Fix a dangling reference in rustc_thread_pool

Author: cuviper

compiler/rustc_thread_pool/src/latch.rs

@@ -388,13 +388,17 @@ 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;
+ // SAFETY: We don't use any references from `this` after this call.
  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 and shadowed these fields beforehand.
+ registry.notify_worker_latch_is_set(worker_index);
  }
  }
  CountLatchKind::Blocking { latch } => unsafe { LockLatch::set(latch) },