test.rs

struct Container<'a> {
    process: &'a mut dyn FnMut(),
}

// ...

struct ContainerOwner<'a> {
    container: Container<'a>,
}

impl<'a> ContainerOwner<'a> {
    fn new(container: Container<'a>) -> Self {
        Self { container }
    }
}

fn use_container(owner: &mut ContainerOwner) {
    (owner.container.process)();
}

// ...

fn foo() {}

fn main() {
    let mut x = ContainerOwner::new(Container { process: &mut foo });
  
    use_container(&mut x);
}