Can we reduce the amount of code further?

Yes. It is possible to completely omit the requirement to create an implementation of a trait if that trait contains a default method:

trait MyTrait 
{
  fn test_code(&self) -> bool;
  fn self_test_code(&self) -> bool { self.test_code() } }

test_code is just the stub which requires an implementation. The self_test_code function doesn't need an implementation as it has a default method already.

Can the default method be overridden?

It can.

Note

The code for this section is in 09/override_default_method.

Let's start the code off by defining a trait. This has a default method for is_not_done. We will still need to implement is_done though, which we do for the UseFirstTime struct:

struct UseFirstTime; 
impl MyTrait for UseFirstTime 
{
  fn is_done(&self) -> bool 
  {
    println!("UseFirstTime.is_done"); 
    true
  } 
}

We next want to override the default method for is_not_done. Again, we create an empty struct and write both the implementations...