refactor rent_due for normal case of exempt (#23350)

runtime/src/rent_collector.rs

@@ -83,6 +83,12 @@ impl RentCollector {
     /// given an account that 'should_collect_rent'
     /// returns (amount rent due, is_exempt_from_rent)
     pub fn get_rent_due(&self, account: &impl ReadableAccount) -> RentDue {
+        if self
+            .rent
+            .is_exempt(account.lamports(), account.data().len())
+        {
+            RentDue::Exempt
+        } else {
             let slots_elapsed: u64 = (account.rent_epoch()..=self.epoch)
                 .map(|epoch| self.epoch_schedule.get_slots_in_epoch(epoch + 1))
                 .sum();
@@ -94,8 +100,9 @@ impl RentCollector {
                 0.0
             };
 
-        self.rent
-            .due(account.lamports(), account.data().len(), years_elapsed)
+            // we know this account is not exempt
+            RentDue::Paying(self.rent.due_amount(account.data().len(), years_elapsed))
+        }
     }
 
     // Updates the account's lamports and status, and returns the amount of rent collected, if any.

sdk/program/src/rent.rs

@@ -68,10 +68,15 @@ impl Rent {
         if self.is_exempt(balance, data_len) {
             RentDue::Exempt
         } else {
+            RentDue::Paying(self.due_amount(data_len, years_elapsed))
+        }
+    }
+
+    /// rent due for account that is known to be not exempt
+    pub fn due_amount(&self, data_len: usize, years_elapsed: f64) -> u64 {
         let actual_data_len = data_len as u64 + ACCOUNT_STORAGE_OVERHEAD;
         let lamports_per_year = self.lamports_per_byte_year * actual_data_len;
-            RentDue::Paying((lamports_per_year as f64 * years_elapsed) as u64)
-        }
+        (lamports_per_year as f64 * years_elapsed) as u64
     }
 
     pub fn free() -> Self {