Rollup merge of #57375 - stjepang:duration-constants, r=joshtriplett

Add duration constants

Add constants `SECOND`, `MILLISECOND`, `MICROSECOND`, and `NANOSECOND` to `core::time`.

This will make working with durations more ergonomic. Compare:

```rust
// Convenient, but deprecated function.
thread::sleep_ms(2000);

// The current canonical way to sleep for two seconds.
thread::sleep(Duration::from_secs(2));

// Sleeping using one of the new constants.
thread::sleep(2 * SECOND);
```

src/libcore/time.rs

@@ -23,6 +23,22 @@ const MILLIS_PER_SEC: u64 = 1_000;
 const MICROS_PER_SEC: u64 = 1_000_000;
 const MAX_NANOS_F64: f64 = ((u64::MAX as u128 + 1)*(NANOS_PER_SEC as u128)) as f64;
 
+/// The duration of one second.
+#[unstable(feature = "duration_constants", issue = "57391")]
+pub const SECOND: Duration = Duration::from_secs(1);
+
+/// The duration of one millisecond.
+#[unstable(feature = "duration_constants", issue = "57391")]
+pub const MILLISECOND: Duration = Duration::from_millis(1);
+
+/// The duration of one microsecond.
+#[unstable(feature = "duration_constants", issue = "57391")]
+pub const MICROSECOND: Duration = Duration::from_micros(1);
+
+/// The duration of one nanosecond.
+#[unstable(feature = "duration_constants", issue = "57391")]
+pub const NANOSECOND: Duration = Duration::from_nanos(1);
+
 /// A `Duration` type to represent a span of time, typically used for system
 /// timeouts.
 ///

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 = "57391")]
+pub use core::time::{SECOND, MILLISECOND, MICROSECOND, NANOSECOND};
+
 /// A measurement of a monotonically nondecreasing clock.
 /// Opaque and useful only with `Duration`.
 ///