trait DoerMaker {
  type T: StuffDoer;
  fn make_doer(&self, f: Foo) -> Self::T;
}

trait StuffDoer {
  fn do_stuff(&self) -> Option<()>;
}

impl<T: StuffDoer + ?Sized> StuffDoer for Box<T> {
    fn do_stuff(&self) -> Option<()> {
        (**self).do_stuff()
    }
}

enum Foo {
  A,
  B,
  C,
}

struct Bar;
impl DoerMaker for Bar {
  type T=Box<StuffDoer>;

  fn make_doer(&self, f: Foo) -> Box<StuffDoer> {
    match f {
      Foo::A => Box::new(Baz{}),
      Foo::B => Box::new(Bat{}),
      Foo::C => Box::new(Bang{}),
    }
  }
}

struct Baz;
impl StuffDoer for Baz {
  fn do_stuff(&self) -> Option<()> {
    None
  }
}

struct Bat;
impl StuffDoer for Bat {
  fn do_stuff(&self) -> Option<()> {
    None
  }
}

struct Bang;
impl StuffDoer for Bang {
  fn do_stuff(&self) -> Option<()> {
    None
  }
}