Add a deprecation notice about async constructors

CHANGELOG.md

@@ -43,6 +43,9 @@
 * `console.*()` calls in tests are now always intercepted by default. To show them use `--nocapture`. When shown they are always printed in-place instead of after test results, analogous to `cargo test`.
   [#4356](https://github.com/rustwasm/wasm-bindgen/pull/4356)
 
+* Deprecation warning when using an async function as a constructor
+  [#4402](https://github.com/rustwasm/wasm-bindgen/pull/4402)
+
 ### Fixed
 
 - Fixed using [JavaScript keyword](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Lexical_grammar#keywords) as identifiers not being handled correctly.

crates/macro-support/src/parser.rs

@@ -1418,6 +1418,11 @@ impl MacroParse<&ClassMarker> for &mut syn::ImplItemFn {
             FunctionPosition::Impl { self_ty: class },
         )?;
         let method_kind = if opts.constructor().is_some() {
+            if function.r#async {
+                self.attrs.push(syn::parse_quote! {
+                    #[deprecated(note = "constructors cannot be async")]
+                });
+            }
             ast::MethodKind::Constructor
         } else {
             let is_static = method_self.is_none();

crates/macro/ui-tests/unsupported-options.rs

@@ -12,7 +12,7 @@ impl RustStruct {
     pub fn static_method() {}
 
     #[wasm_bindgen(constructor)]
-    pub fn new() -> Self {
+    pub async fn new() -> Self {
         Self { data: 0 }
     }
 
@@ -79,4 +79,8 @@ impl NonWasmType {
     pub fn static_method() {}
 }
 
-fn main() {}
+fn main() {
+    async {
+        RustStruct::new().await;
+    };
+}

crates/macro/ui-tests/unsupported-options.stderr

@@ -93,3 +93,11 @@ note: required by a bound in `CheckSupportsStaticProperty`
    | pub struct CheckSupportsStaticProperty<T: SupportsStaticProperty>(T);
    |                                           ^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `CheckSupportsStaticProperty`
    = note: this error originates in the attribute macro `wasm_bindgen::prelude::__wasm_bindgen_class_marker` which comes from the expansion of the attribute macro `wasm_bindgen` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+warning: use of deprecated associated function `RustStruct::new`: constructors cannot be async
+  --> ui-tests/unsupported-options.rs:84:21
+   |
+84 |         RustStruct::new().await;
+   |                     ^^^
+   |
+   = note: `#[warn(deprecated)]` on by default