message: disable compiler warning only for BlockedTaskQueue::add
diff: // http://opensource.org/licenses/MIT>, at your option. This file may not be
  // copied, modified, or distributed except according to those terms.
  -#![allow(unused_assignments)]
  -
  use crate::arch;
  use crate::arch::mm::VirtAddr;
  use crate::arch::percore::*;
  @@ -528,6 +526,7 @@ impl BlockedTaskQueue {
  }
  /// Blocks the given task for `wakeup_time` ticks, or indefinitely if None is given.
  + #[allow(unused_assignments)]
  pub fn add(&mut self, task: Rc<RefCell<Task>>, wakeup_time: Option<u64>) {
  {
  // Set the task status to Blocked.
  