units: Add macros for Add and Sub

Add macros for implementing `ops::Add` and `ops::Sub` for various
combinations of references. Use the macro in `fee_rate`.

These are internal macros so no need to protect `core` using
`$crate::_export::_core`.

units/src/fee_rate.rs

@@ -190,48 +190,14 @@ impl Add for FeeRate {
 
     fn add(self, rhs: FeeRate) -> Self::Output { FeeRate(self.0 + rhs.0) }
 }
-
-impl Add<FeeRate> for &FeeRate {
-    type Output = FeeRate;
-
-    fn add(self, rhs: FeeRate) -> Self::Output { FeeRate(self.0 + rhs.0) }
-}
-
-impl Add<&FeeRate> for FeeRate {
-    type Output = FeeRate;
-
-    fn add(self, rhs: &FeeRate) -> Self::Output { FeeRate(self.0 + rhs.0) }
-}
-
-impl<'a> Add<&'a FeeRate> for &FeeRate {
-    type Output = FeeRate;
-
-    fn add(self, rhs: &'a FeeRate) -> Self::Output { FeeRate(self.0 + rhs.0) }
-}
+crate::internal_macros::impl_add_for_references!(FeeRate);
 
 impl Sub for FeeRate {
     type Output = FeeRate;
 
     fn sub(self, rhs: FeeRate) -> Self::Output { FeeRate(self.0 - rhs.0) }
 }
-
-impl Sub<FeeRate> for &FeeRate {
-    type Output = FeeRate;
-
-    fn sub(self, rhs: FeeRate) -> Self::Output { FeeRate(self.0 - rhs.0) }
-}
-
-impl Sub<&FeeRate> for FeeRate {
-    type Output = FeeRate;
-
-    fn sub(self, rhs: &FeeRate) -> Self::Output { FeeRate(self.0 - rhs.0) }
-}
-
-impl<'a> Sub<&'a FeeRate> for &FeeRate {
-    type Output = FeeRate;
-
-    fn sub(self, rhs: &'a FeeRate) -> Self::Output { FeeRate(self.0 - rhs.0) }
-}
+crate::internal_macros::impl_sub_for_references!(FeeRate);
 
 /// Computes the ceiling so that the fee computation is conservative.
 impl Mul<FeeRate> for Weight {

units/src/internal_macros.rs

@@ -0,0 +1,65 @@
+// SPDX-License-Identifier: CC0-1.0
+
+//! Internal macros.
+//!
+//! Macros meant to be used inside the `bitcoin-units` library.
+
+/// Implements `ops::Add` for various references.
+///
+/// Requires `$ty` it implement `Add` e.g. 'impl Add<T> for T'. Adds impls of:
+///
+/// - Add<T> for &T
+/// - Add<&T> for T
+/// - Add<&T> for &T
+macro_rules! impl_add_for_references {
+    ($ty:ident) => {
+        impl core::ops::Add<$ty> for &$ty {
+            type Output = $ty;
+
+            fn add(self, rhs: $ty) -> Self::Output { *self + rhs }
+        }
+
+        impl core::ops::Add<&$ty> for $ty {
+            type Output = $ty;
+
+            fn add(self, rhs: &$ty) -> Self::Output { self + *rhs }
+        }
+
+        impl<'a> core::ops::Add<&'a $ty> for &$ty {
+            type Output = $ty;
+
+            fn add(self, rhs: &'a $ty) -> Self::Output { *self + *rhs }
+        }
+    }
+}
+pub(crate) use impl_add_for_references;
+
+/// Implement `ops::Sub` for various references.
+///
+/// Requires `$ty` it implement `Sub` e.g. 'impl Sub<T> for T'. Adds impls of:
+///
+/// - Sub<T> for &T
+/// - Sub<&T> for T
+/// - Sub<&T> for &T
+macro_rules! impl_sub_for_references {
+    ($ty:ident) => {
+        impl core::ops::Sub<$ty> for &$ty {
+            type Output = $ty;
+
+            fn sub(self, rhs: $ty) -> Self::Output { *self - rhs }
+        }
+
+        impl core::ops::Sub<&$ty> for $ty {
+            type Output = $ty;
+
+            fn sub(self, rhs: &$ty) -> Self::Output { self - *rhs }
+        }
+
+        impl<'a> core::ops::Sub<&'a $ty> for &$ty {
+            type Output = $ty;
+
+            fn sub(self, rhs: &'a $ty) -> Self::Output { *self - *rhs }
+        }
+    }
+}
+pub(crate) use impl_sub_for_references;

units/src/lib.rs

@@ -23,6 +23,8 @@ extern crate alloc;
 #[cfg(feature = "std")]
 extern crate std;
 
+mod internal_macros;
+
 #[doc(hidden)]
 pub mod _export {
     /// A re-export of core::*