Forcing two generics to be the same in impl

Question

I have a Hittable trait, with a method that returns a generic implementing Material:

pub trait Hittable { fn hit<T>(&self -> Option<HitRecord<T>> where T: Material; }

And an implementation of this trait on a Sphere<T>:

impl<T> Hittable for Sphere<T> where T: Material { fn hit(&self) -> Option<HitRecord<T>> { // snip }

I want the T in HitRecords to be the same as the T in Sphere<T>, but as of now now it will not compile because I need to annotate the hit method in the impl with a T as well. But if I do so, the 2 T's will not be forced to be the same. How do I enforce that?

Chayim Friedman · Accepted Answer · 2022-06-08 09:07:40Z

You should uplift the generic parameter from the method into the trait:

pub trait Hittable<T: Material> { fn hit(&self) -> Option<HitRecord<T>>; } impl<T: Material> Hittable<T> for Sphere<T> { fn hit(&self) -> Option<HitRecord<T>> { // snip } }

Another way is to use an associated type (it models the situation better, but as long as you only implement the trait for one generic parameter it doesn't practically matter):

pub trait Hittable { type T: Material; fn hit(&self) -> Option<HitRecord<Self::T>>; } impl<T: Material> Hittable for Sphere<T> { type T = T; fn hit(&self) -> Option<HitRecord<T>> { // snip } }

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