support empty yield args

Author: Your Name

src/state.rs

@@ -2171,7 +2171,7 @@ impl Lua {
             return Err(Error::runtime("cannot yield across Rust/Lua boundary."));
         }
         unsafe {
-            raw.extra.get().as_mut().unwrap_unchecked().yielded_values = args.into_lua_multi(self)?;
+            raw.extra.get().as_mut().unwrap_unchecked().yielded_values = Some(args.into_lua_multi(self)?);
         }
         Ok(())
     }

src/state/extra.rs

@@ -96,7 +96,7 @@ pub(crate) struct ExtraData {
     pub(super) enable_jit: bool,
 
     // Values currently being yielded from Lua.yield()
-    pub(super) yielded_values: MultiValue,
+    pub(super) yielded_values: Option<MultiValue>,
 }
 
 impl Drop for ExtraData {
@@ -198,7 +198,7 @@ impl ExtraData {
             enable_jit: true,
             #[cfg(feature = "luau")]
             running_gc: false,
-            yielded_values: MultiValue::with_capacity(0),
+            yielded_values: None,
         }));
 
         // Store it in the registry

src/state/util.rs

@@ -188,7 +188,7 @@ where
             let raw = extra.as_ref().unwrap_unchecked().raw_lua();
             let values = take(&mut extra.as_mut().unwrap_unchecked().yielded_values);
 
-            if !values.is_empty() {
+            if let Some(values) = values {
                 if raw.state() == state {
                     // Edge case: main thread is being yielded
                     //

tests/luau/cont.rs

@@ -4,10 +4,84 @@ use mlua::Lua;
 
 #[test]
 fn test_luau_continuation() {
-    // Yielding continuation
-    mlua::Lua::set_fflag("LuauYieldableContinuations", true).unwrap();
-
     let lua = Lua::new();
+    // No yielding continuation fflag test
+    let cont_func = lua
+        .create_function_with_luau_continuation(
+            |lua, a: u64| {
+                match lua.set_yield_args(a) {
+                    Ok(()) => println!("set_yield_args called"),
+                    Err(e) => println!("{:?}", e),
+                };
+                Ok(())
+            },
+            |_lua, _status, a: u64| {
+                println!("Reached cont");
+                Ok(a + 39)
+            },
+        )
+        .expect("Failed to create cont_func");
+
+    let luau_func = lua
+        .load(
+            "
+        local cont_func = ...
+        local res = cont_func(1)
+        return res + 1
+    ",
+        )
+        .into_function()
+        .expect("Failed to create function");
+
+    let th = lua
+        .create_thread(luau_func)
+        .expect("Failed to create luau thread");
+
+    let v = th
+        .resume::<mlua::MultiValue>(cont_func)
+        .expect("Failed to resume");
+    let v = th.resume::<i32>(v).expect("Failed to load continuation");
+
+    assert_eq!(v, 41);
+
+    // empty yield args test
+    let cont_func = lua
+        .create_function_with_luau_continuation(
+            |lua, _: ()| {
+                match lua.set_yield_args(()) {
+                    Ok(()) => println!("set_yield_args called"),
+                    Err(e) => println!("{:?}", e),
+                };
+                Ok(())
+            },
+            |_lua, _status, mv: mlua::MultiValue| Ok(mv.len()),
+        )
+        .expect("Failed to create cont_func");
+
+    let luau_func = lua
+        .load(
+            "
+        local cont_func = ...
+        local res = cont_func(1)
+        return res - 1
+    ",
+        )
+        .into_function()
+        .expect("Failed to create function");
+
+    let th = lua
+        .create_thread(luau_func)
+        .expect("Failed to create luau thread");
+
+    let v = th
+        .resume::<mlua::MultiValue>(cont_func)
+        .expect("Failed to resume");
+    assert!(v.is_empty());
+    let v = th.resume::<i32>(v).expect("Failed to load continuation");
+    assert_eq!(v, -1);
+
+    // Yielding continuation fflag test
+    mlua::Lua::set_fflag("LuauYieldableContinuations", true).unwrap();
 
     let cont_func = lua
         .create_function_with_luau_continuation(