Add test and discussed fix

Author: lyinch

library/std/src/sys/unix/time.rs

@@ -149,9 +149,11 @@ mod inner {
     impl Instant {
         pub fn now() -> Instant {
             extern "C" {
-                fn mach_absolute_time() -> u64;
+                //fn mach_absolute_time() -> u64;
+                fn clock_gettime_nsec_np(clock_type: i64) -> u64;
             }
-            Instant { t: unsafe { mach_absolute_time() } }
+            Instant { t: unsafe { clock_gettime_nsec_np(4) } }
+            //Instant { t: unsafe { mach_absolute_time() } }
         }
 
         pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {

library/std/src/time/tests.rs

@@ -12,6 +12,16 @@ macro_rules! assert_almost_eq {
     }};
 }
 
+#[test]
+#[cfg(all(target_arch = "aarch64", target_os = "macos"))]
+fn wtf() {
+    let t0 = Instant::now();
+    let t1 = t0 + Duration::from_nanos(50);
+    let d = t1 - t0;
+    dbg!(t0, t1, d);
+    assert_eq!(t0 + d, t1);
+}
+
 #[test]
 fn instant_monotonic() {
     let a = Instant::now();