std: use a mutex for ThreadId generation if available

library/std/src/thread/current.rs

@@ -108,7 +108,7 @@ pub(super) mod id {
         get().unwrap_or_else(
             #[cold]
             || {
-                let id = ThreadId::new();
+                let id = ThreadId::desperate_new();
                 id::set(id);
                 id
             },

library/std/src/thread/id.rs

@@ -31,11 +31,15 @@ use crate::sync::atomic::{Atomic, Ordering};
 pub struct ThreadId(NonZero<u64>);
 
 impl ThreadId {
-    // Generate a new unique thread ID.
-    pub(crate) fn new() -> ThreadId {
+    /// Generates a new unique thread ID, and does not rely on the global
+    /// allocator.
+    ///
+    /// This should only be used in contexts where a new `ThreadId` is
+    /// desperately required, but the global allocator might not be available.
+    pub(super) fn desperate_new() -> ThreadId {
         #[cold]
         fn exhausted() -> ! {
-            panic!("failed to generate unique thread ID: bitspace exhausted")
+            rtabort!("failed to generate unique thread ID: bitspace exhausted")
         }
 
         cfg_select! {
@@ -96,6 +100,25 @@ impl ThreadId {
         }
     }
 
+    /// Generates a new unique thread ID.
+    pub(crate) fn new() -> ThreadId {
+        cfg_select! {
+            target_has_atomic = "64" => {
+                // The generation is implemented lock-free anyway.
+                ThreadId::desperate_new()
+            }
+            _ => {
+                // Synchronize the id generation – `Mutex` has better
+                // synchronization behaviour than the spinlock used in
+                // `desperate_new`, but might need allocation.
+                use crate::sync::nonpoison::Mutex;
+                static LOCK: Mutex<()> = Mutex::new(());
+                let _guard = LOCK.lock();
+                ThreadId::desperate_new()
+            }
+        }
+    }
+
     #[cfg(any(not(target_thread_local), target_has_atomic = "64"))]
     pub(super) fn from_u64(v: u64) -> Option<ThreadId> {
         NonZero::new(v).map(ThreadId)