pool allocator

use std::cell::UnsafeCell;

struct Pool(usize);
impl Pool {
  fn new(&mut self) -> Box<Data> {
    // record which pool `data` came from.
    Box::new(Data(self.0))
  }
  fn free(&mut self, data: Box<Data>) {
    // enfore that `data` wwas allocated from this pool
    assert_eq!((*data).0, self.0);
  }
}
struct Data(usize);
struct DataBox {
  data: Box<Data>,
  pool: *mut Pool,
}
impl DataBox {
  fn new(pool: &UnsafeCell<Pool>) -> DataBox {
    let pool: *mut _ = pool.get();
    DataBox {
      data: unsafe { &mut *pool }.new(),
      pool: pool,
    }
  }
}
/*
impl Drop for DataBox {
  fn drop(&mut self) {
    unsafe { &mut *self.pool }.free(self.data);
  }
}
*/
fn drop_databox(db: DataBox) {
  unsafe { &mut *db.pool }.free(db.data);
}

fn main() {
  let pool0 = UnsafeCell::new(Pool(0));
  let b0 = DataBox::new(&pool0);
  drop_databox(b0);
  let pool1 = UnsafeCell::new(Pool(1));
  let b1 = DataBox::new(&pool1);
  drop_databox(b1);
}