Traits & Generics — Polymorphism in Rust
Rust's abstraction tool: interfaces + generics combined
Traits define "this type can do this behavior." Like Java interfaces, but you can implement a trait for existing types after the fact.
trait Summary {
fn summarize(&self) -> String;
}
With where T: Summary trait bounds in generic functions, the compiler generates separate functions per concrete type (monomorphization) — zero vtable cost at runtime.
dyn Trait enables runtime polymorphism (dynamic dispatch) through vtable indirect calls.
Key Points
1
Define common behavior (method signatures) with traits
2
Implement for concrete types with impl Trait for Type
3
Generics + trait bounds for compile-time polymorphism (static dispatch)
4
dyn Trait for runtime polymorphism when needed (dynamic dispatch)
Pros
- ✓ Zero-cost abstraction — generics inlined via monomorphization
- ✓ Retroactive implementation on existing types (within orphan rule)
Cons
- ✗ Monomorphization can increase binary size
- ✗ Complex trait bounds lead to long where clauses
Use Cases
Serialization — serde Serialize/Deserialize traits
Async runtime — Future trait-based async/await