Small optimization and comment about underflow/overflow

crates/pallet-rewards/src/lib.rs

@@ -435,27 +435,26 @@ impl<T: Config> Pallet<T> {
         let is = params.initial_subsidy;
         let mdi = max_decay_issuance;
 
+        let term1 = is * is * t / mdi;
+        let term2 = term1 * is * t / mdi;
+        let term3 = term2 * is * t / mdi;
+        let term4 = term3 * is * t / mdi;
+        let term5 = term4 * is * t / mdi;
         let maybe_subsidy = try {
-            is.checked_sub(&(is * is * t / mdi))?
-                .checked_add(&(is * is * t / mdi * is * t / mdi / 2_u32.into()))?
-                .checked_sub(&(is * is * t / mdi * is * t / mdi * is * t / mdi / 6_u32.into()))?
-                .checked_add(
-                    &(is * is * t / mdi * is * t / mdi * is * t / mdi * is * t
-                        / mdi
-                        / 24_u32.into()),
-                )?
-                .checked_sub(
-                    &(is * is * t / mdi * is * t / mdi * is * t / mdi * is * t / mdi * is * t
-                        / mdi
-                        / 120_u32.into()),
-                )?
+            is.checked_sub(&term1)?
+                .checked_add(&(term2 / 2_u32.into()))?
+                .checked_sub(&(term3 / 6_u32.into()))?
+                .checked_add(&(term4 / 24_u32.into()))?
+                .checked_sub(&(term5 / 120_u32.into()))?
         };
 
         match maybe_subsidy {
             Some(subsidy) => subsidy,
             None => {
+                // Due to use of Taylor series above this can eventually underflow, but not any
+                // time soon
                 warn!(
-                    "Underflow or overflow when calculating reference subsidy, incorrect \
+                    "Underflow or overflow when calculating reference subsidy, likely incorrect \
                     issuance parameters"
                 );