Fixed #1768 (#1820)

This Pull Request fixes/closes #1768.

It adds one extra peeked token in the buffered lexer, since it didn't take into account that the stream might end just after the last peeked token. The panic was only happening in debug mode, but still, this was wrong.

boa/src/syntax/parser/cursor/buffered_lexer/mod.rs

@@ -17,12 +17,12 @@ const MAX_PEEK_SKIP: usize = 3;
 /// The fixed size of the buffer used for storing values that are peeked ahead.
 ///
 /// The size is calculated for a worst case scenario, where we want to peek `MAX_PEEK_SKIP` tokens
-/// skipping line terminators:
+/// skipping line terminators, and the stream ends just after:
 /// ```text
-/// [\n, B, \n, C, \n, D, \n, E, \n, F]
-///   0  0   1  1   2  2   3  3   4  4
+/// [\n, B, \n, C, \n, D, \n, E, \n, F, None]
+///   0  0   1  1   2  2   3  3   4  4  5
 /// ```
-const PEEK_BUF_SIZE: usize = (MAX_PEEK_SKIP + 1) * 2;
+const PEEK_BUF_SIZE: usize = (MAX_PEEK_SKIP + 1) * 2 + 1;
 
 #[derive(Debug)]
 pub(super) struct BufferedLexer<R> {
@@ -49,6 +49,7 @@ where
                 None::<Token>,
                 None::<Token>,
                 None::<Token>,
+                None::<Token>,
             ],
             read_index: 0,
             write_index: 0,

boa/src/syntax/parser/cursor/buffered_lexer/tests.rs

@@ -278,3 +278,11 @@ fn skip_peeked_terminators() {
     // End of stream
     assert!(cur.peek(2, true, &mut interner).unwrap().is_none());
 }
+
+#[test]
+fn issue_1768() {
+    let mut cur = BufferedLexer::from(&b"\n(\nx\n)\n"[..]);
+    let mut interner = Interner::default();
+
+    assert!(cur.peek(3, true, &mut interner).unwrap().is_none());
+}

boa/src/syntax/parser/expression/assignment/mod.rs

@@ -153,8 +153,9 @@ where
                                         .map(Node::ArrowFunctionDecl);
                                     }
                                     TokenKind::Punctuator(Punctuator::CloseParen) => {
-                                        // Need to check if the token after the close paren is an arrow, if so then this is an ArrowFunction
-                                        // otherwise it is an expression of the form (b).
+                                        // Need to check if the token after the close paren is an
+                                        // arrow, if so then this is an ArrowFunction otherwise it
+                                        // is an expression of the form (b).
                                         if let Some(t) = cursor.peek(3, interner)? {
                                             if t.kind() == &TokenKind::Punctuator(Punctuator::Arrow)
                                             {