time: document maximum sleep duration (#3126)

tokio/src/time/sleep.rs

@@ -31,6 +31,8 @@ pub fn sleep_until(deadline: Instant) -> Sleep {
 ///
 /// To run something regularly on a schedule, see [`interval`].
 ///
+/// The maximum duration for a sleep is 68719476734 milliseconds (approximately 2.2 years).
+///
 /// # Cancellation
 ///
 /// Canceling a sleep instance is done by dropping the returned future. No additional

tokio/tests/time_sleep.rs

@@ -182,6 +182,7 @@ const MAX_DURATION: u64 = (1 << (6 * NUM_LEVELS)) - 1;
 #[tokio::test]
 async fn exactly_max() {
     // TODO: this should not panic but `time::ms()` is acting up
+    // If fixed, make sure to update documentation on `time::sleep` too.
     time::sleep(ms(MAX_DURATION)).await;
 }