Merge rust-bitcoin/rust-bitcoin#3627: Mark functions `const` in `fee_rate`

68aac98753 Mark functions `const` (Jamil Lambert, PhD)

Pull request description:

  Following on from #3608 and #1174 more functions have been marked as const.

  Mark `checked_` functions in `units/src/fee_rate.rs` as const.
  Replace `map()` and `?` operators, which are not allowed in const context, with match statements.

ACKs for top commit:
  apoelstra:
    ACK 68aac98753dba77bccc6572c65d441108dc53a17; successfully ran local tests; nice
  tcharding:
    ACK 68aac98753

Tree-SHA512: 2165f0fdb2bbbd68223646c7f788ec3b637cf2bec06a9faee8b252900fac0517985047d5e88ec074b6d9121fcb3a3774a62f326e53e1ceaa041d65a4e2e35fb6

units/src/fee_rate.rs

@@ -79,12 +79,24 @@ impl FeeRate {
     /// Checked multiplication.
     ///
     /// Computes `self * rhs` returning [`None`] if overflow occurred.
-    pub fn checked_mul(self, rhs: u64) -> Option<Self> { self.0.checked_mul(rhs).map(Self) }
+    pub const fn checked_mul(self, rhs: u64) -> Option<Self> {
+        // No `map()` in const context.
+        match self.0.checked_mul(rhs) {
+            Some(res) => Some(Self(res)),
+            None => None,
+        }
+    }
 
     /// Checked division.
     ///
     /// Computes `self / rhs` returning [`None`] if `rhs == 0`.
-    pub fn checked_div(self, rhs: u64) -> Option<Self> { self.0.checked_div(rhs).map(Self) }
+    pub const fn checked_div(self, rhs: u64) -> Option<Self> {
+        // No `map()` in const context.
+        match self.0.checked_div(rhs) {
+            Some(res) => Some(Self(res)),
+            None => None,
+        }
+    }
 
     /// Checked weight multiplication.
     ///
@@ -94,20 +106,38 @@ impl FeeRate {
     /// rounded down.
     ///
     /// [`None`] is returned if an overflow occurred.
-    pub fn checked_mul_by_weight(self, rhs: Weight) -> Option<Amount> {
-        let sats = self.0.checked_mul(rhs.to_wu())?.checked_add(999)? / 1000;
-        Some(Amount::from_sat(sats))
+    pub const fn checked_mul_by_weight(self, rhs: Weight) -> Option<Amount> {
+        // No `?` operator in const context.
+        match self.0.checked_mul(rhs.to_wu()) {
+            Some(mul_res) => match mul_res.checked_add(999) {
+                Some(add_res) => Some(Amount::from_sat(add_res / 1000)),
+                None => None,
+            },
+            None => None,
+        }
     }
 
     /// Checked addition.
     ///
     /// Computes `self + rhs` returning [`None`] if overflow occured.
-    pub fn checked_add(self, rhs: u64) -> Option<Self> { self.0.checked_add(rhs).map(Self) }
+    pub const fn checked_add(self, rhs: u64) -> Option<Self> {
+        // No `map()` in const context.
+        match self.0.checked_add(rhs) {
+            Some(res) => Some(Self(res)),
+            None => None,
+        }
+    }
 
     /// Checked subtraction.
     ///
     /// Computes `self - rhs` returning [`None`] if overflow occured.
-    pub fn checked_sub(self, rhs: u64) -> Option<Self> { self.0.checked_sub(rhs).map(Self) }
+    pub const fn checked_sub(self, rhs: u64) -> Option<Self> {
+        // No `map()` in const context.
+        match self.0.checked_sub(rhs) {
+            Some(res) => Some(Self(res)),
+            None => None,
+        }
+    }
 
     /// Calculates the fee by multiplying this fee rate by weight, in weight units, returning [`None`]
     /// if an overflow occurred.