gistfile1.rs

struct S;

impl S {
  fn a(&self) {
    println!("a");
  }
  fn b(&self) {
    println!("b");
  }
}

trait T {
  fn b(&self);
  fn a(&self);
}

impl T for S {
  fn a(&self) {
    println!("A");
    self.b();
  }
  fn b(&self) {
    println!("B");
    self.a();
  }
}

fn test_trait(t: &T) {
  t.a();
}

fn main() {
  let s = S;
  test_trait(&s);
}

$ rustc test.rs && ./test
A
b