Type casting with a generic integer

Question

I have been working on a function which I would like to make generic over all integer types. I have therefore used the Integer trait from the num crate https://docs.rs/num/latest/num/trait.Integer.html however, my function requires my to be able to type cast both to and from my input type e.g.

fn main() {
    test(54_u8);
}

fn test<T> (x: T)
where T: num::Integer + std::fmt::Display {
    assert_eq!(10_u64, x as u64);
    assert_eq!(54_i32 as T, x);
}

Where the code fails with the error message

an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object

How can I typecast a generic integer? Thanks,

Answer 1

num::FromPrimitive and num::ToPrimitive can help to switch back and forth between num::Integer and a concrete type.

fn main() {
    test(10_u8);
    test(54_u8);
}

fn test<T>(x: T)
where
    T: num::Integer + num::FromPrimitive + num::ToPrimitive + Copy,
{
    if T::from_u64(10) == Some(x) {
        println!("this is ten");
    }
    if let Some(54) = x.to_i32() {
        println!("this is fifty-four");
    }
}