Reducing boilerplate in the "clone to Box" pattern


I’m working on some code that uses dynamic polymorphism extensively. To clone trait instances I’ve end up using many copies of the “clone to Box” pattern suggested in various places (SO: “How to clone a struct storing a trait object?”, users.rust-lang: “Is it possible to clone a boxed trait object?”), viz.:

trait Trait: TraitClone {}

trait TraitClone {
    fn clone_to_box(&self) -> Box<Trait>;

impl<T> TraitClone for T where T: 'static + Trait + Clone {
    fn clone_to_box(&self) -> Box<Trait> {

impl Clone for Box<Trait> {
    fn clone(&self) -> Box<Trait> {

struct Impl {}
impl Trait for Impl {}

As I say I have many uses of this pattern. Each struct that implements Trait just needs a #[derive(Clone)], which is really slick; but every Trait needs a sibling trait and two impl blocks duplicated, which I’d like to improve.

Generic CloneToBox

I managed to write a generic trait CloneToBox<T> to replace TraitClone:

trait CloneToBox {
    type Boxed;
    fn clone_to_box(&self) -> Box<Self::Boxed>;

impl<T> CloneToBox for T where T: Clone {
    type Boxed = T;
    fn clone_to_box(&self) -> Box<Self::Boxed> {

trait Trait: CloneToBox {}

// Rest as before

I’m amazed that this compiles (see Rust playground), as with a plain generic type on CloneToBox (not an associated type) compilation fails citing a circular reference in Trait's supertraits.

This seems a little cleaner than my first approach and is shorter than the other implementations I’ve found while searching. Any thoughts on putting this in a crate or the standard library?


Oops, I spoke too soon!

It seems impossible to write the type of Trait in the version that uses the generic CloneToBox supertrait.

fn example() {
    // This doesn't compile with "missing associated type `Boxed` value"
    let t1: &Trait = &Impl {};
    // This compiles, but is not polymorphic over implementations of Trait
    let t2: &Trait<Boxed = Impl> = &Impl {};
    // This doesn't compile with "missing associated type `Boxed` value" on the
    // inner Trait, i.e. this
    //                      |
    let t3: &Trait<Boxed = Trait> = &Impl {};

Can anyone make this work generically? Any other approaches people have tried?

I also considered using a macro to declare and implement TraitClone in a DRY way, but haven’t started trying to write it yet.