Add a deprecation notice about async constructors

CHANGELOG.md

@@ -1,6 +1,14 @@
 # `wasm-bindgen` Change Log
 --------------------------------------------------------------------------------
 
+## Unreleased
+
+### Changed
+
+* Deprecation warning when using an async function as a constructor
+  [#4402](https://github.com/rustwasm/wasm-bindgen/pull/4402)
+
+
 ## [0.2.100](https://github.com/rustwasm/wasm-bindgen/compare/0.2.99...0.2.100)
 
 Released 2025-01-12

crates/backend/src/codegen.rs

@@ -811,6 +811,18 @@ impl TryToTokens for ast::Export {
                     add_check(quote! {
                         let _: #wasm_bindgen::__rt::marker::CheckSupportsConstructor<#class>;
                     });
+
+                    if self.function.r#async {
+                        (quote_spanned! {
+                            self.function.name_span =>
+                            const _: () = {
+                                #[deprecated(note = "async constructors produce invalid TS code and support will be removed in the future")]
+                                const fn constructor() {}
+                                constructor();
+                            };
+                        })
+                        .to_tokens(into);
+                    }
                 }
                 ast::MethodKind::Operation(operation) => match operation.kind {
                     ast::OperationKind::Getter(_) | ast::OperationKind::Setter(_) => {

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 }
     }
 

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

@@ -1,3 +1,11 @@
+warning: use of deprecated function `RustStruct::new::{closure#0}::_::constructor`: async constructors produce invalid TS code and support will be removed in the future
+  --> ui-tests/unsupported-options.rs:15:18
+   |
+15 |     pub async fn new() -> Self {
+   |                  ^^^
+   |
+   = note: `#[warn(deprecated)]` on by default
+
 error[E0277]: JavaScript constructors are not supported for `RustEnum`
   --> ui-tests/unsupported-options.rs:56:12
    |

tests/wasm/futures.rs

@@ -124,6 +124,7 @@ pub struct AsyncStruct;
 #[wasm_bindgen]
 impl AsyncStruct {
     #[wasm_bindgen(constructor)]
+    #[allow(deprecated)]
     pub async fn new() -> AsyncStruct {
         AsyncStruct
     }