Matching a generic parameter to an associated type in an impl

Question

I have a trait with an associated type and a generic struct::

trait Generator {
    type Foo;
    fn generate(&self) -> Self::Foo;
}

struct Baz

        

Answer 1

You could get rid of the generic argument B and instead of constraining B, directly pass A::Foo as the second generic argument to Baz, but I'm not sure if your actual problem matches the simplified example you showed.

impl<A: Generator> Baz<A, A::Foo> {
    fn addFoo(&mut self)  {
        self.vec.push(self.generator.generate());
    }
}

Answer 2

The trick is to only have a single generic parameter:

trait Generator {
    type Foo;
    fn generate(&self) -> Self::Foo;
}

struct Baz<G>
where
    G: Generator,
{
    generator: G,
    vec: Vec<G::Foo>,
}

impl<G> Baz<G>
where
    G: Generator,
{
    fn add_foo(&mut self) {
        self.vec.push(self.generator.generate());
    }
}

Since the vector will contain G::Foo, we can actually just say that.

The Rust style is snake_case, so I updated that as well as made the type parameter G to help the reader.

Answer 3

I must explain to the compiler that B is the same as A::Foo

There is a special syntax for it:

impl<A, B> Baz<A, B>
where
    A: Generator<Foo = B>,
{
    fn add_foo(&mut self) {
        self.vec.push(self.generator.generate());
    }
}