Continue panic fixes (#1896)

This PR changes the following:

- Fixes the panics induced by incorrect continues.
- Adds tests which demonstrate the various panics induced.
- Actually rustfmts correctly?

boa_engine/src/bytecompiler.rs

@@ -1489,9 +1489,7 @@ impl<'b> ByteCompiler<'b> {
                         .iter()
                         .rev()
                         .filter(|info| info.kind == JumpControlInfoKind::Loop);
-                    let address = if node.label().is_none() {
-                        items.next().expect("continue target").start_address
-                    } else {
+                    let address = if let Some(label_name) = node.label() {
                         let mut num_loops = 0;
                         let mut emit_for_of_in_exit = 0;
                         let mut address_info = None;
@@ -1505,7 +1503,14 @@ impl<'b> ByteCompiler<'b> {
                                 emit_for_of_in_exit += 1;
                             }
                         }
-                        let address = address_info.expect("continue target").start_address;
+                        let address = address_info
+                            .ok_or_else(|| {
+                                self.context.construct_syntax_error(format!(
+                                    "Cannot use the undeclared label '{}'",
+                                    self.context.interner().resolve_expect(label_name)
+                                ))
+                            })?
+                            .start_address;
                         for _ in 0..emit_for_of_in_exit {
                             self.emit_opcode(Opcode::Pop);
                             self.emit_opcode(Opcode::Pop);
@@ -1514,6 +1519,14 @@ impl<'b> ByteCompiler<'b> {
                             self.emit_opcode(Opcode::LoopEnd);
                         }
                         address
+                    } else {
+                        items
+                            .next()
+                            .ok_or_else(|| {
+                                self.context
+                                    .construct_syntax_error("continue must be inside loop")
+                            })?
+                            .start_address
                     };
                     self.emit_opcode(Opcode::LoopEnd);
                     self.emit_opcode(Opcode::LoopStart);

boa_engine/src/tests.rs

@@ -442,6 +442,31 @@ fn test_invalid_break_target() {
     assert!(Context::default().eval(src).is_err());
 }
 
+#[test]
+fn test_invalid_continue_target() {
+    let mut context = Context::default();
+    let src = r#"
+        while (false) {
+          continue nonexistent;
+        }
+        "#;
+    let string = forward(&mut context, src);
+    assert_eq!(
+        string,
+        "Uncaught \"SyntaxError\": \"Cannot use the undeclared label 'nonexistent'\""
+    );
+}
+
+#[test]
+fn test_invalid_continue() {
+    let mut context = Context::default();
+    let string = forward(&mut context, r"continue;");
+    assert_eq!(
+        string,
+        "Uncaught \"SyntaxError\": \"continue must be inside loop\""
+    );
+}
+
 #[test]
 fn unary_pre() {
     let unary_inc = r#"