Into and As and Generic Type Constraints

help
1

hi,

I'm having trouble with type constraints.

In this example into() fails but casting with as works:

fn example() {
    let a: u64 = 0;
    let b: f64 = a.into(); // Error - From<u64> not satisifed
    let c: f64 = a as f64; // Works
}

I have a generic function, but it's wrong because you can't cast to and from f64:

fn example_generic<T>(v: T) -> T
where
    T: Into<f64>,
    f64: Into<T>,
{
    let x: f64 = v.into();
    x.into()
}

Since as works, is it possible to add a type constraint to say "can cast with as"? Instead of Into<T>.

I also tried implementing Into with as for the number types, but you can't do that.

Sorry if this has been asked before.

2

as far as i know no, as is a special keyword without a trait behind it. in our project we worked around it by defining our own trait to convert between numeric types

3

ok, thank you @giocri

4

it is possible to implement these casts as library, but it is not built into the language, nor provided by the standard library.

you can use NumCast from the num crate:

~~https://docs.rs/num/latest/num/trait.NumCast.html~~

EDIT:

actually, you may want to use AsPrimitive<f64>, which is the generic equivalence of cast with as f64:

5

thank you @nerditation

here is a quick test using your solution:

fn example_generic<T>(v: T) -> T
where
    T: AsPrimitive<f64>,
    f64: AsPrimitive<T>,
{
    let x: f64 = v.as_();
    let y: T = x.as_();
    y
}