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.
cargo add ambassador
#[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):
```rust use ambassador::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:
```rust use ambassador::Delegate;
pub struct Cat;
impl Shout for Cat { fn shout(&self, input: &str) -> String { format!("{} - meow!", input) } }
pub struct WrappedCat(Cat); ```
#[delegate(..., target = "foo")] - target keyFor structs with multiple fields, the field that should act as delegation target can be specified via the target key:
```rust
pub struct WrappedCats { foo: Cat, bar: Cat, } ```
This also works for tuple structs with multiple fields, by using their index as a target key:
```rust
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.
```rust
pub struct Cat;
impl Cat { fn shout(&self, input: &str) -> String { format!("{} - meow!", input) } } ```
#[delegate(..., where = "A: Shout")] - where keyTo 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 ```rust
pub struct WrappedFoo { foo: A, } ```
#[delegate(Shout<X>)] - trait genericsWe 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.
```rust use ambassador::{delegatable_trait, Delegate}; use std::fmt::Display;
pub trait Shout
pub struct Cat;
impl
// 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); ```
#[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):
```rust use ambassador::delegatabletraitremote; use std::fmt::Display;
trait Display { fn fmt(&self, f: &mut ::std::fmt::Formatter) -> Result<(), ::std::fmt::Error>; }
pub struct WrappedCat(Cat); ```
#[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.
```rust mod wrapped { use super::*; pub struct WrappedAnimals { pub foo: Cat, pub bar: A, baz: u32, // private field } }
use wrapped::*;
struct WrappedAnimals
Note: Because of the orphan rule #[delegatable_trait_remote] and #[delegate_remote] can't be combined
In this example we have a trait Shout that is implemented for both Cat and Dog.
We now add an Animal enum and add a delegated trait implementation for Shout,
that calls the respective implementations of the enum variants:
```rust use ambassador::{delegatable_trait, Delegate};
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) } }
pub enum Animal { Cat(Cat), Dog(Dog), }
pub fn main() { let fooanimal = Animal::Cat(Cat); println!("{}", fooanimal.shout("BAR")); } ```
Delegating a trait implementation for a tuple struct (only single-field tuples supported for now), for e.g. a newtype pattern.
```rust use ambassador::{delegatable_trait, Delegate};
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 WrappedCat(Cat);
pub fn main() { let fooanimal = WrappedCat(Cat); println!("{}", fooanimal.shout("BAR")); } ```
Delegating a trait implementation for a normal struct
```rust use ambassador::{delegatable_trait, Delegate};
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 WrappedCat { inner_cat: Cat, }
pub fn main() { let fooanimal = WrappedCat { innercat: Cat }; println!("{}", foo_animal.shout("BAR")); } ```
Licensed under either of Apache License, Version 2.0 or MIT license at your option.
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.