Fix implementation of Duration::checked_div

Author: jhpratt

library/core/src/time.rs

@@ -656,10 +656,10 @@ impl Duration {
     #[rustc_const_stable(feature = "duration_consts_2", since = "1.58.0")]
     pub const fn checked_div(self, rhs: u32) -> Option<Duration> {
         if rhs != 0 {
-            let secs = self.secs / (rhs as u64);
-            let carry = self.secs - secs * (rhs as u64);
-            let extra_nanos = carry * (NANOS_PER_SEC as u64) / (rhs as u64);
-            let nanos = self.nanos.0 / rhs + (extra_nanos as u32);
+            let (secs, extra_secs) = (self.secs / (rhs as u64), self.secs % (rhs as u64));
+            let (mut nanos, extra_nanos) = (self.nanos.0 / rhs, self.nanos.0 % rhs);
+            nanos +=
+                ((extra_secs * (NANOS_PER_SEC as u64) + extra_nanos as u64) / (rhs as u64)) as u32;
             debug_assert!(nanos < NANOS_PER_SEC);
             Some(Duration::new(secs, nanos))
         } else {

library/core/tests/time.rs

@@ -170,6 +170,7 @@ fn saturating_mul() {
 fn div() {
     assert_eq!(Duration::new(0, 1) / 2, Duration::new(0, 0));
     assert_eq!(Duration::new(1, 1) / 3, Duration::new(0, 333_333_333));
+    assert_eq!(Duration::new(1, 1) / 7, Duration::new(0, 142_857_143));
     assert_eq!(Duration::new(99, 999_999_000) / 100, Duration::new(0, 999_999_990));
 }