Bug 1331092 - Part 7: Implement Async Generator yield*.

Tag #1287

14 files changed, 451 insertions, 43 deletions

diff --git a/js/public/Class.h b/js/public/Class.h
index a7250eac05..f4fa9ccafb 100644
--- a/js/public/Class.h
+++ b/js/public/Class.h
@@ -779,7 +779,7 @@ struct JSClass {
 // application.
 #define JSCLASS_GLOBAL_APPLICATION_SLOTS 5
 #define JSCLASS_GLOBAL_SLOT_COUNT                                             \
-    (JSCLASS_GLOBAL_APPLICATION_SLOTS + JSProto_LIMIT * 2 + 44)
+    (JSCLASS_GLOBAL_APPLICATION_SLOTS + JSProto_LIMIT * 2 + 45)
 #define JSCLASS_GLOBAL_FLAGS_WITH_SLOTS(n)                                    \
     (JSCLASS_IS_GLOBAL | JSCLASS_HAS_RESERVED_SLOTS(JSCLASS_GLOBAL_SLOT_COUNT + (n)))
 #define JSCLASS_GLOBAL_FLAGS                                                  \
diff --git a/js/src/builtin/Promise.cpp b/js/src/builtin/Promise.cpp
index 59e710935e..d2b7b543b1 100644
--- a/js/src/builtin/Promise.cpp
+++ b/js/src/builtin/Promise.cpp
@@ -2329,6 +2329,165 @@ js::AsyncGeneratorAwait(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj
     return PerformPromiseThenWithReaction(cx, promise, reaction);
 }
 
+// Async Iteration proposal 6.1.3.2.1 %AsyncFromSyncIteratorPrototype%.next
+// Async Iteration proposal 6.1.3.2.2 %AsyncFromSyncIteratorPrototype%.return
+// Async Iteration proposal 6.1.3.2.3 %AsyncFromSyncIteratorPrototype%.throw
+bool
+js::AsyncFromSyncIteratorMethod(JSContext* cx, CallArgs& args, CompletionKind completionKind)
+{
+    // Step 1.
+    RootedValue thisVal(cx, args.thisv());
+
+    // Step 2.
+    RootedObject resultPromise(cx, CreatePromiseObjectWithoutResolutionFunctions(cx));
+    if (!resultPromise)
+        return false;
+
+    // Step 3.
+    if (!thisVal.isObject() || !thisVal.toObject().is<AsyncFromSyncIteratorObject>()) {
+        // Step 3.a.
+        RootedValue badGeneratorError(cx);
+        if (!GetTypeError(cx, JSMSG_NOT_AN_ASYNC_ITERATOR, &badGeneratorError))
+            return false;
+
+        // Step 3.b.
+        if (!RejectMaybeWrappedPromise(cx, resultPromise, badGeneratorError))
+            return false;
+
+        // Step 3.c.
+        args.rval().setObject(*resultPromise);
+        return true;
+    }
+
+    Rooted<AsyncFromSyncIteratorObject*> asyncIter(
+        cx, &thisVal.toObject().as<AsyncFromSyncIteratorObject>());
+
+    // Step 4.
+    RootedObject iter(cx, asyncIter->iterator());
+
+    RootedValue resultVal(cx);
+    RootedValue func(cx);
+    if (completionKind == CompletionKind::Normal) {
+        // 6.1.3.2.1 steps 5-6 (partially).
+        if (!GetProperty(cx, iter, iter, cx->names().next, &func))
+            return AbruptRejectPromise(cx, args, resultPromise, nullptr);
+    } else if (completionKind == CompletionKind::Return) {
+        // 6.1.3.2.2 steps 5-6.
+        if (!GetProperty(cx, iter, iter, cx->names().return_, &func))
+            return AbruptRejectPromise(cx, args, resultPromise, nullptr);
+
+        // Step 7.
+        if (func.isNullOrUndefined()) {
+            // Step 7.a.
+            RootedObject resultObj(cx, CreateIterResultObject(cx, args.get(0), true));
+            if (!resultObj)
+                return AbruptRejectPromise(cx, args, resultPromise, nullptr);
+
+            RootedValue resultVal(cx, ObjectValue(*resultObj));
+
+            // Step 7.b.
+            if (!ResolvePromiseInternal(cx, resultPromise, resultVal))
+                return AbruptRejectPromise(cx, args, resultPromise, nullptr);
+
+            // Step 7.c.
+            args.rval().setObject(*resultPromise);
+            return true;
+        }
+    } else {
+        // 6.1.3.2.3 steps 5-6.
+        MOZ_ASSERT(completionKind == CompletionKind::Throw);
+        if (!GetProperty(cx, iter, iter, cx->names().throw_, &func))
+            return AbruptRejectPromise(cx, args, resultPromise, nullptr);
+
+        // Step 7.
+        if (func.isNullOrUndefined()) {
+            // Step 7.a.
+            if (!RejectMaybeWrappedPromise(cx, resultPromise, args.get(0)))
+                return AbruptRejectPromise(cx, args, resultPromise, nullptr);
+
+            // Step 7.b.
+            args.rval().setObject(*resultPromise);
+            return true;
+        }
+    }
+
+    // 6.1.3.2.1 steps 5-6 (partially).
+    // 6.1.3.2.2, 6.1.3.2.3 steps 8-9.
+    RootedValue iterVal(cx, ObjectValue(*iter));
+    FixedInvokeArgs<1> args2(cx);
+    args2[0].set(args.get(0));
+    if (!js::Call(cx, func, iterVal, args2, &resultVal))
+        return AbruptRejectPromise(cx, args, resultPromise, nullptr);
+
+    // 6.1.3.2.1 steps 5-6 (partially).
+    // 6.1.3.2.2, 6.1.3.2.3 steps 10.
+    if (!resultVal.isObject()) {
+        CheckIsObjectKind kind;
+        switch (completionKind) {
+          case CompletionKind::Normal:
+            kind = CheckIsObjectKind::IteratorNext;
+            break;
+          case CompletionKind::Throw:
+            kind = CheckIsObjectKind::IteratorThrow;
+            break;
+          case CompletionKind::Return:
+            kind = CheckIsObjectKind::IteratorReturn;
+            break;
+        }
+        MOZ_ALWAYS_FALSE(ThrowCheckIsObject(cx, kind));
+        return AbruptRejectPromise(cx, args, resultPromise, nullptr);
+    }
+
+    RootedObject resultObj(cx, &resultVal.toObject());
+
+    // Following step numbers are for 6.1.3.2.1.
+    // For 6.1.3.2.2 and 6.1.3.2.3, steps 7-16 corresponds to steps 11-20.
+
+    // Steps 7-8.
+    RootedValue value(cx);
+    if (!GetProperty(cx, resultObj, resultObj, cx->names().value, &value))
+        return AbruptRejectPromise(cx, args, resultPromise, nullptr);
+
+    // Steps 9-10.
+    RootedValue doneVal(cx);
+    if (!GetProperty(cx, resultObj, resultObj, cx->names().done, &doneVal))
+        return AbruptRejectPromise(cx, args, resultPromise, nullptr);
+    bool done = ToBoolean(doneVal);
+
+    // Step 11.
+    Rooted<PromiseObject*> promise(cx, CreatePromiseObjectWithoutResolutionFunctions(cx));
+    if (!promise)
+        return false;
+
+    // Step 12.
+    if (!ResolvePromiseInternal(cx, promise, value))
+        return false;
+
+    // Steps 13-14.
+    RootedValue onFulfilled(cx, Int32Value(done
+                                           ? PromiseHandlerAsyncIteratorValueUnwrapDone
+                                           : PromiseHandlerAsyncIteratorValueUnwrapNotDone));
+
+    RootedObject incumbentGlobal(cx);
+    if (!GetObjectFromIncumbentGlobal(cx, &incumbentGlobal))
+        return false;
+
+    // Step 15.
+    Rooted<PromiseReactionRecord*> reaction(cx, NewReactionRecord(cx, resultPromise, onFulfilled,
+                                                                  UndefinedHandleValue,
+                                                                  nullptr, nullptr,
+                                                                  incumbentGlobal));
+    if (!reaction)
+        return false;
+
+    if (!PerformPromiseThenWithReaction(cx, promise, reaction))
+        return false;
+
+    // Step 16.
+    args.rval().setObject(*resultPromise);
+    return true;
+}
+
 // Async Iteration proposal 6.4.3.3.
 MOZ_MUST_USE bool
 js::AsyncGeneratorResolve(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
diff --git a/js/src/builtin/Promise.h b/js/src/builtin/Promise.h
index 680ab2abd9..b04bd0e2a6 100644
--- a/js/src/builtin/Promise.h
+++ b/js/src/builtin/Promise.h
@@ -168,6 +168,9 @@ MOZ_MUST_USE bool
 AsyncGeneratorEnqueue(JSContext* cx, HandleValue asyncGenVal, CompletionKind completionKind,
                       HandleValue completionValue, MutableHandleValue result);
 
+bool
+AsyncFromSyncIteratorMethod(JSContext* cx, CallArgs& args, CompletionKind completionKind);
+
 /**
  * A PromiseTask represents a task that can be dispatched to a helper thread
  * (via StartPromiseTask), executed (by implementing PromiseTask::execute()),
diff --git a/js/src/frontend/BytecodeEmitter.cpp b/js/src/frontend/BytecodeEmitter.cpp
index 9d6a686095..d926e130c1 100644
--- a/js/src/frontend/BytecodeEmitter.cpp
+++ b/js/src/frontend/BytecodeEmitter.cpp
@@ -2118,7 +2118,7 @@ class ForOfLoopControl : public LoopControl
     bool emitIteratorClose(BytecodeEmitter* bce,
                            CompletionKind completionKind = CompletionKind::Normal) {
         ptrdiff_t start = bce->offset();
-        if (!bce->emitIteratorClose(completionKind, allowSelfHosted_))
+        if (!bce->emitIteratorClose(IteratorKind::Sync, completionKind, allowSelfHosted_))
             return false;
         ptrdiff_t end = bce->offset();
         return bce->tryNoteList.append(JSTRY_FOR_OF_ITERCLOSE, 0, start, end);
@@ -3722,6 +3722,18 @@ BytecodeEmitter::emitFinishIteratorResult(bool done)
 }
 
 bool
+BytecodeEmitter::emitToIteratorResult(bool done)
+{
+    if (!emitPrepareIteratorResult())    // VALUE OBJ
+        return false;
+    if (!emit1(JSOP_SWAP))               // OBJ VALUE
+        return false;
+    if (!emitFinishIteratorResult(done)) // RESULT
+        return false;
+    return true;
+}
+
+bool
 BytecodeEmitter::emitGetNameAtLocation(JSAtom* name, const NameLocation& loc, bool callContext)
 {
     switch (loc.kind()) {
@@ -5255,7 +5267,8 @@ BytecodeEmitter::emitIteratorNext(ParseNode* pn, bool allowSelfHosted /* = false
 }
 
 bool
-BytecodeEmitter::emitIteratorClose(CompletionKind completionKind /* = CompletionKind::Normal */,
+BytecodeEmitter::emitIteratorClose(IteratorKind iterKind /* = IteratorKind::Sync */,
+                                   CompletionKind completionKind /* = CompletionKind::Normal */,
                                    bool allowSelfHosted /* = false */)
 {
     MOZ_ASSERT(allowSelfHosted || emitterMode != BytecodeEmitter::SelfHosting,
@@ -5340,6 +5353,11 @@ BytecodeEmitter::emitIteratorClose(CompletionKind completionKind /* = Completion
         return false;
     checkTypeSet(JSOP_CALL);
 
+    if (iterKind == IteratorKind::Async) {
+        if (!emitAwait())                                 // ... ... RESULT
+            return false;
+    }
+
     if (completionKind == CompletionKind::Throw) {
         if (!emit1(JSOP_SWAP))                            // ... RET ITER RESULT UNDEF
             return false;
@@ -5349,7 +5367,7 @@ BytecodeEmitter::emitIteratorClose(CompletionKind completionKind /* = Completion
         if (!tryCatch->emitCatch())                       // ... RET ITER RESULT
             return false;
 
-        // Just ignore the exception thrown by call.
+        // Just ignore the exception thrown by call and await.
         if (!emit1(JSOP_EXCEPTION))                       // ... RET ITER RESULT EXC
             return false;
         if (!emit1(JSOP_POP))                             // ... RET ITER RESULT
@@ -6972,6 +6990,63 @@ BytecodeEmitter::emitIterator()
 }
 
 bool
+BytecodeEmitter::emitAsyncIterator()
+{
+    // Convert iterable to iterator.
+    if (!emit1(JSOP_DUP))                                         // OBJ OBJ
+        return false;
+    if (!emit2(JSOP_SYMBOL, uint8_t(JS::SymbolCode::asyncIterator))) // OBJ OBJ @@ASYNCITERATOR
+        return false;
+    if (!emitElemOpBase(JSOP_CALLELEM))                           // OBJ ITERFN
+        return false;
+
+    IfThenElseEmitter ifAsyncIterIsUndefined(this);
+    if (!emit1(JSOP_DUP))                                         // OBJ ITERFN ITERFN
+        return false;
+    if (!emit1(JSOP_UNDEFINED))                                   // OBJ ITERFN ITERFN UNDEF
+        return false;
+    if (!emit1(JSOP_EQ))                                          // OBJ ITERFN EQ
+        return false;
+    if (!ifAsyncIterIsUndefined.emitIfElse())                     // OBJ ITERFN
+        return false;
+
+    if (!emit1(JSOP_POP))                                         // OBJ
+        return false;
+    if (!emit1(JSOP_DUP))                                         // OBJ OBJ
+        return false;
+    if (!emit2(JSOP_SYMBOL, uint8_t(JS::SymbolCode::iterator)))   // OBJ OBJ @@ITERATOR
+        return false;
+    if (!emitElemOpBase(JSOP_CALLELEM))                           // OBJ ITERFN
+        return false;
+    if (!emit1(JSOP_SWAP))                                        // ITERFN OBJ
+        return false;
+    if (!emitCall(JSOP_CALLITER, 0))                              // ITER
+        return false;
+    checkTypeSet(JSOP_CALLITER);
+    if (!emitCheckIsObj(CheckIsObjectKind::GetIterator))          // ITER
+        return false;
+
+    if (!emit1(JSOP_TOASYNCITER))                                 // ITER
+        return false;
+
+    if (!ifAsyncIterIsUndefined.emitElse())                       // OBJ ITERFN
+        return false;
+
+    if (!emit1(JSOP_SWAP))                                        // ITERFN OBJ
+        return false;
+    if (!emitCall(JSOP_CALLITER, 0))                              // ITER
+        return false;
+    checkTypeSet(JSOP_CALLITER);
+    if (!emitCheckIsObj(CheckIsObjectKind::GetIterator))          // ITER
+        return false;
+
+    if (!ifAsyncIterIsUndefined.emitEnd())                        // ITER
+        return false;
+
+    return true;
+}
+
+bool
 BytecodeEmitter::emitSpread(bool allowSelfHosted)
 {
     LoopControl loopInfo(this, StatementKind::Spread);
@@ -8593,6 +8668,16 @@ BytecodeEmitter::emitYield(ParseNode* pn)
 }
 
 bool
+BytecodeEmitter::emitAwait()
+{
+    if (!emitGetDotGenerator())
+        return false;
+    if (!emitYieldOp(JSOP_AWAIT))
+        return false;
+    return true;
+}
+
+bool
 BytecodeEmitter::emitAwait(ParseNode* pn)
 {
     MOZ_ASSERT(sc->isFunctionBox());
@@ -8600,11 +8685,7 @@ BytecodeEmitter::emitAwait(ParseNode* pn)
 
     if (!emitTree(pn->pn_kid))
         return false;
-    if (!emitGetDotGenerator())
-        return false;
-    if (!emitYieldOp(JSOP_AWAIT))
-        return false;
-    return true;
+    return emitAwait();
 }
 
 bool
@@ -8613,10 +8694,17 @@ BytecodeEmitter::emitYieldStar(ParseNode* iter)
     MOZ_ASSERT(sc->isFunctionBox());
     MOZ_ASSERT(sc->asFunctionBox()->isStarGenerator());
 
+    bool isAsyncGenerator = sc->asFunctionBox()->isAsync();
+
     if (!emitTree(iter))                                  // ITERABLE
         return false;
-    if (!emitIterator())                                  // ITER
-        return false;
+    if (isAsyncGenerator) {
+        if (!emitAsyncIterator())                         // ITER
+            return false;
+    } else {
+        if (!emitIterator())                              // ITER
+            return false;
+    }
 
     // Initial send value is undefined.
     if (!emit1(JSOP_UNDEFINED))                           // ITER RECEIVED
@@ -8674,7 +8762,8 @@ BytecodeEmitter::emitYieldStar(ParseNode* iter)
     //
     // If the iterator does not have a "throw" method, it calls IteratorClose
     // and then throws a TypeError.
-    if (!emitIteratorClose())                             // ITER RESULT EXCEPTION
+    IteratorKind iterKind = isAsyncGenerator ? IteratorKind::Async : IteratorKind::Sync;
+    if (!emitIteratorClose(iterKind))
         return false;
     if (!emitUint16Operand(JSOP_THROWMSG, JSMSG_ITERATOR_NO_THROW)) // throw
         return false;
@@ -8690,6 +8779,12 @@ BytecodeEmitter::emitYieldStar(ParseNode* iter)
     if (!emitCall(JSOP_CALL, 1, iter))                    // ITER OLDRESULT RESULT
         return false;
     checkTypeSet(JSOP_CALL);
+
+    if (isAsyncGenerator) {
+        if (!emitAwait())                                 // ITER OLDRESULT RESULT
+            return false;
+    }
+
     if (!emitCheckIsObj(CheckIsObjectKind::IteratorThrow)) // ITER OLDRESULT RESULT
         return false;
     if (!emit1(JSOP_SWAP))                                // ITER RESULT OLDRESULT
@@ -8756,6 +8851,11 @@ BytecodeEmitter::emitYieldStar(ParseNode* iter)
         return false;
     checkTypeSet(JSOP_CALL);
 
+    if (iterKind == IteratorKind::Async) {
+        if (!emitAwait())                                 // ... FTYPE FVALUE RESULT
+            return false;
+    }
+
     // Step v.
     if (!emitCheckIsObj(CheckIsObjectKind::IteratorReturn)) // ITER OLDRESULT FTYPE FVALUE RESULT
         return false;
@@ -8830,9 +8930,15 @@ BytecodeEmitter::emitYieldStar(ParseNode* iter)
         return false;
     if (!emitCall(JSOP_CALL, 1, iter))                           // ITER RESULT
         return false;
+    checkTypeSet(JSOP_CALL);
+
+    if (isAsyncGenerator) {
+        if (!emitAwait())                                        // ITER RESULT RESULT
+            return false;
+    }
+
     if (!emitCheckIsObj(CheckIsObjectKind::IteratorNext))        // ITER RESULT
         return false;
-    checkTypeSet(JSOP_CALL);
     MOZ_ASSERT(this->stackDepth == startDepth);
 
     if (!emitJumpTargetAndPatch(checkResult))                    // checkResult:
diff --git a/js/src/frontend/BytecodeEmitter.h b/js/src/frontend/BytecodeEmitter.h
index 67e3294890..5eaf5ce146 100644
--- a/js/src/frontend/BytecodeEmitter.h
+++ b/js/src/frontend/BytecodeEmitter.h
@@ -10,6 +10,7 @@
 #define frontend_BytecodeEmitter_h
 
 #include "jscntxt.h"
+#include "jsiter.h"
 #include "jsopcode.h"
 #include "jsscript.h"
 
@@ -606,6 +607,7 @@ struct MOZ_STACK_CLASS BytecodeEmitter
     MOZ_MUST_USE bool emitPrepareIteratorResult();
     MOZ_MUST_USE bool emitFinishIteratorResult(bool done);
     MOZ_MUST_USE bool iteratorResultShape(unsigned* shape);
+    MOZ_MUST_USE bool emitToIteratorResult(bool done);
 
     MOZ_MUST_USE bool emitGetDotGenerator();
 
@@ -613,6 +615,7 @@ struct MOZ_STACK_CLASS BytecodeEmitter
     MOZ_MUST_USE bool emitYield(ParseNode* pn);
     MOZ_MUST_USE bool emitYieldOp(JSOp op);
     MOZ_MUST_USE bool emitYieldStar(ParseNode* iter);
+    MOZ_MUST_USE bool emitAwait();
     MOZ_MUST_USE bool emitAwait(ParseNode* pn);
 
     MOZ_MUST_USE bool emitPropLHS(ParseNode* pn);
@@ -708,10 +711,13 @@ struct MOZ_STACK_CLASS BytecodeEmitter
     // It will replace that stack value with the corresponding iterator
     MOZ_MUST_USE bool emitIterator();
 
+    MOZ_MUST_USE bool emitAsyncIterator();
+
     // Pops iterator from the top of the stack. Pushes the result of |.next()|
     // onto the stack.
     MOZ_MUST_USE bool emitIteratorNext(ParseNode* pn, bool allowSelfHosted = false);
-    MOZ_MUST_USE bool emitIteratorClose(CompletionKind completionKind = CompletionKind::Normal,
+    MOZ_MUST_USE bool emitIteratorClose(IteratorKind iterKind = IteratorKind::Sync,
+                                        CompletionKind completionKind = CompletionKind::Normal,
                                         bool allowSelfHosted = false);
 
     template <typename InnerEmitter>
diff --git a/js/src/js.msg b/js/src/js.msg
index 2778085c7e..d22265ba82 100644
--- a/js/src/js.msg
+++ b/js/src/js.msg
@@ -596,3 +596,5 @@ MSG_DEF(JSMSG_ITERATOR_NO_THROW,       0, JSEXN_TYPEERR, "iterator does not have
 
 // Async Iteration
 MSG_DEF(JSMSG_NOT_AN_ASYNC_GENERATOR,  0, JSEXN_TYPEERR, "Not an async generator")
+MSG_DEF(JSMSG_NOT_AN_ASYNC_ITERATOR,   0, JSEXN_TYPEERR, "Not an async from sync iterator")
+MSG_DEF(JSMSG_GET_ASYNC_ITER_RETURNED_PRIMITIVE, 0, JSEXN_TYPEERR, "[Symbol.asyncIterator]() returned a non-object value")
diff --git a/js/src/jsiter.h b/js/src/jsiter.h
index 17b7df38bf..0b324a0140 100644
--- a/js/src/jsiter.h
+++ b/js/src/jsiter.h
@@ -227,6 +227,8 @@ InitLegacyIteratorClass(JSContext* cx, HandleObject obj);
 extern JSObject*
 InitStopIterationClass(JSContext* cx, HandleObject obj);
 
+enum class IteratorKind { Sync, Async };
+
 } /* namespace js */
 
 #endif /* jsiter_h */
diff --git a/js/src/vm/AsyncIteration.cpp b/js/src/vm/AsyncIteration.cpp
index 04effe1b10..74f6389c10 100644
--- a/js/src/vm/AsyncIteration.cpp
+++ b/js/src/vm/AsyncIteration.cpp
@@ -148,6 +148,69 @@ js::AsyncGeneratorAwaitedRejected(JSContext* cx, Handle<AsyncGeneratorObject*> a
     return AsyncGeneratorResume(cx, asyncGenObj, CompletionKind::Throw, reason);
 }
 
+const Class AsyncFromSyncIteratorObject::class_ = {
+    "AsyncFromSyncIteratorObject",
+    JSCLASS_HAS_RESERVED_SLOTS(AsyncFromSyncIteratorObject::Slots)
+};
+
+// Async Iteration proposal 6.1.3.1.
+JSObject*
+js::CreateAsyncFromSyncIterator(JSContext* cx, HandleObject iter)
+{
+    // Step 1 (implicit).
+    // Done in bytecode emitted by emitAsyncIterator.
+
+    // Steps 2-4.
+    return AsyncFromSyncIteratorObject::create(cx, iter);
+}
+
+// Async Iteration proposal 6.1.3.1 steps 2-4.
+/* static */ JSObject*
+AsyncFromSyncIteratorObject::create(JSContext* cx, HandleObject iter)
+{
+    // Step 2.
+    RootedObject proto(cx, GlobalObject::getOrCreateAsyncFromSyncIteratorPrototype(cx,
+                                                                                   cx->global()));
+    if (!proto)
+        return nullptr;
+
+    RootedObject obj(cx, NewNativeObjectWithGivenProto(cx, &class_, proto));
+    if (!obj)
+        return nullptr;
+
+    Handle<AsyncFromSyncIteratorObject*> asyncIter = obj.as<AsyncFromSyncIteratorObject>();
+
+    // Step 3.
+    asyncIter->setIterator(iter);
+
+    // Step 4.
+    return asyncIter;
+}
+
+// Async Iteration proposal 6.1.3.2.1 %AsyncFromSyncIteratorPrototype%.next.
+static bool
+AsyncFromSyncIteratorNext(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return AsyncFromSyncIteratorMethod(cx, args, CompletionKind::Normal);
+}
+
+// Async Iteration proposal 6.1.3.2.2 %AsyncFromSyncIteratorPrototype%.return.
+static bool
+AsyncFromSyncIteratorReturn(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return AsyncFromSyncIteratorMethod(cx, args, CompletionKind::Return);
+}
+
+// Async Iteration proposal 6.1.3.2.3 %AsyncFromSyncIteratorPrototype%.throw.
+static bool
+AsyncFromSyncIteratorThrow(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return AsyncFromSyncIteratorMethod(cx, args, CompletionKind::Throw);
+}
+
 // Async Iteration proposal 6.4.1.2 AsyncGenerator.prototype.next.
 static bool
 AsyncGeneratorNext(JSContext* cx, unsigned argc, Value* vp)
@@ -413,17 +476,17 @@ js::AsyncGeneratorResumeNext(JSContext* cx, Handle<AsyncGeneratorObject*> asyncG
     return AsyncGeneratorResume(cx, asyncGenObj, completionKind, argument);
 }
 
-// Async Iteration proposal 6.2.1.2 (partially).
+// Async Iteration proposal 6.2.1.3 (partially).
 // Most steps are done in generator.
 static MOZ_MUST_USE bool
 AsyncGeneratorYield(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
-                    HandleValue value, bool done)
+                    HandleValue value)
 {
-    // Step 6.
+    // Step 5.
     asyncGenObj->setSuspendedYield();
 
-    // Step 10.c.
-    return AsyncGeneratorResolve(cx, asyncGenObj, value, done);
+    // Step 8.
+    return AsyncGeneratorResolve(cx, asyncGenObj, value, false);
 }
 
 // Async Iteration proposal 6.4.3.5 steps 12-14, 16-20.
@@ -451,41 +514,30 @@ AsyncGeneratorResume(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
         return AsyncGeneratorThrown(cx, asyncGenObj);
     }
 
+    if (asyncGenObj->generatorObj()->isAfterAwait())
+        return AsyncGeneratorAwait(cx, asyncGenObj, result);
+
     // The following code corresponds to the following 3 cases:
     //   * yield
-    //   * await
+    //   * yield*
     //   * return
-    // For await and return, property access is done on an internal result
+    // For yield and return, property access is done on an internal result
     // object and it's not observable.
-    // For yield, it's done on an user-provided result object, and it's
-    // observable, so perform in that order in all cases.
+    // For yield*, it's done on a possibly user-provided result object, and
+    // it's observable.
 
+    // 2.2.1 yield* steps 6.a.vii, 6.b.ii.7, 6.c.ix.
     RootedObject resultObj(cx, &result.toObject());
     RootedValue value(cx);
-    RootedValue doneVal(cx);
 
-    // 6.2.1.2 step 10.a.
     if (!GetProperty(cx, resultObj, resultObj, cx->names().value, &value))
         return false;
 
-    // 6.2.1.2 step 10.b.
-    if (!GetProperty(cx, resultObj, resultObj, cx->names().done, &doneVal))
-        return false;
-
-    // 6.2.1.2 step 10.c.
     if (asyncGenObj->generatorObj()->isAfterYield())
-        return AsyncGeneratorYield(cx, asyncGenObj, value, ToBoolean(doneVal));
+        return AsyncGeneratorYield(cx, asyncGenObj, value);
 
     // 6.4.3.2 step 5.d-g.
-    if (ToBoolean(doneVal)) {
-        MOZ_ASSERT(!asyncGenObj->generatorObj()->isAfterAwait());
-        return AsyncGeneratorReturned(cx, asyncGenObj, value);
-    }
-
-    MOZ_ASSERT(asyncGenObj->generatorObj()->isAfterAwait());
-
-    // 5.1 steps 2-9.
-    return AsyncGeneratorAwait(cx, asyncGenObj, value);
+    return AsyncGeneratorReturned(cx, asyncGenObj, value);
 }
 
 static const JSFunctionSpec async_iterator_proto_methods[] = {
@@ -493,6 +545,13 @@ static const JSFunctionSpec async_iterator_proto_methods[] = {
     JS_FS_END
 };
 
+static const JSFunctionSpec async_from_sync_iter_methods[] = {
+    JS_FN("next", AsyncFromSyncIteratorNext, 1, 0),
+    JS_FN("throw", AsyncFromSyncIteratorThrow, 1, 0),
+    JS_FN("return", AsyncFromSyncIteratorReturn, 1, 0),
+    JS_FS_END
+};
+
 static const JSFunctionSpec async_generator_methods[] = {
     JS_FN("next", AsyncGeneratorNext, 1, 0),
     JS_FN("throw", AsyncGeneratorThrow, 1, 0),
@@ -513,6 +572,19 @@ GlobalObject::initAsyncGenerators(JSContext* cx, Handle<GlobalObject*> global)
     if (!DefinePropertiesAndFunctions(cx, asyncIterProto, nullptr, async_iterator_proto_methods))
         return false;
 
+    // Async Iteration proposal 6.1.3.2 %AsyncFromSyncIteratorPrototype%.
+    RootedObject asyncFromSyncIterProto(
+        cx, GlobalObject::createBlankPrototypeInheriting(cx, global, &PlainObject::class_,
+                                                         asyncIterProto));
+    if (!asyncFromSyncIterProto)
+        return false;
+    if (!DefinePropertiesAndFunctions(cx, asyncFromSyncIterProto, nullptr,
+                                      async_from_sync_iter_methods) ||
+        !DefineToStringTag(cx, asyncFromSyncIterProto, cx->names().AsyncFromSyncIterator))
+    {
+        return false;
+    }
+
     // Async Iteration proposal 6.4.1 %AsyncGeneratorPrototype%.
     RootedObject asyncGenProto(
         cx, GlobalObject::createBlankPrototypeInheriting(cx, global, &PlainObject::class_,
@@ -557,6 +629,7 @@ GlobalObject::initAsyncGenerators(JSContext* cx, Handle<GlobalObject*> global)
     }
 
     global->setReservedSlot(ASYNC_ITERATOR_PROTO, ObjectValue(*asyncIterProto));
+    global->setReservedSlot(ASYNC_FROM_SYNC_ITERATOR_PROTO, ObjectValue(*asyncFromSyncIterProto));
     global->setReservedSlot(ASYNC_GENERATOR, ObjectValue(*asyncGenerator));
     global->setReservedSlot(ASYNC_GENERATOR_FUNCTION, ObjectValue(*asyncGenFunction));
     global->setReservedSlot(ASYNC_GENERATOR_PROTO, ObjectValue(*asyncGenProto));
diff --git a/js/src/vm/AsyncIteration.h b/js/src/vm/AsyncIteration.h
index c0135283fa..974c209a01 100644
--- a/js/src/vm/AsyncIteration.h
+++ b/js/src/vm/AsyncIteration.h
@@ -196,6 +196,32 @@ class AsyncGeneratorObject : public NativeObject
     }
 };
 
+JSObject*
+CreateAsyncFromSyncIterator(JSContext* cx, HandleObject iter);
+
+class AsyncFromSyncIteratorObject : public NativeObject
+{
+  private:
+    enum AsyncFromSyncIteratorObjectSlots {
+        Slot_Iterator = 0,
+        Slots
+    };
+
+    void setIterator(HandleObject iterator_) {
+        setFixedSlot(Slot_Iterator, ObjectValue(*iterator_));
+    }
+
+  public:
+    static const Class class_;
+
+    static JSObject*
+    create(JSContext* cx, HandleObject iter);
+
+    JSObject* iterator() const {
+        return &getFixedSlot(Slot_Iterator).toObject();
+    }
+};
+
 MOZ_MUST_USE bool
 AsyncGeneratorResumeNext(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj);
 
diff --git a/js/src/vm/CommonPropertyNames.h b/js/src/vm/CommonPropertyNames.h
index 4a9c03e884..6a8afb56b5 100644
--- a/js/src/vm/CommonPropertyNames.h
+++ b/js/src/vm/CommonPropertyNames.h
@@ -29,6 +29,7 @@
     macro(ArrayValuesAt, ArrayValuesAt, "ArrayValuesAt") \
     macro(as, as, "as") \
     macro(Async, Async, "Async") \
+    macro(AsyncFromSyncIterator, AsyncFromSyncIterator, "Async-from-Sync Iterator") \
     macro(AsyncFunction, AsyncFunction, "AsyncFunction") \
     macro(AsyncGenerator, AsyncGenerator, "AsyncGenerator") \
     macro(AsyncGeneratorFunction, AsyncGeneratorFunction, "AsyncGeneratorFunction") \
diff --git a/js/src/vm/GlobalObject.h b/js/src/vm/GlobalObject.h
index ce802daba9..3fd2762f84 100644
--- a/js/src/vm/GlobalObject.h
+++ b/js/src/vm/GlobalObject.h
@@ -101,6 +101,7 @@ class GlobalObject : public NativeObject
         ASYNC_FUNCTION_PROTO,
         ASYNC_FUNCTION,
         ASYNC_ITERATOR_PROTO,
+        ASYNC_FROM_SYNC_ITERATOR_PROTO,
         ASYNC_GENERATOR,
         ASYNC_GENERATOR_FUNCTION,
         ASYNC_GENERATOR_PROTO,
@@ -636,6 +637,12 @@ class GlobalObject : public NativeObject
     }
 
     static NativeObject*
+    getOrCreateAsyncFromSyncIteratorPrototype(JSContext* cx, Handle<GlobalObject*> global) {
+        return MaybeNativeObject(getOrCreateObject(cx, global, ASYNC_FROM_SYNC_ITERATOR_PROTO,
+                                                   initAsyncGenerators));
+    }
+
+    static NativeObject*
     getOrCreateAsyncGenerator(JSContext* cx, Handle<GlobalObject*> global) {
         return MaybeNativeObject(getOrCreateObject(cx, global, ASYNC_GENERATOR,
                                                    initAsyncGenerators));
diff --git a/js/src/vm/Interpreter.cpp b/js/src/vm/Interpreter.cpp
index 4d59273243..3cf9b57f60 100644
--- a/js/src/vm/Interpreter.cpp
+++ b/js/src/vm/Interpreter.cpp
@@ -1940,7 +1940,6 @@ CASE(EnableInterruptsPseudoOpcode)
 CASE(JSOP_NOP)
 CASE(JSOP_NOP_DESTRUCTURING)
 CASE(JSOP_UNUSED126)
-CASE(JSOP_UNUSED210)
 CASE(JSOP_UNUSED211)
 CASE(JSOP_TRY_DESTRUCTURING_ITERCLOSE)
 CASE(JSOP_UNUSED221)
@@ -3596,6 +3595,17 @@ CASE(JSOP_TOASYNCGEN)
 }
 END_CASE(JSOP_TOASYNCGEN)
 
+CASE(JSOP_TOASYNCITER)
+{
+    ReservedRooted<JSObject*> iter(&rootObject1, &REGS.sp[-1].toObject());
+    JSObject* asyncIter = CreateAsyncFromSyncIterator(cx, iter);
+    if (!asyncIter)
+        goto error;
+
+    REGS.sp[-1].setObject(*asyncIter);
+}
+END_CASE(JSOP_TOASYNCITER)
+
 CASE(JSOP_SETFUNNAME)
 {
     MOZ_ASSERT(REGS.stackDepth() >= 2);
@@ -5138,6 +5148,10 @@ js::ThrowCheckIsObject(JSContext* cx, CheckIsObjectKind kind)
       case CheckIsObjectKind::GetIterator:
         JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_GET_ITER_RETURNED_PRIMITIVE);
         break;
+      case CheckIsObjectKind::GetAsyncIterator:
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_GET_ASYNC_ITER_RETURNED_PRIMITIVE);
+        break;
       default:
         MOZ_CRASH("Unknown kind");
     }
diff --git a/js/src/vm/Interpreter.h b/js/src/vm/Interpreter.h
index 9fefd75ccd..5e7087f7fa 100644
--- a/js/src/vm/Interpreter.h
+++ b/js/src/vm/Interpreter.h
@@ -561,7 +561,8 @@ enum class CheckIsObjectKind : uint8_t {
     IteratorNext,
     IteratorReturn,
     IteratorThrow,
-    GetIterator
+    GetIterator,
+    GetAsyncIterator
 };
 
 bool
diff --git a/js/src/vm/Opcodes.h b/js/src/vm/Opcodes.h
index fc7fbbe662..e13cd62219 100644
--- a/js/src/vm/Opcodes.h
+++ b/js/src/vm/Opcodes.h
@@ -2136,7 +2136,15 @@
      *   Stack: promise, gen => resolved
      */ \
     macro(JSOP_AWAIT,         209, "await",        NULL,  4,  2,  1,  JOF_UINT24) \
-    macro(JSOP_UNUSED210,     210, "unused210",    NULL,  1,  0,  0,  JOF_BYTE) \
+    /*
+     * Pops the iterator from the top of the stack, and create async iterator
+     * from it and push the async iterator back onto the stack.
+     *   Category: Statements
+     *   Type: Generator
+     *   Operands:
+     *   Stack: iter => asynciter
+     */ \
+    macro(JSOP_TOASYNCITER,   210, "toasynciter",  NULL,  1,  1,  1,  JOF_BYTE) \
     macro(JSOP_UNUSED211,     211, "unused211",    NULL,  1,  0,  0,  JOF_BYTE) \
     /*
      * Initializes generator frame, creates a generator and pushes it on the