Add iterator done check in spread destructuring

This fixes the logic in the desugaring of destructuring assignments. In
particular, a spread element would not check if previous `next` results
had already been done, and would always call `next()` again.

Change-Id: I1bd384678722e6cf51c5777fc3b0dd965360291a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2430488
Commit-Queue: Gus Caplan <snek@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70144}

src/interpreter/bytecode-generator.cc

@@ -3793,6 +3793,7 @@ void BytecodeGenerator::BuildDestructuringArrayAssignment(
 
         if (spread) {
           RegisterAllocationScope scope(this);
+          BytecodeLabel is_done;
 
           // A spread is turned into a loop over the remainer of the iterator.
           Expression* target = spread->expression();
@@ -3809,6 +3810,10 @@ void BytecodeGenerator::BuildDestructuringArrayAssignment(
               feedback_index(feedback_spec()->AddLiteralSlot()));
           builder()->StoreAccumulatorInRegister(array);
 
+          // If done, jump to assigning empty array
+          builder()->LoadAccumulatorWithRegister(done);
+          builder()->JumpIfTrue(ToBooleanMode::kConvertToBoolean, &is_done);
+
           // var index = 0;
           Register index = register_allocator()->NewRegister();
           builder()->LoadLiteral(Smi::zero());
@@ -3826,6 +3831,7 @@ void BytecodeGenerator::BuildDestructuringArrayAssignment(
                                      next_value_load_slot, next_done_load_slot,
                                      index_slot, element_slot);
 
+          builder()->Bind(&is_done);
           // Assign the array to the LHS.
           builder()->LoadAccumulatorWithRegister(array);
           BuildAssignment(lhs_data, op, lookup_hoisting_mode);

test/cctest/interpreter/bytecode_expectations/DestructuringAssignment.golden

@@ -100,7 +100,7 @@ snippet: "
 "
 frame size: 14
 parameter count: 1
-bytecode array length: 232
+bytecode array length: 236
 bytecodes: [
   /*   48 S> */ B(CreateArrayLiteral), U8(0), U8(0), U8(37),
                 B(Star), R(2),
@@ -149,6 +149,8 @@ bytecodes: [
                 B(Star), R(0),
   /*   63 S> */ B(CreateEmptyArrayLiteral), U8(15),
                 B(Star), R(11),
+                B(Ldar), R(6),
+                B(JumpIfToBooleanTrue), U8(44),
                 B(LdaZero),
                 B(Star), R(12),
                 B(LdaTrue),
@@ -211,8 +213,8 @@ constant pool: [
   ONE_BYTE_INTERNALIZED_STRING_TYPE ["return"],
 ]
 handlers: [
-  [34, 162, 170],
-  [186, 207, 209],
+  [34, 166, 174],
+  [190, 211, 213],
 ]
 
 ---

test/mjsunit/destruct-array-spread-done.js

@@ -0,0 +1,26 @@
+// Copyright 2020 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+'use strict';
+
+let called = 0
+const it = {
+  [Symbol.iterator]() {
+    return this;
+  },
+  next() {
+    called += 1;
+    return {
+      value: 42,
+      done: true,
+    };
+  },
+};
+
+const [a, b, ...c] = it;
+
+assertEquals(called, 1);
+assertEquals(a, undefined);
+assertEquals(b, undefined);
+assertEquals(c.length, 0);