Understanding the From and To traits

mvolkmann · February 15, 2021, 12:41pm

I created this repo to help me understand using the From and Into traits: GitHub - mvolkmann/rust-from-into

Parts of it work fine, but I haven't been able to determine the correct way to call the into method here:

mvolkmann/rust-from-into/blob/6cf369e9920f445517599d38e713ce49d84ce1a9/src/main.rs#L60


            y: p2.y,
            z: 0.0,
        }
    }
}
fn main() {
    let p2 = Point2D::new(3.0, 4.0);
    let p3 = Point3D::from(&p2);
    println!("p2 to 3D = {:?}", p3);
    let p3: Point3D = &p2.into();
    println!("p2 to 3D = {:?}", p3);
    let distance: f64 = p2.into();
    println!("p2 distance from origin = {}", distance);
    let p3 = Point3D::new(3.0, 4.0, 5.0);
    let p2 = Point2D::from(&p3);
    println!("p3 to 2D = {:?}", p2);
    let p2: Point2D = &p3.into();
    println!("p3 to 2D = {:?}", p3);
    let distance: f64 = p3.into();

The error is "mismatched types
expected struct Point3D
found reference &_rustcE0308" but it's not clear to me how to fix this. It seems like I only want to call into on references because I don't want this to transfer ownership.

bjorn3 · February 15, 2021, 12:43pm

Does let p3 = p2.into() work? That should be equivlent to let p3 = (&p2).into() due to an auto ref for receivers.

mvolkmann · February 15, 2021, 1:08pm

When I try let p3 = p2.into(); it says "error: type annotations needed". It wants me to specify the type of p3. But when I try let p3: Point3D = p2.into(); it says "error: the trait bound Point3D: From<Point2D> is not satisfied. My code implements the From trait for &Point2D, but not Point2D. It seems like I only want to implement it for &Point2D though.

m51 · February 15, 2021, 2:00pm

Method calls (dot operator) have higher precedence than references (& operator).

Just put parentheses around your references:

(&p2).into()

The built-in precedence rules means you are effectively doing this:

&(p2.into())

which makes the compiler error make sense.