refactor rent_due for normal case of exempt

runtime/src/rent_collector.rs

@@ -83,19 +83,26 @@ 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 {
-        let slots_elapsed: u64 = (account.rent_epoch()..=self.epoch)
-            .map(|epoch| self.epoch_schedule.get_slots_in_epoch(epoch + 1))
-            .sum();
-
-        // avoid infinite rent in rust 1.45
-        let years_elapsed = if self.slots_per_year != 0.0 {
-            slots_elapsed as f64 / self.slots_per_year
+        if self
+            .rent
+            .is_exempt(account.lamports(), account.data().len())
+        {
+            RentDue::Exempt
         } else {
-            0.0
-        };
-
-        self.rent
-            .due(account.lamports(), account.data().len(), years_elapsed)
+            let slots_elapsed: u64 = (account.rent_epoch()..=self.epoch)
+                .map(|epoch| self.epoch_schedule.get_slots_in_epoch(epoch + 1))
+                .sum();
+
+            // avoid infinite rent in rust 1.45
+            let years_elapsed = if self.slots_per_year != 0.0 {
+                slots_elapsed as f64 / self.slots_per_year
+            } else {
+                0.0
+            };
+
+            // 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

@@ -68,12 +68,17 @@ impl Rent {
         if self.is_exempt(balance, data_len) {
             RentDue::Exempt
         } else {
-            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)
+            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;
+        (lamports_per_year as f64 * years_elapsed) as u64
+    }
+
     pub fn free() -> Self {
         Self {
             lamports_per_byte_year: 0,