I’m experimenting with const expressions and generics in Rust and stumbled across the following problem. When I want to use the const generic argument (here N) indirectly like for example N + 1, the compiler tells me that I need to add a “where-bound” like this one:

where [(); N + 1]:,.

I don’t understand the meaning of this. To me it looks like a constraint that says: “There is an array of type “empty tuple / unit” and “N + 1″ entries which is bound to (nothing; as there is nothing after the colon)”.

Can anyone help me understand the parts of this constraint?

Minimal example:

#![allow(unused)]
#![allow(incomplete_features)]
#![feature(generic_const_exprs)]

struct Foo<const N: usize> {}

impl<const N: usize> Foo<N>
where
    [(); N + 1]:,
{
    const BAR: [u32; N + 1] = [0u32; N + 1];
}

fn main() {}