To dispatch on traits relating two types, where the second type that co-satisfies the trait is uniquely determined by the first, you can use associated types and generics in your code. Here’s an example illustrating how this can be achieved:

trait Trait {
    type AssociatedType;

    fn method(&self) -> Self::AssociatedType;
}

struct TypeA;

struct TypeB;

impl Trait for TypeA {
    type AssociatedType = TypeB;

    fn method(&self) -> Self::AssociatedType {
        TypeB
    }
}

struct TypeC;

struct TypeD;

impl Trait for TypeC {
    type AssociatedType = TypeD;

    fn method(&self) -> Self::AssociatedType {
        TypeD
    }
}

fn main() {
    let a = TypeA;
    let b = a.method(); // TypeB

    let c = TypeC;
    let d = c.method(); // TypeD
}

In this example, the Trait has an associated type called AssociatedType. Each type that implements Trait specifies its own unique associated type. By using generics and associated types, you can dispatch on the trait and retrieve the associated type specific to each implementing type.

Note that the example uses two different structs (TypeA, TypeC) that implement Trait and have their own unique associated types (TypeB, TypeD). When calling the method function on an instance of each struct, the appropriate associated type is returned.

By leveraging associated types and generics in this manner, you can achieve dispatch based on traits relating two types, where the second type that co-satisfies the trait is uniquely determined by the first.