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);
```

1 files changed, 16 insertions, 0 deletions

diff --git a/src/libcore/time.rs b/src/libcore/time.rs
index b12ee0497d2..a751965dffa 100644
--- a/src/libcore/time.rs
+++ b/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.
 ///