Lower async return primitives to js_sys wrappers#14
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guybedford merged 2 commits intoApr 29, 2026
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`pub async fn -> Result<T, JsValue>` requires `T: JsGeneric` (an externref-backed type) under the workers-rs wasm-bindgen fork. Rust primitives don't satisfy that bound, so emitting `Result<bool|f64|String, JsValue>` for `Promise<boolean|number|string>` fails to compile under that flavour. Lower these primitives to their `js_sys` wrapper types — `Boolean`, `Number`, `JsString` — in async return position only. The wrappers ARE externref-backed and pass the JsGeneric bound. Callers recover the Rust primitive via `.value_of()` (Boolean / Number) or `String::from(_)` (JsString). The lowering is scoped to async returns: arguments, properties, and sync returns continue to use Rust primitives. Void returns stay as `()`, named JS types stay as themselves, JsValue stays as JsValue — the constraint only bites for primitives wrapped by `Promise<T>`. Snapshot impact: 36 async primitive returns across the four fixtures shift to JS wrapper types. Public API for callers changes from `Result<bool, JsValue>` to `Result<Boolean, JsValue>` etc., trading one `.value_of()` call at the call site for portability across wasm-bindgen flavours and a single extern (no Rust-side cast logic).
guybedford
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Apr 29, 2026
guybedford
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This is correct - and was even the nature of async fn in Wasm Bindgen before we added generics. JsGeneric just does require a JsValue / externref ABI so these types are the representations to do that.
If we hit real ergonomic issues though, please do bring them up.
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Promise<T>/JsFuture<T>inwasm-bindgenrequireT: JsGeneric,which bare Rust primitives (
bool,f64,String) don't satisfy.primitives lower to
js_syswrappers (Boolean,Number,JsString,Undefined) andNullable<T>becomesJsOption<T>.is_asyncthroughto_return_typefrom the three call sitesin
codegen/{functions,classes}.rs(free fns, methods, static methods,constructors).
CONVENTIONS.md,including how callers recover primitives (
.value_of()forBoolean/Number,String::from(_)forJsString).basic,coverage,cloudflare-worker, andworkers-typesreflect the new wrapper-typed async signatures.