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, 19 insertions, 0 deletions

diff --git a/src/libstd/sys/common/mod.rs b/src/libstd/sys/common/mod.rs
index 44c55d1e2c4..5062be8cd63 100644
--- a/src/libstd/sys/common/mod.rs
+++ b/src/libstd/sys/common/mod.rs
@@ -98,3 +98,22 @@ pub fn cleanup() {
         at_exit_imp::cleanup();
     });
 }
+
+// Computes (value*numer)/denom without overflow, as long as both
+// (numer*denom) and the overall result fit into i64 (which is the case
+// for our time conversions).
+#[allow(dead_code)] // not used on all platforms
+pub fn mul_div_u64(value: u64, numer: u64, denom: u64) -> u64 {
+    let q = value / denom;
+    let r = value % denom;
+    // Decompose value as (value/denom*denom + value%denom),
+    // substitute into (value*numer)/denom and simplify.
+    // r < denom, so (denom*numer) is the upper bound of (r*numer)
+    q * numer + r * numer / denom
+}
+
+#[test]
+fn test_muldiv() {
+    assert_eq!(mul_div_u64( 1_000_000_000_001, 1_000_000_000, 1_000_000),
+               1_000_000_000_001_000);
+}