Use faster thread_local in current_thread_id()

library/std/src/sync/mpmc/mod.rs

@@ -1382,3 +1382,6 @@ impl<T> fmt::Debug for Receiver<T> {
         f.pad("Receiver { .. }")
     }
 }
+
+#[cfg(test)]
+mod tests;

library/std/src/sync/mpmc/tests.rs

@@ -0,0 +1,14 @@
+// Ensure that thread_local init with `const { 0 }` still has unique address at run-time
+#[test]
+fn waker_current_thread_id() {
+    let first = super::waker::current_thread_id();
+    let t = crate::thread::spawn(move || {
+        let second = super::waker::current_thread_id();
+        assert_ne!(first, second);
+        assert_eq!(second, super::waker::current_thread_id());
+    });
+
+    assert_eq!(first, super::waker::current_thread_id());
+    t.join().unwrap();
+    assert_eq!(first, super::waker::current_thread_id());
+}

library/std/src/sync/mpmc/waker.rs

@@ -204,6 +204,6 @@ impl Drop for SyncWaker {
 pub fn current_thread_id() -> usize {
     // `u8` is not drop so this variable will be available during thread destruction,
     // whereas `thread::current()` would not be
-    thread_local! { static DUMMY: u8 = 0 }
+    thread_local! { static DUMMY: u8 = const { 0 } }
     DUMMY.with(|x| (x as *const u8).addr())
 }