LibJS/Bytecode: Implement async generators

[Lubrsi]

Bytecode diff:

Summary:
    Diff Tests:
        +3960 ✅ (8.018%) +32 ❌ (0.064%) -3992 📝 (8.082%)

Tests passing: 47122/49388 (95.4%)

AST diff:

Summary:
    Diff Tests:
        +3964 ✅ (8.026%) +27 ❌ (0.056%) +1 💥️ (0.002%) -3992 📝 (8.082%)

Tests passing: 47322/49388 (95.8%)

https://www.youtube.com/watch?v=_ovdm2yX4MA

[trflynn89]

Couple of high level questions without getting into the diff:
Is this really a 1 commit change?
test-js tests?
Any idea what the new crashes are?

[alimpfard]

bytecode changes mostly LGTM, just one comment about a comment:

[trflynn89]

Any idea what the new crashes are?

Seems like a spec bug to me. In bytecode mode, the crashes are:

test/built-ins/AsyncGeneratorPrototype/return/return-state-completed-broken-promise.js 📝 -> 💥
test/built-ins/AsyncGeneratorPrototype/return/return-suspendedStart-broken-promise.js  📝 -> 💥

These are asserting on infallible invocations to AsyncGeneratorAwaitReturn:

// ii. Perform ! AsyncGeneratorAwaitReturn(generator).
MUST(await_return());

However this operation is trivially fallible in step 6:

// 6. Let promise be ? PromiseResolve(%Promise%, completion.[[Value]]).
auto* promise = TRY(promise_resolve(vm, realm.intrinsics().promise_constructor(), completion.value().value()));

The current form of this AO was written here. Before that, abrupt completions from PromiseResolve were specifically handled to reject the promise and prevent further throwing.

[trflynn89]

Ah there's a spec PR open for this already: tc39/ecma262#2683

[Lubrsi]

All comments addressed except the 1 AST crash, which is:

💥️ /home/lukew/libjs-test262/test262/test/built-ins/AsyncGeneratorPrototype/next/request-queue-await-order.js                                                                                                                                                                     
                                                                                                                                                                                                                                                                                  
/home/lukew/Desktop/serenity-project/serenity/Userland/Libraries/LibJS/Runtime/Completion.cpp:117: JS::ThrowCompletionOr<JS::Value> JS::await(VM&, Value): Assertion `success.has_value()' failed.

Though I don't know if it's worth fixing this given that AST's await is very ad-hoc and AST is on it's way out.