^Syntax
Trait :
   unsafe^? trait IDENTIFIER  GenericsDecl^? WhereClause^? {
     TraitItem^*
   }

TraitItem :
   TraitMethod | TraitConst | TraitType

TraitMethod :
   TypeMethod | Method

A trait describes an abstract interface that types can implement. This interface consists of associated items, which come in three varieties:

• functions
• types
• constants

All traits define an implicit type parameter Self that refers to "the type that is implementing this interface". Traits may also contain additional type parameters. These type parameters (including Self) may be constrained by other traits and so forth as usual.

Traits are implemented for specific types through separate [implementations].

Generic functions may use traits as bounds on their type parameters. This will have three effects:

• Only types that have the trait may instantiate the parameter.
• Within the generic function, the methods of the trait can be called on values that have the parameter's type. Associated types can be used in the function's signature, and associated constants can be used in expressions within the function body.
• Generic functions and types with the same or weaker bounds can use the generic type in the function body or signature.

For example:

# #![allow(unused_variables)]
#fn main() {
# type Surface = i32;
# trait Shape { fn draw(&self, Surface); }
struct Figure<S: Shape>(S, S);
fn draw_twice<T: Shape>(surface: Surface, sh: T) {
    sh.draw(surface);
    sh.draw(surface);
}
fn draw_figure<U: Shape>(surface: Surface, Figure(sh1, sh2): Figure<U>) {
    sh1.draw(surface);
    draw_twice(surface, sh2); // Can call this since U: Shape
}
#}

Type parameters can be specified for a trait to make it generic. These appear after the trait name, using the same syntax used in generic functions.

# #![allow(unused_variables)]
#fn main() {
trait Seq<T> {
    fn len(&self) -> u32;
    fn elt_at(&self, n: u32) -> T;
    fn iter<F>(&self, F) where F: Fn(T);
}
#}

Object safe traits can be the base trait of a trait object. A trait is object safe if it has the following qualities (defined in RFC 255):

• It must not require Self: Sized
• All associated functions must either have a where Self: Sized bound or
  • Not have any type parameters (although lifetime parameters are allowed)
  • Must be a method: its first parameter must be called self, with type Self, &Self, &mut Self, Box<Self>.
  • Self may only be used in the type of the receiver.
• It must not have any associated constants.

Trait bounds on Self are considered "supertraits". These are required to be acyclic. Supertraits are somewhat different from other constraints in that they affect what methods are available in the vtable when the trait is used as a trait object. Consider the following example:

# #![allow(unused_variables)]
#fn main() {
trait Shape { fn area(&self) -> f64; }
trait Circle : Shape { fn radius(&self) -> f64; }
#}

The syntax Circle : Shape means that types that implement Circle must also have an implementation for Shape. Multiple supertraits are separated by +, trait Circle : Shape + PartialEq { }. In an implementation of Circle for a given type T, methods can refer to Shape methods, since the typechecker checks that any type with an implementation of Circle also has an implementation of Shape:

# #![allow(unused_variables)]
#fn main() {
struct Foo;

trait Shape { fn area(&self) -> f64; }
trait Circle : Shape { fn radius(&self) -> f64; }
impl Shape for Foo {
    fn area(&self) -> f64 {
        0.0
    }
}
impl Circle for Foo {
    fn radius(&self) -> f64 {
        println!("calling area: {}", self.area());

        0.0
    }
}

let c = Foo;
c.radius();
#}

In type-parameterized functions, methods of the supertrait may be called on values of subtrait-bound type parameters. Referring to the previous example of trait Circle : Shape:

# #![allow(unused_variables)]
#fn main() {
# trait Shape { fn area(&self) -> f64; }
# trait Circle : Shape { fn radius(&self) -> f64; }
fn radius_times_area<T: Circle>(c: T) -> f64 {
    // `c` is both a Circle and a Shape
    c.radius() * c.area()
}
#}

Likewise, supertrait methods may also be called on trait objects.

# #![allow(unused_variables)]
#fn main() {
# trait Shape { fn area(&self) -> f64; }
# trait Circle : Shape { fn radius(&self) -> f64; }
# impl Shape for i32 { fn area(&self) -> f64 { 0.0 } }
# impl Circle for i32 { fn radius(&self) -> f64 { 0.0 } }
# let mycircle = 0i32;
let mycircle = Box::new(mycircle) as Box<Circle>;
let nonsense = mycircle.radius() * mycircle.area();
#}