Ambassador - Delegate trait implementations via procedural macros

Delegating the implementation of traits to enum variants or fields of a struct normally requires a lot of boilerplate code. Ambassador is an attempt to eliminate that boilerplate by deriving the delegating trait implementation via procedural macros.

The minimum supported Rust version is 1.40.0.

Installation

cargo add ambassador

General Usage

#[delegatable_trait]

First we need to make our trait available for delegation by adding a #[delegatable_trait] attribute to it (this also makes your trait delegatable in other crates):

use ambassador::delegatable_trait;

#[delegatable_trait] // <-------
pub trait Shout {
    fn shout(&self, input: &str) -> String;
}

#[derive(Delegate)] & #[delegate(Trait)]

Now we can delegate the implementation of our trait to a struct field/enum variants by adding #[derive(Delegate)] and its associated attribute #[delegate(Trait)] to it:

use ambassador::Delegate;

pub struct Cat;

impl Shout for Cat {
    fn shout(&self, input: &str) -> String {
        format!("{} - meow!", input)
    }
}

#[derive(Delegate)] // <-------
#[delegate(Shout)] // <-------- Delegate implementation of Shout to struct field
pub struct WrappedCat(Cat);

#[delegate(..., target = "foo")] - target key

For structs with multiple fields, the field that should act as delegation target can be specified via the target key:

#[derive(Delegate)]
#[delegate(Shout, target = "foo")] // <-------- Delegate implementation of Shout to struct field .foo
pub struct WrappedCats {
  foo: Cat,
  bar: Cat,
}

This also works for tuple structs with multiple fields, by using their index as a target key:

#[derive(Delegate)]
#[delegate(Shout, target = "1")] // <-------- Delegate implementation of Shout to second field of type Dog
pub struct WrappedAnimals(Cat, Dog);

For remote traits: #[delegatable_trait_remote]

If you want to make an existing trait that lives outside you crate available for delegation, you can do so by copy-pasting it's signature into your code and using the #[delegatable_trait_remote] attribute (see full code sample):

use ambassador::delegatable_trait_remote;
use std::fmt::Display;

#[delegatable_trait_remote]
trait Display {
    fn fmt(&self, f: &mut ::std::fmt::Formatter) -> Result<(), ::std::fmt::Error>;
}

#[derive(Delegate)]
#[delegate(Display)] // <-------- Delegate implementation of Display to struct field
pub struct WrappedCat(Cat);

Usage Examples

In this example we have a trait Shout that is implemented for both Cat and Dog.

Delegate to enum variants

We now add an Animal enum and add a delegated trait implementation for Shout, that calls the respective implementations of the enum variants:

use ambassador::{delegatable_trait, Delegate};

#[delegatable_trait]
pub trait Shout {
    fn shout(&self, input: &str) -> String;
}

pub struct Cat;

impl Shout for Cat {
    fn shout(&self, input: &str) -> String {
        format!("{} - meow!", input)
    }
}

pub struct Dog;

impl Shout for Dog {
    fn shout(&self, input: &str) -> String {
        format!("{} - wuff!", input)
    }
}

#[derive(Delegate)]
#[delegate(Shout)]
pub enum Animal {
    Cat(Cat),
    Dog(Dog),
}

pub fn main() {
    let foo_animal = Animal::Cat(Cat);
    println!("{}", foo_animal.shout("BAR"));
}

Delegate to tuple struct field

Delegating a trait implementation for a tuple struct (only single-field tuples supported for now), for e.g. a newtype pattern.

use ambassador::{delegatable_trait, Delegate};

#[delegatable_trait]
pub trait Shout {
    fn shout(&self, input: &str) -> String;
}

pub struct Cat;

impl Shout for Cat {
    fn shout(&self, input: &str) -> String {
        format!("{} - meow!", input)
    }
}

#[derive(Delegate)]
#[delegate(Shout)]
pub struct WrappedCat(Cat);

pub fn main() {
    let foo_animal = WrappedCat(Cat);
    println!("{}", foo_animal.shout("BAR"));
}

Delegate to struct field

Delegating a trait implementation for a normal struct

use ambassador::{delegatable_trait, Delegate};

#[delegatable_trait]
pub trait Shout {
    fn shout(&self, input: &str) -> String;
}

pub struct Cat;

impl Shout for Cat {
    fn shout(&self, input: &str) -> String {
        format!("{} - meow!", input)
    }
}

#[derive(Delegate)]
#[delegate(Shout)]
pub struct WrappedCat {
    inner_cat: Cat,
}

pub fn main() {
    let foo_animal = WrappedCat { inner_cat: Cat };
    println!("{}", foo_animal.shout("BAR"));
}

License