Attribute Macro trait_variant::make

#[make]

Creates a specialized version of a base trait that adds bounds to async fn and/or -> impl Trait return types.

#[trait_variant::make(Send)]
trait IntFactory {
    async fn make(&self) -> i32;
    fn stream(&self) -> impl Iterator<Item = i32>;
    fn call(&self) -> u32;
}

The above example causes the trait to be rewritten as:

trait IntFactory: Send {
    fn make(&self) -> impl Future<Output = i32> + Send;
    fn stream(&self) -> impl Iterator<Item = i32> + Send;
    fn call(&self) -> u32;
}

Note that ordinary methods such as call are not affected.

If you want to preserve an original trait untouched, make can be used to create a new trait with bounds on async fn and/or -> impl Trait return types.

#[trait_variant::make(IntFactory: Send)]
trait LocalIntFactory {
    async fn make(&self) -> i32;
    fn stream(&self) -> impl Iterator<Item = i32>;
    fn call(&self) -> u32;
}

The example causes a second trait called IntFactory to be created. Implementers of the trait can choose to implement the variant instead of the original trait. The macro creates a blanket impl which ensures that any type which implements the variant also implements the original trait.