std: allow after-main use of synchronization primitives

By creating an unnamed thread handle when the actual one has already been destroyed, synchronization primitives using thread parking can be used even outside the Rust runtime.

This also fixes an inefficiency in the queue-based `RwLock`: if `thread::current` was not initialized yet, it will create a new handle on every parking attempt without initializing `thread::current`. The private `current_or_unnamed` function introduced here fixes this.

1 files changed, 2 insertions, 4 deletions

diff --git a/library/std/src/sys/sync/rwlock/queue.rs b/library/std/src/sys/sync/rwlock/queue.rs
index 51330f8fafe..bd15f8ee952 100644
--- a/library/std/src/sys/sync/rwlock/queue.rs
+++ b/library/std/src/sys/sync/rwlock/queue.rs
@@ -118,7 +118,7 @@ use crate::mem;
 use crate::ptr::{self, NonNull, null_mut, without_provenance_mut};
 use crate::sync::atomic::Ordering::{AcqRel, Acquire, Relaxed, Release};
 use crate::sync::atomic::{AtomicBool, AtomicPtr};
-use crate::thread::{self, Thread, ThreadId};
+use crate::thread::{self, Thread};
 
 /// The atomic lock state.
 type AtomicState = AtomicPtr<()>;
@@ -217,9 +217,7 @@ impl Node {
     /// Prepare this node for waiting.
     fn prepare(&mut self) {
         // Fall back to creating an unnamed `Thread` handle to allow locking in TLS destructors.
-        self.thread.get_or_init(|| {
-            thread::try_current().unwrap_or_else(|| Thread::new_unnamed(ThreadId::new()))
-        });
+        self.thread.get_or_init(thread::current_or_unnamed);
         self.completed = AtomicBool::new(false);
     }