Fitting a SquarePeg into a RoundHole, in Rust

SquarePeg.rs

trait Square {
    fn size(&self) -> f64;
    fn perimeter(&self) -> f64;
}

trait Circle {
    fn radius(&self) -> f64;
    fn perimeter(&self) -> f64;
}

struct RoundHole;

impl RoundHole {
    fn fit( circle_shaped_thing: &impl Circle ) {
        println!("Fit with radius: {}", circle_shaped_thing.radius() );
    }
}




// Square is defined with no mention of traits
struct SquarePeg {
    size: f64
}


// We can add implementations for specific traits later - even to third-party code
// (as long as either the trait or the struct is local)
// Therefore traits are non-intrusive
impl Square for SquarePeg {
    fn size(&self) -> f64 {
        return self.size;
    }
    fn perimeter(&self) -> f64 {
        return self.size*4.0;
    }
}


// --- Don't immediately implement this
impl Circle for SquarePeg {
    fn radius(&self) -> f64 {
        return (2.0*(self.size/2.0).powf(2.0)).sqrt();
    }
    fn perimeter(&self) -> f64 {
        return 2.0*(std::f64::consts::PI*self.radius());
    }
}
// ----


fn main() {

    let square_peg = SquarePeg{ size: 42.0 };

    println!("size: {}", square_peg.size() );

    // doesn't work until Circle implemented for SquarePeg
    RoundHole::fit( &square_peg );

    // Doesn't work until we add the 'as' casts
    println!("perimeter: {}", (&square_peg as &dyn Square).perimeter() );
    
    // Or, more idiomatically (or more cleanly), call the perimeter method explicity from the trait
    println!("perimeter: {}", Circle::perimeter(&square_peg ) );
}