Return Result type instead of Option

Result allows for a more descriptive error messaging.

Author: yancyribbens

units/src/amount/tests.rs

@@ -1466,8 +1466,8 @@ fn math_op_errors() {
 fn to_effective_value() {
     let amt = "1 cBTC".parse::<Amount>().unwrap();
     let fee_rate = FeeRate::from_sat_per_kwu(10);
-    let effective_value =
-        amt.to_effective_value(fee_rate, Weight::from_wu(272)).unwrap();
+    let effective_value = amt.to_effective_value(
+        fee_rate, Weight::from_wu(272)).unwrap();
 
     // 10 sat/kwu * 272 wu = 4 sats (rounding up)
     let expected_fee = "3 sats".parse::<SignedAmount>().unwrap();
@@ -1476,7 +1476,7 @@ fn to_effective_value() {
 }
 
 #[test]
-fn effective_value_fee_rate_does_not_overflow() {
-    let eff_value =Amount::ZERO.to_effective_value(FeeRate::MAX, Weight::from_wu(272));
-    assert!(eff_value.is_none());
+fn to_effective_value_error() {
+    let eff_val = Amount::ZERO.to_effective_value(FeeRate::MAX, Weight::from_wu(272));
+    assert_eq!(eff_val, Err(OutOfRangeError::too_big(false).into()));
 }

units/src/amount/unsigned.rs

@@ -250,11 +250,17 @@ impl Amount {
     /// * `input_weight_prediction` - the predicted input weight.
     pub fn to_effective_value(
         &self,
-        fee_rate: FeeRate,
-        weight: Weight,
-    ) -> Option<SignedAmount> {
-        let signed_input_fee = fee_rate.to_fee(weight)?.to_signed();
-        self.to_signed().checked_sub(signed_input_fee)
+        fee_rate: crate::FeeRate,
+        weight: crate::Weight
+    ) -> Result<SignedAmount, OutOfRangeError> {
+        let signed_input_fee = fee_rate.to_fee(weight).ok_or(
+            OutOfRangeError { is_signed: false, is_greater_than_max: true }
+        )?.to_signed();
+
+        // it's not possible to overflow when subtracted by the smallest given:
+        // Amount::MIN - SignedAmount::MAX = SignedAmount::MIN
+        let eff_val = (self.to_signed() - signed_input_fee).unwrap();  //unwrap ok
+        Ok(eff_val)
     }
 
     /// Converts this [`Amount`] in floating-point notation in the given [`Denomination`].