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Supertraits / Trait Dependencies

Traits can be extended by new traits.

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

pub trait Animal {
    /* methods common to all animals */
}

pub trait Mammal: Animal {
    /* methods only for mammals */
}

// From stdlib

pub trait Ord: Eq + PartialOrd {
    /* methods for Ord */
}

  • When authoring a trait, you can specify traits that a type must also. These are called supertraits.

    For the example above, any type that implements Mammal must also implement Animal.

  • These hierarchies of traits let us design systems around the behavior of complex real-world taxonomies (like fauna, machine hardware, operating system specifics, etc).

  • This is distinct from object inheritance! But it looks similar.

    • Object inheritance allows for overrides and brings in the behavior of the inherited types by default.

    • A trait having a supertrait doesn’t mean that trait can override method implementations as default implementations.

ref:

  • https://doc.rust-lang.org/reference/items/traits.html?highlight=supertrait#r-items.traits.supertraits