From / Into Conversions

struct Celsius(f64);
struct Fahrenheit(f64);

impl From<Celsius> for Fahrenheit {
    fn from(c: Celsius) -> Self {
        Fahrenheit(c.0 * 9.0 / 5.0 + 32.0)
    }
}

fn main() {
    let boiling = Celsius(100.0);
    let f: Fahrenheit = boiling.into();          // uses From impl above
    println!("{} °F", f.0);                      // 212 °F

    // Or call From directly
    let body = Fahrenheit::from(Celsius(37.0));
    println!("{:.1} °F", body.0);                // 98.6 °F

    // From is also why ? upcasts errors — every Error type into Box<dyn Error>
}