Add explicit tests of has_i128-derived methods

tests/newtype.rs

@@ -34,11 +34,25 @@ fn test_from_primitive() {
     assert_eq!(MyFloat::from_u32(25), Some(MyFloat(25.0)));
 }
 
+#[test]
+#[cfg(has_i128)]
+fn test_from_primitive_128() {
+    assert_eq!(MyFloat::from_i128(std::i128::MIN), Some(MyFloat(-2.0.powi(127))));
+}
+
 #[test]
 fn test_to_primitive() {
     assert_eq!(MyFloat(25.0).to_u32(), Some(25));
 }
 
+#[test]
+#[cfg(has_i128)]
+fn test_to_primitive_128() {
+    let f = MyFloat::from_f32(std::f32::MAX).unwrap();
+    assert_eq!(f.to_i128(), None);
+    assert_eq!(f.to_u128(), Some(0xffff_ff00_0000_0000_0000_0000_0000_0000));
+}
+
 #[test]
 fn test_num_ops() {
     assert_eq!(MyFloat(25.0) + MyFloat(10.0), MyFloat(35.0));