traits at length, but compared to enums and sealed traits they allow users to extend an API by implementing the behavior that API asks of them.

This ability for users to extend is powerful for a number of domains, from serialization to abstract representations of hardware and type safe linear algebra.

• If a trait is exposed publicly in a crate, a user depending on that crate can implement that trait for types they define.