sleep_until: improve maintainability

Co-authored-by: Jonas Böttiger <jonasboettiger@icloud.com>

- use the internal platform specific Instant::CLOCK_ID
- skip allow(unused) on on platform that uses it such that it
can not become dead code
- sleep_until: remove leftover links

Author: dvdsk

library/std/src/sys/pal/unix/thread.rs

@@ -326,8 +326,12 @@ impl Thread {
         // If we're awoken with a signal and the return value is -1
         // clock_nanosleep needs to be called again.
         unsafe {
-            while libc::clock_nanosleep(libc::CLOCK_MONOTONIC, libc::TIMER_ABSTIME, ts_ptr, ts_ptr)
-                == -1
+            while libc::clock_nanosleep(
+                super::time::Instant::CLOCK_ID,
+                libc::TIMER_ABSTIME,
+                ts_ptr,
+                ts_ptr,
+            ) == -1
             {
                 assert_eq!(
                     os::errno(),

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> {

library/std/src/thread/mod.rs

@@ -920,8 +920,6 @@ pub fn sleep(dur: Duration) {
 ///
 /// [currently]: crate::io#platform-specific-behavior
 /// [clock_nanosleep]: https://linux.die.net/man/3/clock_nanosleep
-/// [subscription_clock]: https://github.com/WebAssembly/WASI/blob/main/legacy/preview1/docs.md#-subscription_clock-record
-/// [mach_wait_until]: https://developer.apple.com/library/archive/technotes/tn2169/_index.html
 ///
 /// **Disclaimer:** These system calls might change over time.
 ///

library/std/src/time.rs

@@ -408,8 +408,11 @@ impl Instant {
         self.0.checked_sub_duration(&duration).map(Instant)
     }
 
-    // used by platform specific `sleep_until` implementations.
-    #[allow(unused, reason = "not every platform has a specific `sleep_until`")]
+    // Used by platform specific `sleep_until` implementations such as the one used on Linux.
+    #[cfg_attr(
+        not(target_os = "linux"),
+        allow(unused, reason = "not every platform has a specific `sleep_until`")
+    )]
     pub(crate) fn into_inner(self) -> time::Instant {
         self.0
     }