Fix JitterRng on Mac OS

.travis.yml

@@ -78,9 +78,11 @@ matrix:
       rust: nightly
 
     # Testing other channels
-    # Don't run nightly tests on darwin: JitterRng bench fails due to low-res timer
     - env: TARGET=x86_64-unknown-linux-gnu NIGHTLY=1
       rust: nightly
+    - env: TARGET=x86_64-apple-darwin NIGHTLY=1
+      os: osx
+      rust: nightly
 
 before_install:
   - set -e

src/jitter_rng.rs

@@ -611,7 +611,7 @@ impl JitterRng {
     /// times. This measures the absolute worst-case, and gives a lower bound
     /// for the available entropy.
     ///
-    /// ```no_run
+    /// ```rust,no_run
     /// use rand::JitterRng;
     ///
     /// # use std::error::Error;
@@ -670,7 +670,7 @@ impl JitterRng {
     }
 }
 
-#[cfg(feature="std")]
+#[cfg(all(feature="std", not(any(target_os = "macos", target_os = "ios"))))]
 fn get_nstime() -> u64 {
     use std::time::{SystemTime, UNIX_EPOCH};
 
@@ -682,6 +682,18 @@ fn get_nstime() -> u64 {
     dur.as_secs() << 30 | dur.subsec_nanos() as u64
 }
 
+#[cfg(all(feature="std", any(target_os = "macos", target_os = "ios")))]
+fn get_nstime() -> u64 {
+    extern crate libc;
+    // On Mac OS and iOS std::time::SystemTime only has 1000ns resolution.
+    // We use `mach_absolute_time` instead. This provides a CPU dependent unit,
+    // to get real nanoseconds the result should by multiplied by numer/denom
+    // from `mach_timebase_info`.
+    // But we are not interested in the exact nanoseconds, just entropy. So we
+    // use the raw result.
+    unsafe { libc::mach_absolute_time() }
+}
+
 // A function that is opaque to the optimizer to assist in avoiding dead-code
 // elimination. Taken from `bencher`.
 fn black_box<T>(dummy: T) -> T {