Rollup merge of rust-lang#50167 - fintelia:duration-nanos, r=sfackler

 Add as_nanos function to Duration

Duration has historically lacked a way to get the actual number of nanoseconds it contained as a normal Rust type because u64 was of insufficient range, and f64 of insufficient precision. The u128 type solves both issues, so I propose adding an `as_nanos` function to expose the capability.

src/libcore/time.rs

@@ -268,6 +268,57 @@ impl Duration {
     #[inline]
     pub const fn subsec_nanos(&self) -> u32 { self.nanos }
 
+    /// Returns the total number of milliseconds contained by this `Duration`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(duration_as_u128)]
+    /// use std::time::Duration;
+    ///
+    /// let duration = Duration::new(5, 730023852);
+    /// assert_eq!(duration.as_millis(), 5730);
+    /// ```
+    #[unstable(feature = "duration_as_u128", issue = "50202")]
+    #[inline]
+    pub fn as_millis(&self) -> u128 {
+        self.secs as u128 * MILLIS_PER_SEC as u128 + (self.nanos / NANOS_PER_MILLI) as u128
+    }
+
+    /// Returns the total number of microseconds contained by this `Duration`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(duration_as_u128)]
+    /// use std::time::Duration;
+    ///
+    /// let duration = Duration::new(5, 730023852);
+    /// assert_eq!(duration.as_micros(), 5730023);
+    /// ```
+    #[unstable(feature = "duration_as_u128", issue = "50202")]
+    #[inline]
+    pub fn as_micros(&self) -> u128 {
+        self.secs as u128 * MICROS_PER_SEC as u128 + (self.nanos / NANOS_PER_MICRO) as u128
+    }
+
+    /// Returns the total number of nanoseconds contained by this `Duration`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(duration_as_u128)]
+    /// use std::time::Duration;
+    ///
+    /// let duration = Duration::new(5, 730023852);
+    /// assert_eq!(duration.as_nanos(), 5730023852);
+    /// ```
+    #[unstable(feature = "duration_as_u128", issue = "50202")]
+    #[inline]
+    pub fn as_nanos(&self) -> u128 {
+        self.secs as u128 * NANOS_PER_SEC as u128 + self.nanos as u128
+    }
+
     /// Checked `Duration` addition. Computes `self + other`, returning [`None`]
     /// if overflow occurred.
     ///