adds sleep_until test

Author: dvdsk

library/std/tests/thread.rs

@@ -1,6 +1,8 @@
+#![feature(thread_sleep_until)]
+
 use std::sync::{Arc, Mutex};
 use std::thread;
-use std::time::Duration;
+use std::time::{Duration, Instant};
 
 #[test]
 #[cfg_attr(target_os = "emscripten", ignore)]
@@ -14,3 +16,19 @@ fn sleep() {
     thread::sleep(Duration::from_millis(100));
     assert_eq!(*finished.lock().unwrap(), false);
 }
+
+#[test]
+fn sleep_until() {
+    let now = Instant::now();
+    let period = Duration::from_millis(100);
+    let deadline = now + period;
+    thread::sleep_until(deadline);
+
+    let margin = Duration::from_micros(100);
+    let elapsed = now.elapsed();
+    assert!(
+        (period..period + margin).contains(&elapsed),
+        "elapsed: {} microseconds, expected 100_000 to 100_100",
+        elapsed.as_micros()
+    );
+}