Auto merge of #29894 - alexcrichton:stdtime, r=brson

This commit is an implementation of [RFC 1288][rfc] which adds two new unstable
types to the `std::time` module. The `Instant` type is used to represent
measurements of a monotonically increasing clock suitable for measuring time
withing a process for operations such as benchmarks or just the elapsed time to
do something. An `Instant` favors panicking when bugs are found as the bugs are
programmer errors rather than typical errors that can be encountered.

[rfc]: https://github.com/rust-lang/rfcs/pull/1288

The `SystemTime` type is used to represent a system timestamp and is not
monotonic. Very few guarantees are provided about this measurement of the system
clock, but a fixed point in time (`UNIX_EPOCH`) is provided to learn about the
relative distance from this point for any particular time stamp.

This PR takes the same implementation strategy as the `time` crate on crates.io,
namely:

|  Platform  |  Instant                 |  SystemTime              |
|------------|--------------------------|--------------------------|
| Windows    | QueryPerformanceCounter  | GetSystemTimeAsFileTime  |
| OSX        | mach_absolute_time       | gettimeofday             |
| Unix       | CLOCK_MONOTONIC          | CLOCK_REALTIME           |

These implementations can perhaps be refined over time, but they currently
satisfy the requirements of the `Instant` and `SystemTime` types while also
being portable across implementations and revisions of each platform.

cc #29866

1 files changed, 3 insertions, 5 deletions

diff --git a/src/libstd/sync/condvar.rs b/src/libstd/sync/condvar.rs
index a18370dc68e..389c9c4a066 100644
--- a/src/libstd/sync/condvar.rs
+++ b/src/libstd/sync/condvar.rs
@@ -12,11 +12,10 @@ use prelude::v1::*;
 
 use sync::atomic::{AtomicUsize, Ordering};
 use sync::{mutex, MutexGuard, PoisonError};
-use sys::time::SteadyTime;
 use sys_common::condvar as sys;
 use sys_common::mutex as sys_mutex;
 use sys_common::poison::{self, LockResult};
-use time::Duration;
+use time::{Instant, Duration};
 
 /// A type indicating whether a timed wait on a condition variable returned
 /// due to a time out or not.
@@ -345,14 +344,13 @@ impl StaticCondvar {
             where F: FnMut(LockResult<&mut T>) -> bool {
         // This could be made more efficient by pushing the implementation into
         // sys::condvar
-        let start = SteadyTime::now();
+        let start = Instant::now();
         let mut guard_result: LockResult<MutexGuard<'a, T>> = Ok(guard);
         while !f(guard_result
                     .as_mut()
                     .map(|g| &mut **g)
                     .map_err(|e| PoisonError::new(&mut **e.get_mut()))) {
-            let now = SteadyTime::now();
-            let consumed = &now - &start;
+            let consumed = start.elapsed();
             let guard = guard_result.unwrap_or_else(|e| e.into_inner());
             let (new_guard_result, timed_out) = if consumed > dur {
                 (Ok(guard), WaitTimeoutResult(true))