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 1.53.0.

Installation

cargo add ambassador

General Usage

More examples are also available https://github.com/hobofan/ambassador/tree/master/ambassador/tests/run-pass

#[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);

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

Types that implement all the methods of a trait without implementing the trait itself, can be made to implement that trait by setting target="self". This doesn't work for traits with associated types and constants, and requires the where clause to be added explicitly (see where key). If the type doesn't actually implement the methods (possibly due to an incomplete where clause) this can cause a [unconditional_recursion] error.

A possible use case of this is when refactoring some methods of a public type into a trait, the type still needs to implement the methods outside the trait for semver reasons, and using this feature reduces the boilderplate of implementing the trait with the same methods.

#[derive(Delegate)]
#[delegate(Shout, target="self")]
pub struct Cat;

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

#[delegate(..., where = "A: Shout")] - where key

To make a delegation apply only for certain generic bounds, similar to a native where clause, you can specify a where attribute:

A where clause is automatically applied that makes sure the target field implements the trait being delegated

#[derive(Delegate)]
#[delegate(Shout, where = "A: Debug")] // <---- Delegate implementation of Shout to .foo field if foo field implements Debug
pub struct WrappedFoo<A> {
  foo: A,
}

#[delegate(Shout<X>)] - trait generics

We can also delegate traits with generics. When doing this all instances of X and 'x followed by arbitrary digits eg. X0 X12 'x3 are treated as maximally generic. The automatically added where clause ensures they are valid for the inner type being delegated to. Explict where clauses to further refine these types can be added as normal. Specific types can be used instead of X to only derive for those.

use ambassador::{delegatable_trait, Delegate};
use std::fmt::Display;

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

pub struct Cat;

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

#[derive(Delegate)]
#[delegate(Shout<X>, generics = "X")] // <-------- X is fully generic
// The automatic where clause ensures X: Display
// We could also use #[delegate(Shout<& 'a str>, generics = "'a")] to only delegate for &str
pub struct WrappedCat(Cat);

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);

For remote types #[delegate_remote]

If you want to make an existing type that lives outside you crate delegate, you can do so by copy-pasting it's definition into your code and using the #[delegate_remote] attribute (see full code sample):

If the type is a struct, not all the fields have to be public, only the ones being delegated to.

mod wrapped {
    use super::*;
    pub struct WrappedAnimals<A> {
        pub foo: Cat,
        pub bar: A,
        baz: u32, // private field
    }
}

use wrapped::*;

#[delegate_remote]
#[delegate(Shout, target = "bar")]
struct WrappedAnimals<A: Shout> {
    foo: Cat,
    bar: A,
    // We don't even have to include baz since we don't delegate to it
}

Note: Because of the orphan rule #[delegatable_trait_remote] and #[delegate_remote] can't be combined

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