Move async function arguments into spawned futures

crates/backend/src/codegen.rs

@@ -470,19 +470,15 @@ impl TryToTokens for ast::Export {
                     quote! { () },
                     quote! { () },
                     quote! {
-                        wasm_bindgen_futures::spawn_local(async move {
-                            <#syn_ret as wasm_bindgen::__rt::Start>::start(#ret.await);
-                        })
+                        <#syn_ret as wasm_bindgen::__rt::Start>::start(#ret.await)
                     },
                 )
             } else {
                 (
                     quote! { wasm_bindgen::JsValue },
                     quote! { #syn_ret },
                     quote! {
-                        wasm_bindgen_futures::future_to_promise(async move {
-                            <#syn_ret as wasm_bindgen::__rt::IntoJsResult>::into_js_result(#ret.await)
-                        }).into()
+                        <#syn_ret as wasm_bindgen::__rt::IntoJsResult>::into_js_result(#ret.await)
                     },
                 )
             }
@@ -496,8 +492,32 @@ impl TryToTokens for ast::Export {
             (quote! { #syn_ret }, quote! { #syn_ret }, quote! { #ret })
         };
 
+        let mut call = quote! {
+            {
+                #(#arg_conversions)*
+                let #ret = #receiver(#(#converted_arguments),*);
+                #ret_expr
+            }
+        };
+
+        if self.function.r#async {
+            if self.start {
+                call = quote! {
+                    wasm_bindgen_futures::spawn_local(async move {
+                        #call
+                    })
+                }
+            } else {
+                call = quote! {
+                    wasm_bindgen_futures::future_to_promise(async move {
+                        #call
+                    }).into()
+                }
+            }
+        }
+
         let projection = quote! { <#ret_ty as wasm_bindgen::convert::ReturnWasmAbi> };
-        let convert_ret = quote! { #projection::return_abi(#ret_expr) };
+        let convert_ret = quote! { #projection::return_abi(#ret) };
         let describe_ret = quote! {
             <#ret_ty as WasmDescribe>::describe();
             <#inner_ret_ty as WasmDescribe>::describe();
@@ -523,13 +543,8 @@ impl TryToTokens for ast::Export {
                 )]
                 pub unsafe extern "C" fn #generated_name(#(#args),*) -> #projection::Abi {
                     #start_check
-                    // Scope all local variables to be destroyed after we call
-                    // the function to ensure that `#convert_ret`, if it panics,
-                    // doesn't leak anything.
-                    let #ret = {
-                        #(#arg_conversions)*
-                        #receiver(#(#converted_arguments),*)
-                    };
+
+                    let #ret = #call;
                     #convert_ret
                 }
             };

tests/wasm/futures.js

@@ -15,6 +15,9 @@ exports.call_exports = async function() {
   await assert.rejects(wasm.async_throw_message(), /async message/);
   await assert.rejects(wasm.async_throw_jserror(), /async message/);
   await assert.rejects(wasm.async_throw_custom_error(), /custom error/);
+  assert.strictEqual("Hi, Jim!", await wasm.async_take_reference("Jim"));
+  const foo = await new wasm.AsyncStruct();
+  assert.strictEqual(42, await foo.method());
 };
 
 exports.call_promise = async function() {

tests/wasm/futures.rs

@@ -106,6 +106,26 @@ pub async fn async_throw_custom_error() -> Result<AsyncCustomReturn, AsyncCustom
     })
 }
 
+#[wasm_bindgen]
+pub async fn async_take_reference(x: &str) -> String {
+    format!("Hi, {x}!")
+}
+
+#[wasm_bindgen]
+pub struct AsyncStruct;
+
+#[wasm_bindgen]
+impl AsyncStruct {
+    #[wasm_bindgen(constructor)]
+    pub async fn new() -> AsyncStruct {
+        AsyncStruct
+    }
+
+    pub async fn method(&self) -> u32 {
+        42
+    }
+}
+
 #[wasm_bindgen_test]
 async fn test_promise() {
     assert_eq!(call_promise().await.as_string(), Some(String::from("ok")))