A Rust version of Sean Parent's example of concepts-based polymorphism in C++

concepts_polymorphism.rs

// ...from 'Inheritance Is The Base Class of Evil'

use std::fmt;
use std::rc::Rc;

fn white_space(count: usize) -> String {
    std::iter::repeat(' ').take(count).collect::<String>()
}

trait Drawable {
    fn draw(&self, position: usize);
}

impl<T> Drawable for T where T: fmt::Display {
    fn draw(&self, position: usize) {
        println!("{}{}", white_space(position), self);
    }
}

#[derive(Clone)]
struct Document {
    data: Vec<Rc<Drawable>>
}

impl Document {
    fn new() -> Document {
        Document{ data: Vec::new() }
    }

    fn push<T: Drawable + 'static>(&mut self, x: T) {
        self.data.push(Rc::new(x));
    }

    fn replace<T: Drawable + 'static>(&mut self, index: usize, x: T) {
        self.data[index] = Rc::new(x);
    }
}

impl Drawable for Document {
    fn draw(&self, position: usize) {
        println!("{}<document>", white_space(position));
        for &ref e in self.data.iter() {
            e.draw(position + 2);
        }
        println!("{}</document>", white_space(position));
    }
}

struct History {
    history: Vec<Document>
}

impl History {
    fn new() -> History {
        History{ history: vec![Document::new()] }
    }

    fn commit(&mut self) {
        let clone = self.current().clone();
        self.history.push(clone);
    }

    fn undo(&mut self) {
        self.history.pop();
    }

    fn current(&mut self) -> &mut Document {
        self.history.last_mut().unwrap()
    }
}

struct MyClass;

impl Drawable for MyClass {
    fn draw(&self, position: usize) {
        println!("{}MyClass", white_space(position));
    }
}

fn main() {
    let mut h = History::new();

    h.current().push(0);
    h.current().push("Hello!");

    h.commit();
    h.current().draw(0);
    println!("==============================");

    let clone = h.current().clone();
    h.current().push(clone);
    h.current().push(MyClass);
    h.current().replace(1, "World");

    h.current().draw(0);
    println!("==============================");

    h.undo();
    h.current().draw(0);
}