Iterable / iterator support and generic type parameter propagation

[guybedford]

Adds first-class handling for the TS iterability protocol and bare generic type parameters in method signatures, so APIs like SyncKvStorage.put<T>(key: string, value: T) and SyncKvStorage.list<T>(): Iterable<[string, T]> round-trip into typed wasm-bindgen externs without erasing the generics.

Iterators

Iterator<T> and IterableIterator<T> map directly to js_sys::Iterator<T>; AsyncIterator<T> and AsyncIterableIterator<T> map to js_sys::AsyncIterator<T>. New IR variants TypeRef::Iterator and TypeRef::AsyncIterator emit through the existing generic container machinery alongside Promise<T>.

Iterable wrapper synthesis

Iterable<T> describes the protocol — an object exposing [Symbol.iterator](): Iterator<T> — distinct from Iterator<T> itself. To preserve that at the binding layer, top-level Iterable<T> returns now synthesize a wrapper extern type:

interface SyncKvStorage {
  list<T>(): Iterable<[string, T]>;
}

becomes:

pub type SyncKvStorageList<T: ::wasm_bindgen::JsGeneric>;
#[wasm_bindgen(method, js_name = "[Symbol.iterator]")]
pub fn iterator<T: ::wasm_bindgen::JsGeneric>(
    this: &SyncKvStorageList<T>,
) -> Iterator<ArrayTuple<(JsString, T)>>;

#[wasm_bindgen(method)]
pub fn list<T: ::wasm_bindgen::JsGeneric>(this: &SyncKvStorage) -> SyncKvStorageList<T>;

Naming follows the existing <Parent><Member> convention with dedup, mirroring anonymous-interface parameter synthesis. AsyncIterable<T> synthesizes the analogous wrapper keyed on [Symbol.asyncIterator]. The bracketed js_name matches wasm-bindgen's computed-property syntax for symbol-keyed methods. Nested occurrences inside unions or arrays are not synthesized — they erase to JsValue, matching the existing parameter-synthesis limitation.

Generic type parameter propagation

Bare TS generics on methods now survive codegen as <T: ::wasm_bindgen::JsGeneric> declarations rather than being erased to JsValue:

• New TypeRef::TypeParam(String) IR variant for in-scope generic type parameter references.
• convert_ts_type_scoped returns TypeParam(name) instead of Any for names in the method's type-parameter scope.
• convert_formal_params_with_synthesis threads scope through to parameter type conversion (previously called convert_ts_type, which ignored scope — a pre-existing bug that erased every generic argument type to JsValue and emitted dummy use JsValue as T; aliases).
• Method codegen collects every TypeParam reachable from the signature and emits the bound declaration. Synthesized iterable wrappers propagate any TypeParam from their item type onto the wrapper type itself.
• pub type X<T, …> declarations carry their generics so type aliases that mention generic parameters compile.

Before:

#[allow(dead_code)]
use JsValue as T;
#[wasm_bindgen(method)]
pub fn put(this: &SyncKvStorage, key: &str, value: &T);

After:

#[wasm_bindgen(method)]
pub fn put<T: ::wasm_bindgen::JsGeneric>(this: &SyncKvStorage, key: &str, value: &T);

External non-generic type protection

GenericInstantiation codegen now consults the codegen context's recorded type-parameter arity per local type. References like ReadableStream<Uint8Array> whose target accepts zero generics (non-generic web-sys / external types) drop the args and emit the bare base type, preventing E0107 at the use site.

Symbol-keyed methods

Symbol JS names like [Symbol.iterator] previously snake_cased to symboliterator. base_rust_name now strips the [Symbol. prefix and trailing ] when computing the Rust name, so the synthesized iterator methods read as iterator / async_iterator while js_name keeps the bracketed [Symbol.iterator] for wasm-bindgen.

Dedup numbering

dedupe_name now starts collision suffixes at _2 instead of _1 — the unsuffixed candidate is the implicit _1, so the first collision becomes _2. Matches the convention already used by parse::members::unique_type_name (Foo, Foo2, Foo3, …). User-visible: Response::json_1 (instance method colliding with the static json factory) becomes json_2, etc.

Header comment

Generated files now begin with // Generated by ts-gen. Do not edit. — prepended after rustfmt since quote! strips line comments from token streams.

Tests

• New tests/fixtures/iterables.d.ts exercises every iterability variant plus generic propagation through put<T> / get<T>.
• New tests/snapshots/iterables.rs blesses the expected output.
• Four unit tests in parse::members::tests cover happy path (sync), async, non-iterable rejection, and dedup.
• Existing snapshots updated: workers-types now produces typed Iterator<...>, AsyncIterator<...>, and per-method generics on put / get style methods that previously aliased T to JsValue.

Conventions

Two new sections, slotted next to Promise<T> since all three share the generic-container shape:

• Iterator<T> / IterableIterator<T> map to js_sys::Iterator<T>
• Iterable<T> returns synthesize a wrapper with [Symbol.iterator]()

AGENTS.md updated to clarify that CONVENTIONS sections are not numbered and are ordered from simplest to most obscure.

Verification

• just test — 123 unit + 1 snapshot pass
• just clippy — clean with -D warnings
• just fmt-check — clean

[guybedford]

Replaced by #24 on the new IR model.