Conversion between the same type but different const generics

help
1

I'm currently running into an issue with the default impl<T> std::convert::From<T> for <T> when it comes to const generics. Currently I have the following:

struct Decimal<const S: u32>(/* fields unimportant */);

impl<const S1: u32, const S2: u32> From<Decimal<S1>> for Decimal<S2> {
    fn from(previous: Decimal<S1>) -> Self {
        // Mathematical conversions from S1 to S2
        Decimal<S2>(/*fields*/)
    }
}

This however conflicts with the default From implementation from one type to the same type. Can I opt out of that default implementation? Or do I really need to create a custom into method for this?

Edit link to playground

2

You can't opt out of the blanket implementation; you need some other way to avoid the coherence conflict. I don't think there is one on stable.

I gave this a shot on nightly but coherence couldn't see through it.

If the set of parameters is small enough, you can implement for individual ordered pairs of const parameters (e.g. via macro).

3

Thats really unfortunate since event the default implementation fails to compile...

Maybe this would make for a good issue upstream?

1 Like
4

No idea, try and see I suppose.

5

done

6

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