如何迭代使用RwLock构建的并发哈希映射的可变值？

Question

是否有一种优雅的方法可以迭代此并发哈希图中的每个键值对？

use std::collections::HashMap;
use std::hash::Hash;
use std::sync::RwLock;

const N_SHARDS: usize = 16;

#[derive(Default)]
struct ConcurrentHashMap<K, V>
where
    K: Eq + Hash,
{
    shards: [RwLock<HashMap<K, V>>; N_SHARDS],
}

希望这表明我正在尝试完成的事情：

impl<K, V> ConcurrentHashMap<K, V>
where
    K: Eq + Hash,
{
    fn iter_mut(&self) -> impl Iterator<Item = (&K, &mut V)> {
        self.shards
            .iter()
            .flat_map(|shard| shard.write().expect("Poisoned").iter_mut())
    }
}

error[E0515]: cannot return value referencing temporary value
  --> src/lib.rs:22:31
   |
22 |             .flat_map(|shard| shard.write().expect("Poisoned").iter_mut())
   |                               --------------------------------^^^^^^^^^^^
   |                               |
   |                               returns a value referencing data owned by the current function
   |                               temporary value created here