Fix codegen for descriptors of async fn returns (#1782)

They erroneously reported returning the original return type, not the
promise! Let's also add a bunch of positive tests while we're at it.

Closes #1781

Cargo.toml

@@ -44,6 +44,7 @@ cfg-if = "0.1.9"
 [target.'cfg(target_arch = "wasm32")'.dev-dependencies]
 js-sys = { path = 'crates/js-sys', version = '0.3.27' }
 wasm-bindgen-test = { path = 'crates/test', version = '=0.3.0' }
+wasm-bindgen-futures = { path = 'crates/futures', version = '=0.4.0' }
 serde_derive = "1.0"
 wasm-bindgen-test-crate-a = { path = 'tests/crates/a', version = '0.1' }
 wasm-bindgen-test-crate-b = { path = 'tests/crates/b', version = '0.1' }

crates/backend/src/codegen.rs

@@ -446,36 +446,31 @@ impl TryToTokens for ast::Export {
         // For an `async` function we always run it through `future_to_promise`
         // since we're returning a promise to JS, and this will implicitly
         // require that the function returns a `Future<Output = Result<...>>`
-        let (ret_expr, projection) = if self.function.r#async {
+        let (ret_ty, ret_expr) = if self.function.r#async {
             (
+                quote! { wasm_bindgen::JsValue },
                 quote! {
                     wasm_bindgen_futures::future_to_promise(async {
                         wasm_bindgen::__rt::IntoJsResult::into_js_result(#ret.await)
                     }).into()
                 },
-                quote! {
-                    <wasm_bindgen::JsValue as wasm_bindgen::convert::ReturnWasmAbi>
-                },
             )
         } else {
             (
+                quote! { #syn_ret },
                 quote! { #ret },
-                quote! {
-                    <#syn_ret as wasm_bindgen::convert::ReturnWasmAbi>
-                },
             )
         };
+        let projection = quote! { <#ret_ty as wasm_bindgen::convert::ReturnWasmAbi> };
         let convert_ret = quote! { #projection::return_abi(#ret_expr) };
         let describe_ret = quote! {
-            <#syn_ret as WasmDescribe>::describe();
+            <#ret_ty as WasmDescribe>::describe();
         };
         let nargs = self.function.arguments.len() as u32;
         let attrs = &self.function.rust_attrs;
 
         let start_check = if self.start {
-            quote! {
-                const _ASSERT: fn() = || -> #projection::Abi { loop {} };
-            }
+            quote! { const _ASSERT: fn() = || -> #projection::Abi { loop {} }; }
         } else {
             quote! {}
         };

tests/wasm/futures.js

@@ -0,0 +1,16 @@
+const assert = require('assert');
+const wasm = require('wasm-bindgen-test');
+
+exports.call_exports = async function() {
+  await wasm.async_do_nothing();
+  assert.strictEqual(1, await wasm.async_return_1());
+  assert.strictEqual(2, await wasm.async_return_2());
+  await wasm.async_nothing_again();
+  assert.strictEqual(3, await wasm.async_return_3());
+  assert.strictEqual(4, await wasm.async_return_4());
+  assert.strictEqual(5, (await wasm.async_return_5()).val);
+  assert.strictEqual(6, (await wasm.async_return_6()).val);
+  assert.strictEqual(7, (await wasm.async_return_7()).val);
+  assert.strictEqual(8, (await wasm.async_return_8()).val);
+  await assert.rejects(wasm.async_throw(), /async message/);
+};

tests/wasm/futures.rs

@@ -0,0 +1,70 @@
+use wasm_bindgen::prelude::*;
+use wasm_bindgen_test::*;
+
+#[wasm_bindgen(module = "tests/wasm/futures.js")]
+extern "C" {
+    fn call_exports() -> js_sys::Promise;
+}
+
+#[wasm_bindgen_test]
+async fn smoke() {
+    wasm_bindgen_futures::JsFuture::from(call_exports()).await.unwrap();
+}
+
+#[wasm_bindgen]
+pub async fn async_do_nothing() {}
+
+#[wasm_bindgen]
+pub async fn async_return_1() -> JsValue {
+    1.into()
+}
+
+#[wasm_bindgen]
+pub async fn async_return_2() -> u32 {
+    2
+}
+
+#[wasm_bindgen]
+pub async fn async_nothing_again() -> Result<(), JsValue> {
+    Ok(())
+}
+
+#[wasm_bindgen]
+pub async fn async_return_3() -> Result<u32, JsValue> {
+    Ok(3)
+}
+
+#[wasm_bindgen]
+pub async fn async_return_4() -> Result<JsValue, JsValue> {
+    Ok(4.into())
+}
+
+#[wasm_bindgen]
+pub struct AsyncCustomReturn {
+    pub val: u32,
+}
+
+#[wasm_bindgen]
+pub async fn async_return_5() -> AsyncCustomReturn {
+    AsyncCustomReturn { val: 5 }
+}
+
+#[wasm_bindgen]
+pub async fn async_return_6() -> Result<AsyncCustomReturn, JsValue> {
+    Ok(AsyncCustomReturn { val: 6 })
+}
+
+#[wasm_bindgen]
+pub async fn async_return_7() -> Result<AsyncCustomReturn, u32> {
+    Ok(AsyncCustomReturn { val: 7 })
+}
+
+#[wasm_bindgen]
+pub async fn async_return_8() -> Result<AsyncCustomReturn, AsyncCustomReturn> {
+    Ok(AsyncCustomReturn { val: 8 })
+}
+
+#[wasm_bindgen]
+pub async fn async_throw() -> Result<(), js_sys::Error> {
+    Err(js_sys::Error::new("async message"))
+}

tests/wasm/main.rs

@@ -23,6 +23,7 @@ pub mod duplicates;
 pub mod enums;
 #[path = "final.rs"]
 pub mod final_;
+pub mod futures;
 pub mod getters_and_setters;
 pub mod import_class;
 pub mod imports;