An implementation is an item that associates items with an implementing type.

There are two types of implementations: inherent implementations and trait implementations.

Implementations are defined with the keyword impl.

An inherent implementation is defined as the sequence of the impl keyword, generic type declarations, a path to a nominal type, a where clause, and a bracketed set of associable items.

The nominal type is called the implementing type and the associable items are the associated items to the implementing type.

Inherent implementations associate the associated items to the implementing type.

The associated item has a path of a path to the implementing type followed by the associate item's path component.

Inherent implementations cannot contain associated type aliases.

A type can have multiple inherent implementations.

The implementing type must be defined within the same crate.

# #![allow(unused_variables)]
#fn main() {
struct Point {x: i32, y: i32}

impl Point {
    fn log(&self) {
        println!("Point is at ({}, {})", self.x, self.y);
    }
}

let my_point = Point {x: 10, y:11};
my_point.log();
#}

A trait implementation is defined like an inherent implementation except that the optional generic type declarations is followed by a trait followed by the keyword for.

The trait is known as the implemented trait.

The implementing type implements the implemented trait.

A trait implementation must define all non-default associated items declared by the implemented trait, may redefine default associated items defined by the implemented trait, and cannot define any other items.

The path to the associated items is < followed by a path to the implementing type followed by as followed by a path to the trait followed by > as a path component followed by the associated item's path component.

# #![allow(unused_variables)]
#fn main() {
# #[derive(Copy, Clone)]
# struct Point {x: f64, y: f64};
# type Surface = i32;
# struct BoundingBox {x: f64, y: f64, width: f64, height: f64};
# trait Shape { fn draw(&self, Surface); fn bounding_box(&self) -> BoundingBox; }
# fn do_draw_circle(s: Surface, c: Circle) { }
struct Circle {
    radius: f64,
    center: Point,
}

impl Copy for Circle {}

impl Clone for Circle {
    fn clone(&self) -> Circle { *self }
}

impl Shape for Circle {
    fn draw(&self, s: Surface) { do_draw_circle(s, *self); }
    fn bounding_box(&self) -> BoundingBox {
        let r = self.radius;
        BoundingBox {
            x: self.center.x - r,
            y: self.center.y - r,
            width: 2.0 * r,
            height: 2.0 * r,
        }
    }
}
#}

A trait implementation is consider incoherent if either the orphan check fails or there are overlapping implementation instances.

Two trait implementations overlap when there is a non-empty intersection of the traits the implementation is for, the implementations can be instantiated with the same type.

The Orphan Check states that every trait implementation must meet either of the following conditions:

1. The trait being implemented is defined in the same crate.

2. At least one of either Self or a generic type parameter of the trait must meet the following grammar, where C is a nominal type defined within the containing crate:

    T = C
      | &C
      | &mut C
      | Box<C>
   

An implementation can take type and lifetime parameters, which can be used in the rest of the implementation. Type parameters declared for an implementation must be used at least once in either the trait or the implementing type of an implementation. Implementation parameters are written directly after the impl keyword.

# #![allow(unused_variables)]
#fn main() {
# trait Seq<T> { fn dummy(&self, _: T) { } }
impl<T> Seq<T> for Vec<T> {
    /* ... */
}
impl Seq<bool> for u32 {
    /* Treat the integer as a sequence of bits */
}
#}