Add support for #[wasm_bindgen] on async fn (#1754)

This commit adds support to attach `#[wasm_bindgen]` on an `async fn`
which will change the return value into a `Promise` in JS. This in
theory has the exact same semantics as an `async` function in JS where
you call it with all the arguments, nothing happens and you get a
promise back, and then later the promise actually resolves.

This commit also adds a helper trait, `IntoJsResult`, to allow `async`
functions with multiple kinds of return values instead of requiring
everything to be `Result<JsValue, JsValue>`.

crates/macro/ui-tests/async-errors.rs

@@ -0,0 +1,39 @@
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen]
+pub struct MyType;
+
+#[wasm_bindgen]
+pub async fn good1() { loop {} }
+#[wasm_bindgen]
+pub async fn good2() -> JsValue { loop {} }
+#[wasm_bindgen]
+pub async fn good3() -> u32 { loop {} }
+#[wasm_bindgen]
+pub async fn good4() -> MyType { loop {} }
+#[wasm_bindgen]
+pub async fn good5() -> Result<(), JsValue> { loop {} }
+#[wasm_bindgen]
+pub async fn good6() -> Result<JsValue, JsValue> { loop {} }
+#[wasm_bindgen]
+pub async fn good7() -> Result<u32, JsValue> { loop {} }
+#[wasm_bindgen]
+pub async fn good8() -> Result<MyType, JsValue> { loop {} }
+#[wasm_bindgen]
+pub async fn good9() -> Result<MyType, u32> { loop {} }
+#[wasm_bindgen]
+pub async fn good10() -> Result<MyType, MyType> { loop {} }
+
+pub struct BadType;
+
+#[wasm_bindgen]
+pub async fn bad1() -> Result<(), ()> { loop {} }
+#[wasm_bindgen]
+pub async fn bad2() -> Result<(), BadType> { loop {} }
+#[wasm_bindgen]
+pub async fn bad3() -> BadType { loop {} }
+#[wasm_bindgen]
+pub async fn bad4() -> Result<BadType, JsValue> { loop {} }
+
+
+fn main() {}

crates/macro/ui-tests/async-errors.stderr

@@ -0,0 +1,50 @@
+error[E0277]: the trait bound `std::result::Result<(), ()>: wasm_bindgen::__rt::IntoJsResult` is not satisfied
+  --> $DIR/async-errors.rs:29:1
+   |
+29 | #[wasm_bindgen]
+   | ^^^^^^^^^^^^^^^ the trait `wasm_bindgen::__rt::IntoJsResult` is not implemented for `std::result::Result<(), ()>`
+   |
+   = help: the following implementations were found:
+             <std::result::Result<(), E> as wasm_bindgen::__rt::IntoJsResult>
+             <std::result::Result<T, E> as wasm_bindgen::__rt::IntoJsResult>
+   = note: required by `wasm_bindgen::__rt::IntoJsResult::into_js_result`
+
+error[E0277]: the trait bound `std::result::Result<(), BadType>: wasm_bindgen::__rt::IntoJsResult` is not satisfied
+  --> $DIR/async-errors.rs:31:1
+   |
+31 | #[wasm_bindgen]
+   | ^^^^^^^^^^^^^^^ the trait `wasm_bindgen::__rt::IntoJsResult` is not implemented for `std::result::Result<(), BadType>`
+   |
+   = help: the following implementations were found:
+             <std::result::Result<(), E> as wasm_bindgen::__rt::IntoJsResult>
+             <std::result::Result<T, E> as wasm_bindgen::__rt::IntoJsResult>
+   = note: required by `wasm_bindgen::__rt::IntoJsResult::into_js_result`
+
+error[E0277]: the trait bound `wasm_bindgen::JsValue: std::convert::From<BadType>` is not satisfied
+  --> $DIR/async-errors.rs:33:1
+   |
+33 | #[wasm_bindgen]
+   | ^^^^^^^^^^^^^^^ the trait `std::convert::From<BadType>` is not implemented for `wasm_bindgen::JsValue`
+   |
+   = help: the following implementations were found:
+             <wasm_bindgen::JsValue as std::convert::From<&'a T>>
+             <wasm_bindgen::JsValue as std::convert::From<&'a std::string::String>>
+             <wasm_bindgen::JsValue as std::convert::From<&'a str>>
+             <wasm_bindgen::JsValue as std::convert::From<MyType>>
+           and 62 others
+   = note: required because of the requirements on the impl of `std::convert::Into<wasm_bindgen::JsValue>` for `BadType`
+   = note: required because of the requirements on the impl of `wasm_bindgen::__rt::IntoJsResult` for `BadType`
+   = note: required by `wasm_bindgen::__rt::IntoJsResult::into_js_result`
+
+error[E0277]: the trait bound `std::result::Result<BadType, wasm_bindgen::JsValue>: wasm_bindgen::__rt::IntoJsResult` is not satisfied
+  --> $DIR/async-errors.rs:35:1
+   |
+35 | #[wasm_bindgen]
+   | ^^^^^^^^^^^^^^^ the trait `wasm_bindgen::__rt::IntoJsResult` is not implemented for `std::result::Result<BadType, wasm_bindgen::JsValue>`
+   |
+   = help: the following implementations were found:
+             <std::result::Result<(), E> as wasm_bindgen::__rt::IntoJsResult>
+             <std::result::Result<T, E> as wasm_bindgen::__rt::IntoJsResult>
+   = note: required by `wasm_bindgen::__rt::IntoJsResult::into_js_result`
+
+For more information about this error, try `rustc --explain E0277`.