cgmath::assert_approx_eq!

macro_rules! assert_approx_eq(
    ($given: expr, $expected: expr) => ({
        let (given_val, expected_val) = (&($given), &($expected));
        if !given_val.approx_eq(expected_val) {
            panic!("assertion failed: `left ≈ right` (left: `{:?}`, right: `{:?}`, tolerance: `{:?}`)",
                *given_val, *expected_val,
                ApproxEq::approx_epsilon(Some(*given_val))
            );
        }
    })
);