refactor: use std::sync::RwLock's try api, remove tokio dependency

src/mapref.rs

@@ -22,7 +22,7 @@
 //! });
 
 use hashbrown::HashTable;
-use tokio::sync::{RwLockReadGuard, RwLockWriteGuard};
+use std::sync::{RwLockReadGuard, RwLockWriteGuard};
 
 /// A reference to a key-value pair in a [`crate::ShardMap`]. Holds a shared (read-only) lock on the shard
 /// associated with the key.

src/shard.rs

@@ -1,5 +1,9 @@
 use hashbrown::HashTable;
-use tokio::sync::RwLock;
+use std::{
+    future::Future,
+    sync::{RwLock, RwLockReadGuard, RwLockWriteGuard, TryLockError},
+    task::Poll,
+};
 
 /// A shard in a [`crate::ShardMap`]. Each shard contains a [`hashbrown::HashTable`] of key-value pairs.
 pub(crate) struct Shard<K, V> {
@@ -15,6 +19,22 @@ where
             data: RwLock::new(HashTable::new()),
         }
     }
+
+    pub fn write<'a>(&'a self) -> impl Future<Output = RwLockWriteGuard<'a, HashTable<(K, V)>>> {
+        std::future::poll_fn(|_ctx| match self.data.try_write() {
+            Ok(guard) => Poll::Ready(guard),
+            Err(TryLockError::WouldBlock) => Poll::Pending,
+            Err(TryLockError::Poisoned(_)) => panic!("lock poisoned"),
+        })
+    }
+
+    pub fn read<'a>(&'a self) -> impl Future<Output = RwLockReadGuard<'a, HashTable<(K, V)>>> {
+        std::future::poll_fn(|_ctx| match self.data.try_read() {
+            Ok(guard) => Poll::Ready(guard),
+            Err(TryLockError::WouldBlock) => Poll::Pending,
+            Err(TryLockError::Poisoned(_)) => panic!("lock poisoned"),
+        })
+    }
 }
 
 impl<K, V> std::ops::Deref for Shard<K, V> {