RFC: Associated traits

[sandersaares]

Allow traits to declare associated traits — named trait constraints that are defined abstractly in a trait and given concrete values in implementations. Just as associated types let a trait abstract over which type is used, associated traits let a trait abstract over which constraints are imposed. This is the trait-level analog of associated types.

#![feature(associated_traits)]

// 1. Declare an associated trait in a trait definition.
trait Container {
    trait Elem;
    fn process<T: Self::Elem>(&self, item: T);
}

// 2. Each impl chooses the concrete constraints.
impl Container for MyContainer {
    trait Elem = Send + Clone;
    fn process<T: Self::Elem>(&self, item: T) { /* ... */ }
}

// 3. Generic code outside the impl uses `C::Elem` as a bound.
//    This is the primary use case: the caller is generic over
//    the constraints without knowing what they are.
fn process_item<C: Container, T: C::Elem>(container: &C, item: T) {
    container.process(item);
}

// 4. Fully-qualified syntax also works.
fn process_qualified<C: Container, T: <C as Container>::Elem>(container: &C, item: T) {
    container.process(item);
}

Previous discussion in #2190.

Prototype implementation: https://github.com/sandersaares/rust/tree/associated-traits

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