Return NumOpResult for FeeRate and Weight

As we did for `Amount` change the fee functions on `FeeRate` and
`Weight` to return `NumOpResult`.

This change does not add that much value but it does not make things
worse and it makes the API more uniform. Also reduces lines of code.

units/src/fee.rs

@@ -27,17 +27,12 @@ use core::ops;
 
 use NumOpResult as R;
 
-use crate::{Amount, FeeRate, MathOp, NumOpError as E, NumOpResult, Weight};
+use crate::{Amount, FeeRate, NumOpResult, Weight};
 
 crate::internal_macros::impl_op_for_references! {
     impl ops::Mul<FeeRate> for Weight {
         type Output = NumOpResult<Amount>;
-        fn mul(self, rhs: FeeRate) -> Self::Output {
-            match rhs.mul_by_weight(self) {
-                Some(amount) => R::Valid(amount),
-                None => R::Error(E::while_doing(MathOp::Mul)),
-            }
-        }
+        fn mul(self, rhs: FeeRate) -> Self::Output { rhs.mul_by_weight(self) }
     }
     impl ops::Mul<FeeRate> for NumOpResult<Weight> {
         type Output = NumOpResult<Amount>;
@@ -72,12 +67,7 @@ crate::internal_macros::impl_op_for_references! {
 
     impl ops::Mul<Weight> for FeeRate {
         type Output = NumOpResult<Amount>;
-        fn mul(self, rhs: Weight) -> Self::Output {
-            match self.mul_by_weight(rhs) {
-                Some(amount) => R::Valid(amount),
-                None => R::Error(E::while_doing(MathOp::Mul)),
-            }
-        }
+        fn mul(self, rhs: Weight) -> Self::Output { self.mul_by_weight(rhs) }
     }
     impl ops::Mul<Weight> for NumOpResult<FeeRate> {
         type Output = NumOpResult<Amount>;
@@ -218,7 +208,7 @@ mod tests {
         assert_eq!(Amount::from_sat_u32(100), fee);
 
         let fee = FeeRate::from_sat_per_kwu(10).unwrap().mul_by_weight(Weight::MAX);
-        assert!(fee.is_none());
+        assert!(fee.is_error());
 
         let weight = Weight::from_vb(3).unwrap();
         let fee_rate = FeeRate::from_sat_per_vb(3).unwrap();

units/src/fee_rate/mod.rs

@@ -196,8 +196,8 @@ impl FeeRate {
     pub const fn to_fee(self, weight: Weight) -> Amount {
         // No `unwrap_or()` in const context.
         match self.mul_by_weight(weight) {
-            Some(fee) => fee,
-            None => Amount::MAX,
+            NumOpResult::Valid(fee) => fee,
+            NumOpResult::Error(_) => Amount::MAX,
         }
     }
 
@@ -207,7 +207,7 @@ impl FeeRate {
     /// This is equivalent to `Self::checked_mul_by_weight()`.
     #[must_use]
     #[deprecated(since = "TBD", note = "use `to_fee()` instead")]
-    pub fn fee_wu(self, weight: Weight) -> Option<Amount> { self.mul_by_weight(weight) }
+    pub fn fee_wu(self, weight: Weight) -> Option<Amount> { self.mul_by_weight(weight).ok() }
 
     /// Calculates the fee by multiplying this fee rate by weight, in virtual bytes, returning [`None`]
     /// if an overflow occurred.
@@ -223,21 +223,18 @@ impl FeeRate {
     /// Computes the absolute fee amount for a given [`Weight`] at this fee rate. When the resulting
     /// fee is a non-integer amount, the amount is rounded up, ensuring that the transaction fee is
     /// enough instead of falling short if rounded down.
-    ///
-    /// Returns [`None`] if overflow occurred.
-    #[must_use]
-    pub const fn mul_by_weight(self, weight: Weight) -> Option<Amount> {
+    pub const fn mul_by_weight(self, weight: Weight) -> NumOpResult<Amount> {
         let wu = weight.to_wu();
         if let Some(fee_kwu) = self.to_sat_per_kwu_floor().checked_mul(wu) {
             // Bump by 999 to do ceil division using kwu.
             if let Some(bump) = fee_kwu.checked_add(999) {
                 let fee = bump / 1_000;
                 if let Ok(fee_amount) = Amount::from_sat(fee) {
-                    return Some(fee_amount);
+                    return NumOpResult::Valid(fee_amount);
                 }
             }
         }
-        None
+        NumOpResult::Error(E::while_doing(MathOp::Mul))
     }
 }
 

units/src/weight.rs

@@ -10,7 +10,7 @@ use arbitrary::{Arbitrary, Unstructured};
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 
-use crate::{Amount, CheckedSum, FeeRate};
+use crate::{Amount, CheckedSum, FeeRate, NumOpResult};
 
 /// The factor that non-witness serialization data is multiplied by during weight calculation.
 pub const WITNESS_SCALE_FACTOR: usize = 4;
@@ -168,10 +168,7 @@ impl Weight {
     /// Computes the absolute fee amount for a given [`FeeRate`] at this weight. When the resulting
     /// fee is a non-integer amount, the amount is rounded up, ensuring that the transaction fee is
     /// enough instead of falling short if rounded down.
-    ///
-    /// Returns [`None`] if overflow occurred.
-    #[must_use]
-    pub const fn mul_by_fee_rate(self, fee_rate: FeeRate) -> Option<Amount> {
+    pub const fn mul_by_fee_rate(self, fee_rate: FeeRate) -> NumOpResult<Amount> {
         fee_rate.mul_by_weight(self)
     }
 }