avoid reentrant lock acquire when ThreadIds run out

library/std/src/thread/mod.rs

@@ -1004,6 +1004,7 @@ impl ThreadId {
             // If we somehow use up all our bits, panic so that we're not
             // covering up subtle bugs of IDs being reused.
             if COUNTER == u64::MAX {
+                drop(_guard); // in case the panic handler ends up calling `ThreadId::new()`, avoid reentrant lock acquire.
                 panic!("failed to generate unique thread ID: bitspace exhausted");
             }