Rollup merge of #114238 - jhpratt:fix-duration-div, r=thomcc

Fix implementation of `Duration::checked_div`

I ran across this while running some sanity checks on `time`. Quickcheck immediately found a bug, and as I'd modified the code from `std` I knew there was a bug here as well.

tl;dr this code fails ([playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=1189a3efcdfc192c27d6d87815359353))

```rust
use std::time::Duration;

fn main() {
    assert_eq!(
        Duration::new(1, 1).checked_div(7),
        Some(Duration::new(0, 142_857_143)),
    );
}
```

The existing code determines that 1/7 = 0 (seconds), 1/7 = 0 (nanoseconds), 1 billion / 7 = 142,857,142 (extra nanoseconds). The billion comes from multiplying the remainder of the seconds (1) by the number of nanoseconds in a second. However, **this wrongly ignores any remaining nanoseconds**. This PR takes that into consideration, adds a test, and also changes the roundabout way of calculating the remainder into directly computing it.

Note: This is _not_ a rounding error. This result divides evenly.

`@rustbot` label +A-time +C-bug +S-waiting-on-reviewer +T-libs

2 files changed, 5 insertions, 4 deletions

diff --git a/library/core/src/time.rs b/library/core/src/time.rs
index b08d5782ab6..1e8d28979e6 100644
--- a/library/core/src/time.rs
+++ b/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 {
diff --git a/library/core/tests/time.rs b/library/core/tests/time.rs
index 872611937cc..bd6e63edbb9 100644
--- a/library/core/tests/time.rs
+++ b/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));
 }