Bug 1931350: Add initial implementation for the Iterator Sequencing p…

…roposal. r=mgaudet

Test PR: tc39/test262#4326

Differential Revision: https://phabricator.services.mozilla.com/D229014

js/src/builtin/Iterator.js

@@ -974,11 +974,84 @@ function IteratorFind(predicate) {
   }
 }
 
-
 #ifdef NIGHTLY_BUILD
-/** 
+/**
+ * Iterator.concat ( ...items )
+ *
+ * https://tc39.es/proposal-iterator-sequencing/
+ */
+function IteratorConcat() {
+  // Step 1.
+  //
+  // Stored in reversed order to simplify removing processed items.
+  var index = ArgumentsLength() * 2;
+  var iterables = std_Array(index);
+
+  // Step 2.
+  for (var i = 0; i < ArgumentsLength(); i++) {
+    var item = GetArgument(i);
+
+    // Step 2.a.
+    if (!IsObject(item)) {
+      ThrowTypeError(JSMSG_OBJECT_REQUIRED, typeof item);
+    }
+
+    // Step 2.b. (Inlined GetMethod)
+    var method = item[GetBuiltinSymbol("iterator")];
+
+    // Step 2.c.
+    if (!IsCallable(method)) {
+      ThrowTypeError(JSMSG_NOT_ITERABLE, ToSource(item));
+    }
+
+    // Step 2.d.
+    DefineDataProperty(iterables, --index, item);
+    DefineDataProperty(iterables, --index, method);
+  }
+  assert(index === 0, "all items stored");
+
+  // Steps 3-5.
+  var result = NewIteratorHelper();
+  var generator = IteratorConcatGenerator(iterables);
+  UnsafeSetReservedSlot(
+    result,
+    ITERATOR_HELPER_GENERATOR_SLOT,
+    generator
+  );
+
+  // Step 6.
+  return result;
+}
+
+/**
+ * Iterator.concat ( ...items )
+ *
+ * https://tc39.es/proposal-iterator-sequencing/
+ */
+function* IteratorConcatGenerator(iterables) {
+  assert(IsArray(iterables), "iterables is an array");
+  assert(iterables.length % 2 === 0, "iterables contains pairs (item, method)");
+
+  // Step 3.a.
+  for (var i = iterables.length; i > 0; ) {
+    var item = iterables[--i];
+    var method = iterables[--i];
+
+    // Remove processed items to avoid keeping them alive.
+    iterables.length -= 2;
+
+    // Steps 3.a.i-v.
+    for (var innerValue of allowContentIterWith(item, method)) {
+      // Steps 3.a.v.1-3. (Implicit through for-of loop)
+
+      yield innerValue;
+    }
+  }
+}
+
+/**
  * Iterator.zip (iterables [, options])
- * 
+ *
  * https://tc39.es/proposal-joint-iteration/#sec-iterator.zip
  */
 function IteratorZip(predicate) {
@@ -993,4 +1066,4 @@ function IteratorZip(predicate) {
 function IteratorZipKeyed(predicate) {
   return false;
 }
-#endif
+#endif

js/src/vm/CommonPropertyNames.h

@@ -98,6 +98,7 @@
   MACRO_(compact, "compact")                                                   \
   MACRO_(compactDisplay, "compactDisplay")                                     \
   MACRO_(compare, "compare")                                                   \
+  MACRO_(concat, "concat")                                                     \
   MACRO_(configurable, "configurable")                                         \
   MACRO_(const_, "const")                                                      \
   MACRO_(constrain, "constrain")                                               \

js/src/vm/Iteration.cpp

@@ -1939,6 +1939,7 @@ void js::AssertDenseElementsNotIterated(NativeObject* obj) {
 static const JSFunctionSpec iterator_static_methods[] = {
     JS_SELF_HOSTED_FN("from", "IteratorFrom", 1, 0),
 #ifdef NIGHTLY_BUILD
+    JS_SELF_HOSTED_FN("concat", "IteratorConcat", 0, 0),
     JS_SELF_HOSTED_FN("range", "IteratorRange", 1, 0),
     JS_SELF_HOSTED_FN("zip", "IteratorZip", 1, 0),
     JS_SELF_HOSTED_FN("zipKeyed", "IteratorZipKeyed", 1, 0),

js/src/vm/JSObject.cpp

@@ -2339,6 +2339,15 @@ JS_PUBLIC_API bool js::ShouldIgnorePropertyDefinition(JSContext* cx,
     return true;
   }
 
+  // It's gently surprising that this is JSProto_Function, but the trick
+  // to realize is that this is a -constructor function-, not a function
+  // on the prototype; and the proto of the constructor is JSProto_Function.
+  if (key == JSProto_Function &&
+      !JS::Prefs::experimental_iterator_sequencing() &&
+      id == NameToId(cx->names().concat)) {
+    return true;
+  }
+
   if (key == JSProto_Atomics && !JS::Prefs::experimental_atomics_pause() &&
       id == NameToId(cx->names().pause)) {
     return true;