From 131e1832766ce8821f96b67a49d4599cb3c2052c Mon Sep 17 00:00:00 2001
From: Berserk <tushar.18246@gmail.com>
Date: Wed, 11 Nov 2026 01:40:21 +0530
Subject: [PATCH] fix(engine): check iterator completion before requesting rest
 values (#5648)

---
 core/engine/src/bytecompiler/iterator.rs |   4 +
 core/engine/src/tests/iterators.rs       | 145 +++++++++++++++++++++++
 2 files changed, 149 insertions(+)

diff --git a/core/engine/src/bytecompiler/iterator.rs b/core/engine/src/bytecompiler/iterator.rs
index d52607d223b..9a5f8347584 100644
--- a/core/engine/src/bytecompiler/iterator.rs
+++ b/core/engine/src/bytecompiler/iterator.rs
@@ -57,6 +57,9 @@ impl ByteCompiler<'_> {
         let loop_ = self.next_opcode_location();
         self.push_loop_control_info(None, loop_, false);
 
+        self.bytecode.emit_iterator_done(temp.variable());
+        let done = self.jump_if_true(&temp);
+
         self.bytecode.emit_increment_loop_iteration();
 
         self.iterator_next(false);
@@ -71,6 +74,7 @@ impl ByteCompiler<'_> {
         self.bytecode.emit_jump(loop_);
         self.pop_loop_control_info();
 
+        self.patch_jump(done);
         self.patch_jump(end);
         self.register_allocator.dealloc(temp);
     }
diff --git a/core/engine/src/tests/iterators.rs b/core/engine/src/tests/iterators.rs
index cab9fe7a54e..454d0419e38 100644
--- a/core/engine/src/tests/iterators.rs
+++ b/core/engine/src/tests/iterators.rs
@@ -217,3 +217,148 @@ fn mixed_iterators_close_in_continue() {
         "#}),
     ]);
 }
+
+#[test]
+fn array_rest_iterator_first_result_done() {
+    run_test_actions([
+        TestAction::run_harness(),
+        TestAction::run(indoc! {r#"
+            let calls = 0;
+            const it = {
+                [Symbol.iterator]() {
+                    return this;
+                },
+                next() {
+                    calls++;
+                    return { done: true };
+                }
+            };
+            const [x, ...y] = it;
+
+            let assignCalls = 0;
+            const it2 = {
+                [Symbol.iterator]() {
+                    return this;
+                },
+                next() {
+                    assignCalls++;
+                    return { done: true };
+                }
+            };
+            let a, b;
+            [a, ...b] = it2;
+        "#}),
+        TestAction::assert_eq("calls", 1),
+        TestAction::assert("x === undefined"),
+        TestAction::assert("arrayEquals(y, [])"),
+        TestAction::assert_eq("assignCalls", 1),
+        TestAction::assert("a === undefined"),
+        TestAction::assert("arrayEquals(b, [])"),
+    ]);
+}
+
+#[test]
+fn array_rest_iterator_multiple_values() {
+    run_test_actions([
+        TestAction::run_harness(),
+        TestAction::run(indoc! {r#"
+            let calls = 0;
+            const values = [1, 2, 3];
+            const it = {
+                [Symbol.iterator]() {
+                    return this;
+                },
+                next() {
+                    calls++;
+                    if (values.length > 0) {
+                        return { value: values.shift(), done: false };
+                    }
+                    return { done: true };
+                }
+            };
+            const [first, ...rest] = it;
+        "#}),
+        TestAction::assert_eq("first", 1),
+        TestAction::assert("arrayEquals(rest, [2, 3])"),
+        TestAction::assert_eq("calls", 4),
+    ]);
+}
+
+#[test]
+fn array_rest_iterator_next_throws() {
+    run_test_actions([
+        TestAction::run(indoc! {r#"
+            let calls = 0;
+            const it = {
+                [Symbol.iterator]() {
+                    return this;
+                },
+                next() {
+                    calls++;
+                    if (calls === 2) {
+                        throw new Error("iterator next error");
+                    }
+                    return { value: calls, done: false };
+                }
+            };
+            let caught = false;
+            let errorMsg = "";
+            try {
+                const [first, ...rest] = it;
+            } catch (err) {
+                caught = true;
+                errorMsg = err.message;
+            }
+        "#}),
+        TestAction::assert("caught"),
+        TestAction::assert_eq("calls", 2),
+        TestAction::assert("errorMsg === 'iterator next error'"),
+    ]);
+}
+
+#[test]
+fn array_rest_normal_destructuring() {
+    run_test_actions([
+        TestAction::run_harness(),
+        TestAction::run(indoc! {r#"
+            const [a, b, ...rest] = [1, 2, 3, 4, 5];
+            const [...all] = [10, 20];
+            const [x, ...empty] = [42];
+            const [...emptyAll] = [];
+        "#}),
+        TestAction::assert_eq("a", 1),
+        TestAction::assert_eq("b", 2),
+        TestAction::assert("arrayEquals(rest, [3, 4, 5])"),
+        TestAction::assert("arrayEquals(all, [10, 20])"),
+        TestAction::assert_eq("x", 42),
+        TestAction::assert("arrayEquals(empty, [])"),
+        TestAction::assert("arrayEquals(emptyAll, [])"),
+    ]);
+}
+
+#[test]
+fn array_rest_iterator_already_done_before_rest() {
+    run_test_actions([
+        TestAction::run_harness(),
+        TestAction::run(indoc! {r#"
+            let calls = 0;
+            const it = {
+                [Symbol.iterator]() {
+                    return this;
+                },
+                next() {
+                    calls++;
+                    if (calls === 1) {
+                        return { value: "first", done: false };
+                    }
+                    return { done: true };
+                }
+            };
+            const [a, b, ...rest] = it;
+        "#}),
+        TestAction::assert_eq("calls", 2),
+        TestAction::assert("a === 'first'"),
+        TestAction::assert("b === undefined"),
+        TestAction::assert("arrayEquals(rest, [])"),
+    ]);
+}