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Traits

Syntax
Trait :
   unsafe? trait IDENTIFIER  GenericParams? ( : TypeParamBounds? )? WhereClause? {
     InnerAttribute*
     AssociatedItem*
   }

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

The trait declaration defines a trait in the type namespace of the module or block where it is located. Associated items are defined as members of the trait within their respective namespaces. Associated types are defined in the type namespace. Associated constants and associated functions are defined in the value namespace.

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.

Trait functions may omit the function body by replacing it with a semicolon. This indicates that the implementation must define the function. If the trait function defines a body, this definition acts as a default for any implementation which does not override it. Similarly, associated constants may omit the equals sign and expression to indicate implementations must define the constant value. Associated types must never define the type, the type may only be specified in an implementation.

// Examples of associated trait items with and without definitions.
trait Example {
    const CONST_NO_DEFAULT: i32;
    const CONST_WITH_DEFAULT: i32 = 99;
    type TypeNoDefault;
    fn method_without_default(&self);
    fn method_with_default(&self) {}
}

Trait functions are not allowed to be const.

Trait bounds

Generic items may use traits as bounds on their type parameters.

Generic Traits

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.

trait Seq<T> {
    fn len(&self) -> u32;
    fn elt_at(&self, n: u32) -> T;
    fn iter<F>(&self, f: F) where F: Fn(T);
}

Object Safety

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):

  • All supertraits must also be object safe.
  • Sized must not be a supertrait. In other words, it must not require Self: Sized.
  • It must not have any associated constants.
  • It must not have any associated types with generics.
  • All associated functions must either be dispatchable from a trait object or be explicitly non-dispatchable:
    • Dispatchable functions must:
      • Not have any type parameters (although lifetime parameters are allowed).
      • Be a method that does not use Self except in the type of the receiver.
      • Have a receiver with one of the following types:
      • Not have an opaque return type; that is,
        • Not be an async fn (which has a hidden Future type).
        • Not have a return position impl Trait type (fn example(&self) -> impl Trait).
      • Not have a where Self: Sized bound (receiver type of Self (i.e. self) implies this).
    • Explicitly non-dispatchable functions require:
      • Have a where Self: Sized bound (receiver type of Self (i.e. self) implies this).
# use std::rc::Rc;
# use std::sync::Arc;
# use std::pin::Pin;
// Examples of object safe methods.
trait TraitMethods {
    fn by_ref(self: &Self) {}
    fn by_ref_mut(self: &mut Self) {}
    fn by_box(self: Box<Self>) {}
    fn by_rc(self: Rc<Self>) {}
    fn by_arc(self: Arc<Self>) {}
    fn by_pin(self: Pin<&Self>) {}
    fn with_lifetime<'a>(self: &'a Self) {}
    fn nested_pin(self: Pin<Arc<Self>>) {}
}
# struct S;
# impl TraitMethods for S {}
# let t: Box<dyn TraitMethods> = Box::new(S);
// This trait is object-safe, but these methods cannot be dispatched on a trait object.
trait NonDispatchable {
    // Non-methods cannot be dispatched.
    fn foo() where Self: Sized {}
    // Self type isn't known until runtime.
    fn returns(&self) -> Self where Self: Sized;
    // `other` may be a different concrete type of the receiver.
    fn param(&self, other: Self) where Self: Sized {}
    // Generics are not compatible with vtables.
    fn typed<T>(&self, x: T) where Self: Sized {}
}

struct S;
impl NonDispatchable for S {
    fn returns(&self) -> Self where Self: Sized { S }
}
let obj: Box<dyn NonDispatchable> = Box::new(S);
obj.returns(); // ERROR: cannot call with Self return
obj.param(S);  // ERROR: cannot call with Self parameter
obj.typed(1);  // ERROR: cannot call with generic type
# use std::rc::Rc;
// Examples of non-object safe traits.
trait NotObjectSafe {
    const CONST: i32 = 1;  // ERROR: cannot have associated const

    fn foo() {}  // ERROR: associated function without Sized
    fn returns(&self) -> Self; // ERROR: Self in return type
    fn typed<T>(&self, x: T) {} // ERROR: has generic type parameters
    fn nested(self: Rc<Box<Self>>) {} // ERROR: nested receiver not yet supported
}

struct S;
impl NotObjectSafe for S {
    fn returns(&self) -> Self { S }
}
let obj: Box<dyn NotObjectSafe> = Box::new(S); // ERROR
// Self: Sized traits are not object-safe.
trait TraitWithSize where Self: Sized {}

struct S;
impl TraitWithSize for S {}
let obj: Box<dyn TraitWithSize> = Box::new(S); // ERROR
// Not object safe if `Self` is a type argument.
trait Super<A> {}
trait WithSelf: Super<Self> where Self: Sized {}

struct S;
impl<A> Super<A> for S {}
impl WithSelf for S {}
let obj: Box<dyn WithSelf> = Box::new(S); // ERROR: cannot use `Self` type parameter

Supertraits

Supertraits are traits that are required to be implemented for a type to implement a specific trait. Furthermore, anywhere a generic or trait object is bounded by a trait, it has access to the associated items of its supertraits.

Supertraits are declared by trait bounds on the Self type of a trait and transitively the supertraits of the traits declared in those trait bounds. It is an error for a trait to be its own supertrait.

The trait with a supertrait is called a subtrait of its supertrait.

The following is an example of declaring Shape to be a supertrait of Circle.

trait Shape { fn area(&self) -> f64; }
trait Circle : Shape { fn radius(&self) -> f64; }

And the following is the same example, except using where clauses.

trait Shape { fn area(&self) -> f64; }
trait Circle where Self: Shape { fn radius(&self) -> f64; }

This next example gives radius a default implementation using the area function from Shape.

# trait Shape { fn area(&self) -> f64; }
trait Circle where Self: Shape {
    fn radius(&self) -> f64 {
        // A = pi * r^2
        // so algebraically,
        // r = sqrt(A / pi)
        (self.area() /std::f64::consts::PI).sqrt()
    }
}

This next example calls a supertrait method on a generic parameter.

# trait Shape { fn area(&self) -> f64; }
# trait Circle : Shape { fn radius(&self) -> f64; }
fn print_area_and_radius<C: Circle>(c: C) {
    // Here we call the area method from the supertrait `Shape` of `Circle`.
    println!("Area: {}", c.area());
    println!("Radius: {}", c.radius());
}

Similarly, here is an example of calling supertrait methods on trait objects.

# trait Shape { fn area(&self) -> f64; }
# trait Circle : Shape { fn radius(&self) -> f64; }
# struct UnitCircle;
# impl Shape for UnitCircle { fn area(&self) -> f64 { std::f64::consts::PI } }
# impl Circle for UnitCircle { fn radius(&self) -> f64 { 1.0 } }
# let circle = UnitCircle;
let circle = Box::new(circle) as Box<dyn Circle>;
let nonsense = circle.radius() * circle.area();

Unsafe traits

Traits items that begin with the unsafe keyword indicate that implementing the trait may be unsafe. It is safe to use a correctly implemented unsafe trait. The trait implementation must also begin with the unsafe keyword.

Sync and Send are examples of unsafe traits.

Parameter patterns

Function or method declarations without a body only allow IDENTIFIER or _ wild card patterns. mut IDENTIFIER is currently allowed, but it is deprecated and will become a hard error in the future.

In the 2015 edition, the pattern for a trait function or method parameter is optional:

// 2015 Edition
trait T {
    fn f(i32);  // Parameter identifiers are not required.
}

The kinds of patterns for parameters is limited to one of the following:

Beginning in the 2018 edition, function or method parameter patterns are no longer optional. Also, all irrefutable patterns are allowed as long as there is a body. Without a body, the limitations listed above are still in effect.

trait T {
    fn f1((a, b): (i32, i32)) {}
    fn f2(_: (i32, i32));  // Cannot use tuple pattern without a body.
}

Item visibility

Trait items syntactically allow a Visibility annotation, but this is rejected when the trait is validated. This allows items to be parsed with a unified syntax across different contexts where they are used. As an example, an empty vis macro fragment specifier can be used for trait items, where the macro rule may be used in other situations where visibility is allowed.

macro_rules! create_method {
    ($vis:vis $name:ident) => {
        $vis fn $name(&self) {}
    };
}

trait T1 {
    // Empty `vis` is allowed.
    create_method! { method_of_t1 }
}

struct S;

impl S {
    // Visibility is allowed here.
    create_method! { pub method_of_s }
}

impl T1 for S {}

fn main() {
    let s = S;
    s.method_of_t1();
    s.method_of_s();
}