Default Methods & Supertraits

Traits may ship default method bodies and require supertraits (trait B: A) so implementors provide prerequisite behavior.

Recipe

trait Animal: std::fmt::Display {
    fn name(&self) -> &str;
    fn describe(&self) -> String {
        format!("{} is a {}", self.name(), self)
    }
}

When to reach for this: DRY for shared trait logic and layering traits (Display before custom summary).

Working Example

struct Cat(&'static str);
impl std::fmt::Display for Cat {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "cat")
    }
}
impl Animal for Cat {
    fn name(&self) -> &str { self.0 }
}
 
fn main() {
    let c = Cat("Milo");
    println!("{}", c.describe());
}

What this demonstrates:

• Default describe calls name and Display
• Supertrait Display bound on Animal
• Minimal impl Animal when defaults suffice

Deep Dive

Default methods are inherited unless overridden. Supertraits compose capability: trait Iterator: IntoIterator style hierarchies (conceptually).

Gotchas

• Missing supertrait impl - Errors at impl site. Fix: Implement Display (etc.) first.
• Object safety with defaults - Some combos break dyn. Fix: Check object safety rules.
• Infinite recursion in default - Default calls itself. Fix: Call required method, not self default.
• Sealed pattern bypass - Document who may implement. Fix: Private supertrait in module.
• Breaking default changes - Semver for public traits. Fix: New trait or version bump.

Alternatives

FAQs

Related

• Traits Basics
• Trait Bounds
• Blanket & Conditional Impls
• Common Standard Traits

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.