Merge rust-bitcoin#4473: Assert num op error

a7d0591 Assert error type (yancy)
2f7e74d Add MathOp helper methods (yancy)

Pull request description:

  Follow up from rust-bitcoin#4428 to assert the error type which I agree improves the test.

  Added some helper functions since it can be nice to see what type of overflow happened.

ACKs for top commit:
  tcharding:
    ACK a7d0591
  apoelstra:
    ACK a7d0591; successfully ran local tests

Tree-SHA512: e1b3eba640de2e4f98e075270fd797582601c541e1eebef2959a4e609bf51129e8ad38baab1253b40474c39f82ee4802658ec545cc5c3590637f2ddb13f873f7

Author: apoelstra

units/src/fee.rs

@@ -345,8 +345,11 @@ mod tests {
 
     #[test]
     fn fee_wu() {
-        let fee_overflow = FeeRate::from_sat_per_kwu(10).to_fee(Weight::MAX);
-        assert!(fee_overflow.is_error());
+        let operation = FeeRate::from_sat_per_kwu(10)
+            .to_fee(Weight::MAX)
+            .unwrap_err()
+            .operation();
+        assert!(operation.is_multiplication());
 
         let fee_rate = FeeRate::from_sat_per_vb(2).unwrap();
         let weight = Weight::from_vb(3).unwrap();

units/src/result.rs

@@ -254,6 +254,18 @@ impl MathOp {
 
     /// Returns `true` if this operation error'ed due to division by zero.
     pub fn is_div_by_zero(self) -> bool { !self.is_overflow() }
+
+    /// Returns `true` if this operation error'ed due to addition.
+    pub fn is_addition(self) -> bool { self == MathOp::Add }
+
+    /// Returns `true` if this operation error'ed due to subtraction.
+    pub fn is_subtraction(self) -> bool { self == MathOp::Sub }
+
+    /// Returns `true` if this operation error'ed due to multiplication.
+    pub fn is_multiplication(self) -> bool { self == MathOp::Mul }
+
+    /// Returns `true` if this operation error'ed due to negation.
+    pub fn is_negation(self) -> bool { self == MathOp::Neg }
 }
 
 impl fmt::Display for MathOp {