clarification

src/main.rs

@@ -22,6 +22,10 @@ fn filter_currencies(exchange_rate_results: &mut [ExchangeRateResult], currencie
             .keys()
             .filter(|x| !currencies.contains(x))
             .for_each(|key_to_remove| {
+                /* This is safe, since we:
+                 * 1. Already have a mutable reference.
+                 * 2. Don't run the code in paralell
+                 */
                 let rates = unsafe { (*rates_ptr).borrow_mut() };
                 rates.remove_entry(key_to_remove);
             });