Function stable_cmp_existential_predicate 

fn stable_cmp_existential_predicate<'db>(
    a: &ExistentialPredicate<'db>,
    b: &ExistentialPredicate<'db>,
) -> Ordering

Compares via an ordering that will not change if modules are reordered or other changes are made to the tree. In particular, this ordering is preserved across incremental compilations.