Fix non-associativity of Instant math on aarch64-apple-darwin tar…

…gets

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

@@ -149,7 +149,11 @@ impl From<libc::timespec> for Timespec {
     }
 }
 
-#[cfg(any(target_os = "macos", target_os = "ios", target_os = "watchos"))]
+#[cfg(any(
+    all(target_os = "macos", not(target_arch = "aarch64")),
+    target_os = "ios",
+    target_os = "watchos"
+))]
 mod inner {
     use crate::sync::atomic::{AtomicU64, Ordering};
     use crate::sys::cvt;
@@ -265,7 +269,11 @@ mod inner {
     }
 }
 
-#[cfg(not(any(target_os = "macos", target_os = "ios", target_os = "watchos")))]
+#[cfg(not(any(
+    all(target_os = "macos", not(target_arch = "aarch64")),
+    target_os = "ios",
+    target_os = "watchos"
+)))]
 mod inner {
     use crate::fmt;
     use crate::mem::MaybeUninit;
@@ -281,7 +289,11 @@ mod inner {
 
     impl Instant {
         pub fn now() -> Instant {
-            Instant { t: Timespec::now(libc::CLOCK_MONOTONIC) }
+            #[cfg(target_os = "macos")]
+            const clock_id: clock_t = libc::CLOCK_UPTIME_RAW;
+            #[cfg(not(target_os = "macos"))]
+            const clock_id: clock_t = libc::CLOCK_MONOTONIC;
+            Instant { t: Timespec::now(clock_id) }
         }
 
         pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {

library/std/src/time/tests.rs

@@ -88,6 +88,14 @@ fn instant_math_is_associative() {
     // Changing the order of instant math shouldn't change the results,
     // especially when the expression reduces to X + identity.
     assert_eq!((now + offset) - now, (now - now) + offset);
+
+    // On any platform, `Instant` should have the same resolution as `Duration` (e.g. 1 nanosecond)
+    // or better. Otherwise, math will be non-associative (see #91417).
+    let now = Instant::now();
+    let provided_offset = Duration::from_nanos(1);
+    let later = now + provided_offset;
+    let measured_offset = later - now;
+    assert_eq!(measured_offset, provided_offset);
 }
 
 #[test]