Function rhs_taints 

fn rhs_taints(
    gcx: Gcx<'_>,
    rhs: &Expr<'_>,
    n: usize,
    tainted: &HashSet<VariableId>,
) -> Vec<bool>

Taint of each of the n slots assigned from rhs: elementwise for a tuple of matching arity, otherwise the taint of the whole expression.