wasm-bindgen Files

Notices

• Threading support now requires -Clink-arg=--export=__heap_base to be set in RUSTFLAGS for nightly toolchains from 2026-05-06 onward, after [rust-lang/rust#156174](https://github.com/rust-lang/rust/issues/156174) removed the implicit __heap_base/__data_end exports on wasm* targets. Atomics CI, CLI reference tests, and the nodejs-threads, raytrace-parallel, and wasm-audio-worklet examples have been updated to pass --export=__heap_base explicitly. The flag is backward-compatible with older nightlies.

• -Cpanic=unwind on wasm targets now emits modern (exnref) exception handling by default after [rust-lang/rust#156061](https://github.com/rust-lang/rust/issues/156061), and requires Node.js 22.22.3+ (for WebAssembly.JSTag). Legacy EH wasm can still be produced on current nightlies by adding -Cllvm-args=-wasm-use-legacy-eh to RUSTFLAGS; Node.js 20 may be supported with legacy exception handling, with a tracking issue in #5151.

Added

• Implemented TryFromJsValue for Vec<T> where T: TryFromJsValue. A JS value converts when it is a real Array (per Array.isArray) and every element converts via T::try_from_js_value. This composes recursively (Vec<Vec<String>>, Vec<Option<T>>) and works for any T with a TryFromJsValue impl, including primitives, String, JsValue, and JsCast types. Array-likes (objects with length and numeric indices) are intentionally rejected to mirror the static ABI representation used by js_value_vector_from_abi.

• New extends_js_class and extends_js_namespace attributes on exported structs to allow defining the parent js_class name when it has been customized by js_name and the parent's own js_namespace as well in turn. New validation is added at code generation time that will now catch these cases instead of emitting invalid code. Example:

```rust #[wasm_bindgen(js_name = "Animal", js_namespace = zoo)] pub struct AnimalImpl { / ... / }

#[wasm_bindgen( extends = AnimalImpl, extends_js_class = "Animal", extends_js_namespace = zoo, )] pub struct DogImpl { / ... / } ``` #5154

Changed

• When an exported struct uses js_namespace, the corresponding value must now be repeated on every impl block. Previously the impl-side defaults silently worked resulting in inconsistent emission. Example:

```rust // Before: #[wasm_bindgen(js_namespace = "default")] pub struct Counter { / ... / }

#[wasm_bindgen] // worked, but fragile impl Counter { / ... / }

// After: #[wasm_bindgen(js_namespace = "default")] pub struct Counter { / ... / }

#[wasm_bindgen(js_namespace = "default")] // now required impl Counter { / ... / } `` To ease this transition forjs_namespace` usage, diagnostic messages now include hints for missing namespaces for easier fixing.

#5154

Fixed

• Fixed the descriptor interpreter panicking on Br and BrIf instructions emitted by recent nightly compilers when building with panic=unwind. #5158

• Emscripten output now works against vanilla upstream emscripten without requiring a fork. Dependency tracking, HEAP_DATA_VIEW setup, function-decl intrinsic inlining, catch-wrapper gating, and imported global handling have all been corrected; ESM imports (#[wasm_bindgen(module = "...")] and snippets) are emitted to a sidecar library_bindgen.extern-pre.js consumers pass to emcc via --extern-pre-js; namespaced exports (js_namespace = [...] on a struct/impl) now attach to Module.<segments> instead of emitting top-level export const (which emcc's library evaluator rejects); the generated .d.ts for namespaced exports is now valid TypeScript (mangled identifiers stay module-internal via declare class / declare enum / declare function plus export { BindgenModule }; to mark the file as a module; no spurious unqualified Calc: property on BindgenModule for namespaced items; namespace shapes land as plain interface members (app: { math: { Calc: typeof app__math__Calc } };) instead of the previously-emitted export let app: { ... }; which was invalid TS1131 syntax inside an interface body). #5156

• Fixed a duplicate phantom class being emitted for an exported struct renamed via js_name (Rust ident != JS class name) and/or placed in a js_namespace, when the struct crosses the boundary as a JsValue (e.g. via .into()). The WrapInExportedClass / UnwrapExportedClass imports were keyed by the Rust ident rather than the qualified JS name that exported_classes is keyed by (a regression from [#5154]), so a fresh empty class entry was minted and emitted alongside the real one, with a free() referencing a nonexistent wasm export. Riding the same release's [#5154] wire-format bump, the now-vestigial rust_name field is dropped from the schema and the namespace-qualified name is no longer cached on AuxStruct, AuxEnum, or ExportedClass (derived on demand from (name, js_namespace)), collapsing three fallback chains that only papered over the pre-#5154 keying.

#5160