sleep_until: use clock_nanosleep where possible

Using clock nanosleep leads to more accurate sleep times on platforms
where it is supported.

To enable using clock_nanosleep this makes `sleep_until` platform
specific. That unfortunatly requires identical placeholder
implementations for the other platforms (windows/mac/wasm etc).

we will land platform specific implementations for those later. See the
`sleep_until` tracking issue.

This requires an accessors for the Instant type. As that accessor is only
used on the platforms that have clock_nanosleep it is marked as allow_unused.

32bit time_t targets do not use clock_nanosleep atm, they instead rely
on the same placeholder as the other platforms. We could make them
use clock_nanosleep too in the future using `__clock_nanosleep_time64`.

__clock_nanosleep_time64 is documented at:
https://www.gnu.org/software/libc/manual/html_node/64_002dbit-time-symbol-handling.html

1 files changed, 13 insertions, 5 deletions

diff --git a/library/std/src/sys/pal/unix/time.rs b/library/std/src/sys/pal/unix/time.rs
index 0074d767474..bd7f74fea6a 100644
--- a/library/std/src/sys/pal/unix/time.rs
+++ b/library/std/src/sys/pal/unix/time.rs
@@ -261,6 +261,10 @@ pub struct Instant {
 }
 
 impl Instant {
+    #[cfg(target_vendor = "apple")]
+    pub(crate) const CLOCK_ID: libc::clockid_t = libc::CLOCK_UPTIME_RAW;
+    #[cfg(not(target_vendor = "apple"))]
+    pub(crate) const CLOCK_ID: libc::clockid_t = libc::CLOCK_MONOTONIC;
     pub fn now() -> Instant {
         // https://www.manpagez.com/man/3/clock_gettime/
         //
@@ -273,11 +277,7 @@ impl Instant {
         //
         // Instant on macos was historically implemented using mach_absolute_time;
         // we preserve this value domain out of an abundance of caution.
-        #[cfg(target_vendor = "apple")]
-        const clock_id: libc::clockid_t = libc::CLOCK_UPTIME_RAW;
-        #[cfg(not(target_vendor = "apple"))]
-        const clock_id: libc::clockid_t = libc::CLOCK_MONOTONIC;
-        Instant { t: Timespec::now(clock_id) }
+        Instant { t: Timespec::now(Self::CLOCK_ID) }
     }
 
     pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
@@ -291,6 +291,14 @@ impl Instant {
     pub fn checked_sub_duration(&self, other: &Duration) -> Option<Instant> {
         Some(Instant { t: self.t.checked_sub_duration(other)? })
     }
+
+    #[cfg_attr(
+        not(target_os = "linux"),
+        allow(unused, reason = "needed by the `sleep_until` on some unix platforms")
+    )]
+    pub(crate) fn into_timespec(self) -> Timespec {
+        self.t
+    }
 }
 
 impl fmt::Debug for Instant {