Rollup merge of #123826 - kornelski:one-in-a-quintillion, r=Amanieu

Move rare overflow error to a cold function

`scoped.spawn()` generates unnecessary inlined panic-formatting code for a branch that will never be taken.

library/std/src/thread/scoped.rs

@@ -47,10 +47,16 @@ pub(super) fn increment_num_running_threads(&self) {
         // chance it overflows to 0, which would result in unsoundness.
         if self.num_running_threads.fetch_add(1, Ordering::Relaxed) > usize::MAX / 2 {
             // This can only reasonably happen by mem::forget()'ing a lot of ScopedJoinHandles.
-            self.decrement_num_running_threads(false);
-            panic!("too many running threads in thread scope");
+            self.overflow();
         }
     }
+
+    #[cold]
+    fn overflow(&self) {
+        self.decrement_num_running_threads(false);
+        panic!("too many running threads in thread scope");
+    }
+
     pub(super) fn decrement_num_running_threads(&self, panic: bool) {
         if panic {
             self.a_thread_panicked.store(true, Ordering::Relaxed);