Implement AddAssign and SubAssign

Author: pitdicker

src/duration.rs

@@ -10,7 +10,7 @@
 
 //! Temporal quantification
 
-use core::ops::{Add, Div, Mul, Neg, Sub};
+use core::ops::{Add, AddAssign, Div, Mul, Neg, Sub, SubAssign};
 use core::time::Duration as StdDuration;
 use core::{fmt, i64};
 #[cfg(feature = "std")]
@@ -380,6 +380,20 @@ impl Sub for Duration {
     }
 }
 
+impl AddAssign for Duration {
+    fn add_assign(&mut self, rhs: Duration) {
+        let new = self.checked_add(&rhs).expect("`Duration + Duration` overflowed");
+        *self = new;
+    }
+}
+
+impl SubAssign for Duration {
+    fn sub_assign(&mut self, rhs: Duration) {
+        let new = self.checked_sub(&rhs).expect("`Duration - Duration` overflowed");
+        *self = new;
+    }
+}
+
 impl Mul<i32> for Duration {
     type Output = Duration;
 
@@ -529,6 +543,11 @@ mod tests {
             -(Duration::days(3) + Duration::seconds(70)),
             Duration::days(-4) + Duration::seconds(86_400 - 70)
         );
+
+        let mut d = Duration::default();
+        d += Duration::minutes(1);
+        d -= Duration::seconds(30);
+        assert_eq!(d, Duration::seconds(30));
     }
 
     #[test]

src/naive/time/mod.rs

@@ -567,11 +567,11 @@ impl NaiveTime {
         if frac >= 1_000_000_000 {
             let rfrac = 2_000_000_000 - frac;
             if rhs >= OldDuration::nanoseconds(i64::from(rfrac)) {
-                rhs = rhs - OldDuration::nanoseconds(i64::from(rfrac));
+                rhs -= OldDuration::nanoseconds(i64::from(rfrac));
                 secs += 1;
                 frac = 0;
             } else if rhs < OldDuration::nanoseconds(-i64::from(frac)) {
-                rhs = rhs + OldDuration::nanoseconds(i64::from(frac));
+                rhs += OldDuration::nanoseconds(i64::from(frac));
                 frac = 0;
             } else {
                 frac = (i64::from(frac) + rhs.num_nanoseconds().unwrap()) as u32;