How to index vectors with integer types (besides usize), without explicit cast?

Question

There are times when indices need to be tightly packed (mesh geometry for example), where its useful to store indices as u32 instead of usize.

Is there a way to index a vector in Rust without having to explicitly cast to usize every time? eg:

vector[some_u32 as usize];

Casting in single instances isn't a problem, its just tedious when needed frequently.

Is there a way to avoid having to cast here?

Answer 1

No.

If it were your own type, you could implement Index<u32>, but it isn't and you can't.

If you're really, pathologically opposed to casting the index, you could write an adaptor type that does the cast, but that's getting a bit silly.

Answer 2

If you only want to index arrays of your own type, and you use a newtype for the index, then you can create an Index impl:

struct MyIndex(u32);
struct MyValue(u64);

impl std::ops::Index<MyIndex> for [MyValue] {
    type Output = MyValue;
    fn index(&self, idx: MyIndex) -> &MyValue {
        &self[idx.0 as usize]
    }
}

Now, you can index any array of MyValues with MyIndex. Instead of using u32 everywhere, you now have to use MyIndex. There's the newtype_derive crate to help you make the MyIndex behave mostly like a u32.