Update sys::time impls to have checked_sub_instant

src/libstd/sys/cloudabi/time.rs

@@ -33,11 +33,9 @@ impl Instant {
         Instant { t: 0 }
     }
 
-    pub fn sub_instant(&self, other: &Instant) -> Duration {
-        let diff = self.t
-            .checked_sub(other.t)
-            .expect("second instant is later than self");
-        Duration::new(diff / NSEC_PER_SEC, (diff % NSEC_PER_SEC) as u32)
+    pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
+        let diff = self.t.checked_sub(other.t)?;
+        Some(Duration::new(diff / NSEC_PER_SEC, (diff % NSEC_PER_SEC) as u32))
     }
 
     pub fn checked_add_duration(&self, other: &Duration) -> Option<Instant> {

src/libstd/sys/redox/time.rs

@@ -137,10 +137,8 @@ impl Instant {
         false
     }
 
-    pub fn sub_instant(&self, other: &Instant) -> Duration {
-        self.t.sub_timespec(&other.t).unwrap_or_else(|_| {
-            panic!("specified instant was later than self")
-        })
+    pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
+        self.t.sub_timespec(&other.t).ok()
     }
 
     pub fn checked_add_duration(&self, other: &Duration) -> Option<Instant> {

src/libstd/sys/sgx/time.rs

@@ -14,8 +14,8 @@ impl Instant {
         Instant(usercalls::insecure_time())
     }
 
-    pub fn sub_instant(&self, other: &Instant) -> Duration {
-        self.0 - other.0
+    pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
+        self.0.checked_sub(other.0)
     }
 
     pub fn checked_add_duration(&self, other: &Duration) -> Option<Instant> {

src/libstd/sys/unix/time.rs

@@ -149,12 +149,11 @@ mod inner {
             true
         }
 
-        pub fn sub_instant(&self, other: &Instant) -> Duration {
+        pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
+            let diff = self.t.checked_sub(other.t)?;
             let info = info();
-            let diff = self.t.checked_sub(other.t)
-                           .expect("second instant is later than self");
             let nanos = mul_div_u64(diff, info.numer as u64, info.denom as u64);
-            Duration::new(nanos / NSEC_PER_SEC, (nanos % NSEC_PER_SEC) as u32)
+            Some(Duration::new(nanos / NSEC_PER_SEC, (nanos % NSEC_PER_SEC) as u32))
         }
 
         pub fn checked_add_duration(&self, other: &Duration) -> Option<Instant> {
@@ -285,10 +284,8 @@ mod inner {
             false // last clause, used so `||` is always trailing above
         }
 
-        pub fn sub_instant(&self, other: &Instant) -> Duration {
-            self.t.sub_timespec(&other.t).unwrap_or_else(|_| {
-                panic!("specified instant was later than self")
-            })
+        pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
+            self.t.sub_timespec(&other.t).ok()
         }
 
         pub fn checked_add_duration(&self, other: &Duration) -> Option<Instant> {

src/libstd/sys/wasm/time.rs

@@ -22,8 +22,8 @@ impl Instant {
         false
     }
 
-    pub fn sub_instant(&self, other: &Instant) -> Duration {
-        self.0 - other.0
+    pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
+        self.0.checked_sub(other.0)
     }
 
     pub fn checked_add_duration(&self, other: &Duration) -> Option<Instant> {

src/libstd/sys/windows/time.rs

@@ -49,17 +49,17 @@ impl Instant {
         Instant { t: Duration::from_secs(0) }
     }
 
-    pub fn sub_instant(&self, other: &Instant) -> Duration {
+    pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
         // On windows there's a threshold below which we consider two timestamps
         // equivalent due to measurement error. For more details + doc link,
         // check the docs on epsilon.
         let epsilon =
             perf_counter::PerformanceCounterInstant::epsilon();
         if other.t > self.t && other.t - self.t <= epsilon {
-            return Duration::new(0, 0)
+            Some(Duration::new(0, 0))
+        } else {
+            self.t.checked_sub(other.t)
         }
-        self.t.checked_sub(other.t)
-              .expect("specified instant was later than self")
     }
 
     pub fn checked_add_duration(&self, other: &Duration) -> Option<Instant> {