Adding in async support for start

crates/backend/src/codegen.rs

@@ -447,17 +447,37 @@ impl TryToTokens for ast::Export {
         // since we're returning a promise to JS, and this will implicitly
         // require that the function returns a `Future<Output = Result<...>>`
         let (ret_ty, ret_expr) = if self.function.r#async {
+            if self.start {
+                (
+                    quote! { () },
+                    quote! {
+                        wasm_bindgen_futures::spawn_local(async move {
+                            <#syn_ret as wasm_bindgen::__rt::Start>::start(#ret.await);
+                        })
+                    },
+                )
+
+            } else {
+                (
+                    quote! { wasm_bindgen::JsValue },
+                    quote! {
+                        wasm_bindgen_futures::future_to_promise(async move {
+                            <#syn_ret as wasm_bindgen::__rt::IntoJsResult>::into_js_result(#ret.await)
+                        }).into()
+                    },
+                )
+            }
+
+        } else if self.start {
             (
-                quote! { wasm_bindgen::JsValue },
-                quote! {
-                    wasm_bindgen_futures::future_to_promise(async {
-                        wasm_bindgen::__rt::IntoJsResult::into_js_result(#ret.await)
-                    }).into()
-                },
+                quote! { () },
+                quote! { <#syn_ret as wasm_bindgen::__rt::Start>::start(#ret) },
             )
+
         } else {
             (quote! { #syn_ret }, quote! { #ret })
         };
+
         let projection = quote! { <#ret_ty as wasm_bindgen::convert::ReturnWasmAbi> };
         let convert_ret = quote! { #projection::return_abi(#ret_expr) };
         let describe_ret = quote! {

crates/macro/ui-tests/start-function.rs

@@ -9,4 +9,25 @@ pub fn foo2(x: u32) {}
 #[wasm_bindgen(start)]
 pub fn foo3<T>() {}
 
+#[wasm_bindgen(start)]
+pub fn foo4() -> Result<(), JsValue> { Ok(()) }
+
+#[wasm_bindgen(start)]
+pub fn foo5() -> Result<JsValue, ()> { Err(()) }
+
+#[wasm_bindgen(start)]
+pub fn foo6() -> Result<JsValue, JsValue> { Ok(JsValue::from(1u32)) }
+
+#[wasm_bindgen(start)]
+pub async fn foo_async1() {}
+
+#[wasm_bindgen(start)]
+pub async fn foo_async2() -> Result<(), JsValue> { Ok(()) }
+
+#[wasm_bindgen(start)]
+pub async fn foo_async3() -> Result<JsValue, ()> { Err(()) }
+
+#[wasm_bindgen(start)]
+pub async fn foo_async4() -> Result<JsValue, JsValue> { Ok(JsValue::from(1u32)) }
+
 fn main() {}

crates/macro/ui-tests/start-function.stderr

@@ -9,3 +9,23 @@ error: the start function cannot have generics
    |
 10 | pub fn foo3<T>() {}
    |            ^^^
+
+error[E0277]: the trait bound `std::result::Result<wasm_bindgen::JsValue, ()>: wasm_bindgen::__rt::Start` is not satisfied
+  --> $DIR/start-function.rs:27:1
+   |
+27 | #[wasm_bindgen(start)]
+   | ^^^^^^^^^^^^^^^^^^^^^^ the trait `wasm_bindgen::__rt::Start` is not implemented for `std::result::Result<wasm_bindgen::JsValue, ()>`
+   |
+   = help: the following implementations were found:
+             <std::result::Result<(), E> as wasm_bindgen::__rt::Start>
+   = note: required by `wasm_bindgen::__rt::Start::start`
+
+error[E0277]: the trait bound `std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue>: wasm_bindgen::__rt::Start` is not satisfied
+  --> $DIR/start-function.rs:30:1
+   |
+30 | #[wasm_bindgen(start)]
+   | ^^^^^^^^^^^^^^^^^^^^^^ the trait `wasm_bindgen::__rt::Start` is not implemented for `std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue>`
+   |
+   = help: the following implementations were found:
+             <std::result::Result<(), E> as wasm_bindgen::__rt::Start>
+   = note: required by `wasm_bindgen::__rt::Start::start`

src/lib.rs

@@ -1116,6 +1116,27 @@ pub mod __rt {
             }
         }
     }
+
+
+    /// An internal helper trait for usage in `#[wasm_bindgen(start)]`
+    /// functions to throw the error (if it is `Err`).
+    pub trait Start {
+        fn start(self);
+    }
+
+    impl Start for () {
+        #[inline]
+        fn start(self) {}
+    }
+
+    impl<E: Into<JsValue>> Start for Result<(), E> {
+        #[inline]
+        fn start(self) {
+            if let Err(e) = self {
+                crate::throw_val(e.into());
+            }
+        }
+    }
 }
 
 /// A wrapper type around slices and vectors for binding the `Uint8ClampedArray`