diff --git a/crates/lb/src/net.rs b/crates/lb/src/net.rs
index 6700a48..0dc07a2 100644
--- a/crates/lb/src/net.rs
+++ b/crates/lb/src/net.rs
@@ -27,7 +27,7 @@ pub enum Error {
     /// IP or socket address syntax error.
     #[error("{0}")]
     InvalidAddr(#[from] AddrParseError),
-    /// Socket was already converted and cannot be used.
+    /// Socket was already converted and cannot be used anymore.
     #[error("socket was already converted")]
     SocketConsumed,
 }
@@ -141,7 +141,7 @@ impl lb_netlib {
         Ok(Some(TcpSocket::new_v6()?).into())
     }
 
-    pub extern "Lua" fn bind_tcp(&self, addr: any, port: any) -> Result<lb_tcpsocket> {
+    pub async extern "Lua" fn bind_tcp(&self, addr: any, port: any) -> Result<lb_tcpsocket> {
         let addr = self.socketaddr(addr, port);
         let socket;
         if addr.ip().is_v6() {
@@ -153,7 +153,7 @@ impl lb_netlib {
         socket
     }
 
-    pub extern "Lua" fn connect_tcp(&self, addr: any, port: any) -> Result<lb_tcpstream> {
+    pub async extern "Lua" fn connect_tcp(&self, addr: any, port: any) -> Result<lb_tcpstream> {
         let addr = self.socketaddr(addr, port);
         let socket;
         if addr.ip().is_v6() {
@@ -164,7 +164,7 @@ impl lb_netlib {
         socket.connect(addr)
     }
 
-    pub extern "Lua" fn listen_tcp(&self, addr: any, port: any) -> Result<lb_tcplistener> {
+    pub async extern "Lua" fn listen_tcp(&self, addr: any, port: any) -> Result<lb_tcplistener> {
         self.bind_tcp(addr, port).listen(1024)
     }
 }
diff --git a/crates/lb/src/task.rs b/crates/lb/src/task.rs
index 518d84b..6f540ae 100644
--- a/crates/lb/src/task.rs
+++ b/crates/lb/src/task.rs
@@ -35,7 +35,7 @@ impl lb_tasklib {
         sleep(Duration::from_secs_f64(ms / 1000.)).await;
     }
 
-    pub extern "Lua" fn spawn(&self, f: function, ...) {
+    pub extern "Lua" fn spawn(&self, f: function, ...) -> lb_task {
         // pack the function and its arguments into a table and pass its ref to rust.
         //
         // this table is used from rust-side to call the function with its args, and it's also
@@ -82,7 +82,7 @@ pub struct lb_task {
 
 #[metatype]
 impl lb_task {
-    pub extern "Lua" fn r#await(&self) -> many {
+    pub async extern "Lua" fn r#await(&self) -> many {
         self.__await();
         let ret = __registry[self.__ref];
         __tunpack(ret, 1, ret.n)
diff --git a/crates/luaffi_impl/src/metatype.rs b/crates/luaffi_impl/src/metatype.rs
index d670ea9..125e87c 100644
--- a/crates/luaffi_impl/src/metatype.rs
+++ b/crates/luaffi_impl/src/metatype.rs
@@ -285,7 +285,7 @@ fn get_ffi_functions(imp: &mut ItemImpl) -> Result<Vec<FfiFunction>> {
 
             let attrs = parse_ffi_function_attrs(&mut func.attrs)?;
             attrs.metamethod.map(|mm| document_metamethod(func, mm));
-
+            func.sig.asyncness.is_some().then(|| document_async(func));
             document_ffi_function(func);
 
             funcs.push(FfiFunction {
@@ -573,6 +573,12 @@ fn get_lua_functions(imp: &mut ItemImpl) -> Result<Vec<LuaFunction>> {
                 "cannot be generic"
             );
 
+            syn_assert!(
+                func.sig.constness.is_none(),
+                func.sig.constness,
+                "cannot be const"
+            );
+
             let mut params: Vec<_> = func
                 .sig
                 .inputs
@@ -593,7 +599,7 @@ fn get_lua_functions(imp: &mut ItemImpl) -> Result<Vec<LuaFunction>> {
 
             let attrs = parse_lua_function_attrs(&mut func.attrs)?;
             attrs.metamethod.map(|mm| document_metamethod(func, mm));
-
+            func.sig.asyncness.is_some().then(|| document_async(func));
             document_lua_function(func);
 
             funcs.push(LuaFunction {
@@ -807,6 +813,12 @@ fn document_lua_function(func: &mut ImplItemFn) {
     ]));
 }
 
+fn document_async(func: &mut ImplItemFn) {
+    func.attrs.insert(0, parse_quote!(#[doc =
+        r#"<span class="stab" title="This function is asynchronous." style="float: right; background: #ebf5ff; margin-left: 3px; padding-left: 5px; padding-right: 5px;">Async</span>"#
+    ]));
+}
+
 fn document_metamethod(func: &mut ImplItemFn, method: Metamethod) {
     let s = match method {
         Metamethod::Eq => "This is a metamethod which is called by the `==` operator.".into(),