Implement round-ties-to-even for Duration Debug for consistency with f64

library/core/src/time.rs

@@ -1316,7 +1316,33 @@ impl fmt::Debug for Duration {
             // normal floating point numbers. However, we only need to do work
             // when rounding up. This happens if the first digit of the
             // remaining ones is >= 5.
-            let integer_part = if fractional_part > 0 && fractional_part >= divisor * 5 {
+            let integer_part = if fractional_part > 0 && fractional_part == divisor * 5 {
+                let last_digit_is_odd = if pos > 0 {
+                    (buf[pos - 1] - b'0') % 2 == 1
+                } else {
+                    // No fractional digits - check the integer part
+                    (integer_part % 2) == 1
+                };
+
+                if last_digit_is_odd {
+                    let mut rev_pos = pos;
+                    let mut carry = true;
+                    while carry && rev_pos > 0 {
+                        rev_pos -= 1;
+
+                        if buf[rev_pos] < b'9' {
+                            buf[rev_pos] += 1;
+                            carry = false;
+                        } else {
+                            buf[rev_pos] = b'0';
+                        }
+                    }
+
+                    if carry { integer_part.checked_add(1) } else { Some(integer_part) }
+                } else {
+                    Some(integer_part)
+                }
+            } else if fractional_part > 0 && fractional_part > divisor * 5 {
                 // Round up the number contained in the buffer. We go through
                 // the buffer backwards and keep track of the carry.
                 let mut rev_pos = pos;

library/coretests/tests/time.rs

@@ -439,7 +439,6 @@ fn debug_formatting_precision_two() {
     assert_eq!(format!("{:.2?}", Duration::new(4, 001_000_000)), "4.00s");
     assert_eq!(format!("{:.2?}", Duration::new(2, 100_000_000)), "2.10s");
     assert_eq!(format!("{:.2?}", Duration::new(2, 104_990_000)), "2.10s");
-    assert_eq!(format!("{:.2?}", Duration::new(2, 105_000_000)), "2.11s");
     assert_eq!(format!("{:.2?}", Duration::new(8, 999_999_999)), "9.00s");
 }
 
@@ -480,6 +479,15 @@ fn debug_formatting_precision_high() {
     assert_eq!(format!("{:.20?}", Duration::new(4, 001_000_000)), "4.00100000000000000000s");
 }
 
+#[test]
+fn debug_formatting_round_to_even() {
+    assert_eq!(format!("{:.0?}", Duration::new(1, 500_000_000)), "2s");
+    assert_eq!(format!("{:.0?}", Duration::new(2, 500_000_000)), "2s");
+    assert_eq!(format!("{:.0?}", Duration::new(0, 1_500_000)), "2ms");
+    assert_eq!(format!("{:.0?}", Duration::new(0, 2_500_000)), "2ms");
+    assert_eq!(format!("{:.2?}", Duration::new(2, 105_000_000)), "2.10s");
+}
+
 #[test]
 fn duration_const() {
     // test that the methods of `Duration` are usable in a const context