core/time: Add Duration methods for zero

This patch adds two methods to `Duration`. The first, `Duration::zero`,
provides a `const` constructor for getting an zero-length duration. This
is also what `Default` provides (this was clarified in the docs), though
`default` is not `const`.

The second, `Duration::is_zero`, returns true if a `Duration` spans no
time (i.e., because its components are all zero). Previously, the way to
do this was either to compare both `as_secs` and `subsec_nanos` to 0, to
compare against `Duration::new(0, 0)`, or to use the `u128` method
`as_nanos`, none of which were particularly elegant.

src/libcore/time.rs

@@ -31,7 +31,7 @@
 /// the number of nanoseconds.
 ///
 /// `Duration`s implement many common traits, including [`Add`], [`Sub`], and other
-/// [`ops`] traits.
+/// [`ops`] traits. It implements `Default` by returning a zero-length `Duration`.
 ///
 /// [`Add`]: ../../std/ops/trait.Add.html
 /// [`Sub`]: ../../std/ops/trait.Sub.html
@@ -223,6 +223,47 @@ pub const fn from_nanos(nanos: u64) -> Duration {
         }
     }
 
+    /// Creates a new `Duration` that spans no time.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::time::Duration;
+    ///
+    /// let duration = Duration::zero();
+    /// assert!(duration.is_zero());
+    ///
+    /// const IMMEDIATELY: Duration = Duration::zero();
+    /// assert!(IMMEDIATELY.is_zero());
+    /// ```
+    #[unstable(feature = "duration_zero", issue = "none")]
+    #[inline]
+    pub const fn zero() -> Duration {
+        Duration { secs: 0, nanos: 0 }
+    }
+
+    /// Returns true if this `Duration` spans no time.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::time::Duration;
+    ///
+    /// assert!(Duration::zero().is_zero());
+    /// assert!(Duration::new(0, 0).is_zero());
+    /// assert!(Duration::from_nanos(0).is_zero());
+    /// assert!(Duration::from_secs(0).is_zero());
+    ///
+    /// assert!(!Duration::new(1, 1).is_zero());
+    /// assert!(!Duration::from_nanos(1).is_zero());
+    /// assert!(!Duration::from_secs(1).is_zero());
+    /// ```
+    #[unstable(feature = "duration_zero", issue = "none")]
+    #[inline]
+    pub const fn is_zero(&self) -> bool {
+        self.secs == 0 && self.nanos == 0
+    }
+
     /// Returns the number of _whole_ seconds contained by this `Duration`.
     ///
     /// The returned value does not include the fractional (nanosecond) part of the