Clarify Duration::MAX depends on Instant

Duration is used in std to represent a difference between two Instants. As such, it has to at least contain that span of time in it. However, Instant can vary by platform. Thus, we should explain the impl of Duration::MAX is sensitive to these vagaries of the platform.
rust-lang · bors · Apr 26, 2021 · Apr 12, 2021 · Apr 24, 2021 · Apr 19, 2021
commit a80dbea918d6dd0516439f399a62512753276da5
diff --git a/library/core/src/time.rs b/library/core/src/time.rs
@@ -136,16 +136,18 @@ impl Duration {
 
     /// The maximum duration.
     ///
-    /// It is roughly equal to a duration of 584,942,417,355 years.
+    /// May vary by platform. At least equal to the number of seconds
+    /// difference between the minimum and maximum [`std::time::Instant`].
+    /// On many platforms this is roughly 584,942,417,355 years.
     ///
     /// # Examples
     ///
     /// ```
-    /// #![feature(duration_constants)]
     /// use std::time::Duration;
     ///
     /// assert_eq!(Duration::MAX, Duration::new(u64::MAX, 1_000_000_000 - 1));
     /// ```
+    /// [`std::time::Instant`]: ../../std/time/struct.Instant.html
     #[stable(feature = "duration_saturating_ops", since = "1.53.0")]
     pub const MAX: Duration = Duration::new(u64::MAX, NANOS_PER_SEC - 1);