From 919272e0856e6871b5a8e2f667f87b5232d6b4fd Mon Sep 17 00:00:00 2001
From: pd <pawan.dogra99@gmail.com>
Date: Tue, 22 Mar 2022 19:28:14 +0000
Subject: [PATCH] Fix panic in do while  (#1968)

 `Node::DoWhileLoop` ast node had a buggy bytecode generation where `self.patch_jump(exit)` was called after emitting `LoopEnd` opcode. This would patch the loop exit to the instruction following the do while code, which would panic in cases where do while was enclosed in a block statement.

This Pull Request fixes #1929.

It changes the following:
- Patch jump before emitting `Opcode::LoopEnd`
- Add test which has do while statement inside a block statement to demonstrate that the change fixes the panic.
---
 boa_engine/src/bytecompiler.rs |  3 +--
 boa_engine/src/tests.rs        | 15 +++++++++++++++
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/boa_engine/src/bytecompiler.rs b/boa_engine/src/bytecompiler.rs
index 3708c1bd810..88ee542b219 100644
--- a/boa_engine/src/bytecompiler.rs
+++ b/boa_engine/src/bytecompiler.rs
@@ -1472,11 +1472,10 @@ impl<'b> ByteCompiler<'b> {
 
                 self.compile_stmt(do_while.body(), false)?;
                 self.emit(Opcode::Jump, &[condition_label_address]);
+                self.patch_jump(exit);
 
                 self.pop_loop_control_info();
                 self.emit_opcode(Opcode::LoopEnd);
-
-                self.patch_jump(exit);
             }
             Node::Continue(node) => {
                 let next = self.next_opcode_location();
diff --git a/boa_engine/src/tests.rs b/boa_engine/src/tests.rs
index 5500d3d34e6..e5e8d9dcbec 100644
--- a/boa_engine/src/tests.rs
+++ b/boa_engine/src/tests.rs
@@ -381,6 +381,21 @@ fn do_while_post_inc() {
     assert_eq!(&exec(with_post_incrementors), "11");
 }
 
+#[test]
+fn do_while_in_block() {
+    let in_block = r#"
+        {
+            var i = 0;
+            do {
+                i += 1;
+            }
+            while(false);
+            i;
+        }
+    "#;
+    assert_eq!(&exec(in_block), "1");
+}
+
 #[test]
 fn for_loop() {
     let simple = r#"