I think this is not going far enough. This is way too much code to show allowed transitions. IMO a state machine should be define first in a list of valid state transitions.

Perhaps something like this:

  /// the states
  states States {
    Green,
    Orange,
    Red,
  }
  
  /// the valid transitions
  transitions SomeTransitions for States {
    fn green_to_orange: Green => Orange,
    orange_to_red: Orange => Red,
    red_to_green: Red => Green,
  }
  
  // now create a context
  struct Context { /\* ... */ }
  
  // implement the transitions
  impl SomeTransitions for Context {
    pub fn green_to_orange(self) -> self {
      /* ... \*/
    }
  }