Questionable use of "punning" variable indexes about chalk

[aturon]

@nikomatsakis provided a probable test-case, which panics:

#[test]
fn mixed_indices() {
    test! {
        program {
            struct Ref<'a, T> { }
        }

        // Check that `'a` (here, `'?0`) is not unified
        // with `'!1`, because they belong to incompatible
        // universes.
        goal {
            exists<T> {
                exists<'a> {
                    exists<U> {
                        Ref<'a, T> = Ref<'a, U>
                    }
                }
            }
        } yields {
            ""
        }
    }
}

He also mentions that this kind of punning trick is used throughout the code.