Re-export constants from core into std

Author: Stjepan Glavina

src/libstd/lib.rs

@@ -248,6 +248,7 @@
 #![feature(const_cstr_unchecked)]
 #![feature(core_intrinsics)]
 #![feature(dropck_eyepatch)]
+#![feature(duration_constants)]
 #![feature(exact_size_is_empty)]
 #![feature(external_doc)]
 #![feature(fixed_size_array)]

src/libstd/time.rs

@@ -21,6 +21,9 @@ use sys_common::FromInner;
 #[stable(feature = "time", since = "1.3.0")]
 pub use core::time::Duration;
 
+#[unstable(feature = "duration_constants", issue = "0")]
+pub use core::time::{SECOND, MILLISECOND, MICROSECOND, NANOSECOND};
+
 /// A measurement of a monotonically nondecreasing clock.
 /// Opaque and useful only with `Duration`.
 ///