Implement async support in metatype

crates/luaffi/src/future.rs

@@ -1,7 +1,7 @@
 use crate::{
     __internal::{display, type_id},
-    Cdef, CdefBuilder, IntoFfi, Metatype, MetatypeBuilder, Type, TypeBuilder, TypeType,
-    UnsafeExternCFn,
+    Cdef, CdefBuilder, FfiReturnConvention, IntoFfi, Metatype, MetatypeBuilder, Type, TypeBuilder,
+    TypeType, UnsafeExternCFn,
 };
 use luaify::luaify;
 use std::{
@@ -168,6 +168,11 @@ unsafe impl<F: Future<Output: IntoFfi> + 'static> Metatype for lua_future<F> {
 unsafe impl<F: Future<Output: IntoFfi> + 'static> IntoFfi for lua_future<F> {
     type Into = lua_future<F>;
 
+    fn convention() -> FfiReturnConvention {
+        // futures are always returned by-value due to rust type inference limitations
+        FfiReturnConvention::ByValue
+    }
+
     fn convert(self) -> Self::Into {
         self
     }