Add a deprecation notice about async constructors

[danielronnkvist]

Adds a deprecation attribute to the method marked as constructor if it is async.

Appreciate suggestions on wording in the deprecation notice.

Resolves #3976

[daxpedda]

Please add a test in crates/macro/ui-tests. We can take a second look at the wording then.

EDIT: A changelog entry is missing as well.

[daxpedda]

Ah, unfortunately this doesn't seem to look right after all. Sorry!
The problem is we need a deprecation warning at the definition site, not the call site. Because realistically the call size will be in JS, not Rust.

This should work:

diff --git a/crates/backend/src/codegen.rs b/crates/backend/src/codegen.rs
index c8decb4a1..19bc75afd 100644
--- a/crates/backend/src/codegen.rs
+++ b/crates/backend/src/codegen.rs
@@ -806,6 +806,14 @@ 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 =>
+                            let async_constructors_deprecated: ();
+                        })
+                        .to_tokens(into);
+                    }
                 }
                 ast::MethodKind::Operation(operation) => match operation.kind {
                     ast::OperationKind::Getter(_) | ast::OperationKind::Setter(_) => {

I wanted to push this directly but didn't have permissions, but I can open a PR in your repo if this makes things easier.

[danielronnkvist]

@daxpedda I see, good point! But what do you think about still using the deprecated attribute? In my opinion, that outputs a better type of warning for this scenario than an unused variable.

diff --git a/crates/backend/src/codegen.rs b/crates/backend/src/codegen.rs
index c8decb4a..e8ff0bcf 100644
--- a/crates/backend/src/codegen.rs
+++ b/crates/backend/src/codegen.rs
@@ -806,6 +806,16 @@ 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 =>
+                            #[deprecated(note = "async constructors are not supported")]
+                            fn constructor() {}
+                            constructor();
+                        })
+                        .to_tokens(into);
+                    }
                 }
                 ast::MethodKind::Operation(operation) => match operation.kind {
                     ast::OperationKind::Getter(_) | ast::OperationKind::Setter(_) => {

[daxpedda]

Yes, definitely better!

[daxpedda]

Apart from the outstanding issue it LGTM!
Thank you again!

[daxpedda]

Thank you!