Fix a dangling reference in rustc_thread_pool

Author: cuviper

compiler/rustc_thread_pool/src/latch.rs

@@ -388,13 +388,16 @@ impl Latch for CountLatch {
     #[inline]
     unsafe fn set(this: *const Self) {
         if unsafe { (*this).counter.fetch_sub(1, Ordering::SeqCst) == 1 } {
-            // NOTE: Once we call `set` on the internal `latch`,
+            // SAFETY: Once we call `set` on the internal `latch`,
             // the target may proceed and invalidate `this`!
             match unsafe { &(*this).kind } {
                 CountLatchKind::Stealing { latch, registry, worker_index } => {
                     let registry = Arc::clone(registry);
+                    let worker_index = *worker_index;
                     if unsafe { CoreLatch::set(latch) } {
-                        registry.notify_worker_latch_is_set(*worker_index);
+                        // We **must not** access any part of `this` anymore,
+                        // which is why we read the fields beforehand.
+                        registry.notify_worker_latch_is_set(worker_index);
                     }
                 }
                 CountLatchKind::Blocking { latch } => unsafe { LockLatch::set(latch) },