fix(parser): handle nested comments (#6526)

handle nested comments

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

src/sqlparser/src/tokenizer.rs

@@ -661,22 +661,24 @@ impl<'a> Tokenizer<'a> {
         chars: &mut Peekable<Chars<'_>>,
     ) -> Result<Option<Token>, TokenizerError> {
         let mut s = String::new();
-        let mut maybe_closing_comment = false;
-        // TODO: deal with nested comments
+
+        let mut nested = 1;
+        let mut last_ch = ' ';
+
         loop {
             match chars.next() {
                 Some(ch) => {
-                    if maybe_closing_comment {
-                        if ch == '/' {
+                    if last_ch == '/' && ch == '*' {
+                        nested += 1;
+                    } else if last_ch == '*' && ch == '/' {
+                        nested -= 1;
+                        if nested == 0 {
+                            s.pop();
                             break Ok(Some(Token::Whitespace(Whitespace::MultiLineComment(s))));
-                        } else {
-                            s.push('*');
                         }
                     }
-                    maybe_closing_comment = ch == '*';
-                    if !maybe_closing_comment {
-                        s.push(ch);
-                    }
+                    s.push(ch);
+                    last_ch = ch;
                 }
                 None => break self.tokenizer_error("Unexpected EOF while in a multi-line comment"),
             }
@@ -1149,6 +1151,21 @@ mod tests {
         compare(expected, tokens);
     }
 
+    #[test]
+    fn tokenize_nested_multiline_comment() {
+        let sql = String::from("0/*multi-line\n* \n/* comment \n /*comment*/*/ */ /comment*/1");
+        let mut tokenizer = Tokenizer::new(&sql);
+        let tokens = tokenizer.tokenize().unwrap();
+        let expected = vec![
+            Token::Number("0".to_string()),
+            Token::Whitespace(Whitespace::MultiLineComment(
+                "multi-line\n* \n/* comment \n /*comment*/*/ */ /comment".to_string(),
+            )),
+            Token::Number("1".to_string()),
+        ];
+        compare(expected, tokens);
+    }
+
     #[test]
     fn tokenize_multiline_comment_with_even_asterisks() {
         let sql = String::from("\n/** Comment **/\n");