Correctly handle nested comments

The tokenizer currently throws EOF error for

`select 'foo' /*/**/*/`

`last_ch` causes problems when tokenizing nested comments, we have to
consume the combination of /* or */

The existing `tokenize_nested_multiline_comment` test fails after fixing
this logic:

/*multi-line\n* \n/* comment \n /*comment*/*/ */ /comment*/
^^ Start                                      ^^ End nested comment

Relevant: #726

src/tokenizer.rs

@@ -1855,22 +1855,30 @@ impl<'a> Tokenizer<'a> {
     ) -> Result<Option<Token>, TokenizerError> {
         let mut s = String::new();
         let mut nested = 1;
-        let mut last_ch = ' ';
 
         loop {
             match chars.next() {
                 Some(ch) => {
-                    if last_ch == '/' && ch == '*' {
+                    if ch == '/' && matches!(chars.peek(), Some('*')) {
+                        s.push(ch);
+                        s.push(chars.next().unwrap()); // consume the '*'
                         nested += 1;
-                    } else if last_ch == '*' && ch == '/' {
+                        continue;
+                    }
+
+                    if ch == '*' && matches!(chars.peek(), Some('/')) {
+                        s.push(ch);
+                        let slash = chars.next();
                         nested -= 1;
                         if nested == 0 {
-                            s.pop();
+                            s.pop(); // remove the last '/'
                             break Ok(Some(Token::Whitespace(Whitespace::MultiLineComment(s))));
                         }
+                        s.push(slash.unwrap());
+                        continue;
                     }
+
                     s.push(ch);
-                    last_ch = ch;
                 }
                 None => {
                     break self.tokenizer_error(
@@ -2718,17 +2726,65 @@ mod tests {
 
     #[test]
     fn tokenize_nested_multiline_comment() {
-        let sql = String::from("0/*multi-line\n* \n/* comment \n /*comment*/*/ */ /comment*/1");
-
         let dialect = GenericDialect {};
+
+        let sql = String::from("0/*multi-line\n* \n/* comment \n /*comment*/*/ */ /comment*/1");
         let tokens = Tokenizer::new(&dialect, &sql).tokenize().unwrap();
         let expected = vec![
             Token::Number("0".to_string(), false),
             Token::Whitespace(Whitespace::MultiLineComment(
-                "multi-line\n* \n/* comment \n /*comment*/*/ */ /comment".to_string(),
+                "multi-line\n* \n/* comment \n /*comment*/*/ ".into(),
+            )),
+            Token::Whitespace(Whitespace::Space),
+            Token::Div,
+            Token::Word(Word {
+                value: "comment".to_string(),
+                quote_style: None,
+                keyword: Keyword::COMMENT,
+            }),
+            Token::Mul,
+            Token::Div,
+            Token::Number("1".to_string(), false),
+        ];
+        compare(expected, tokens);
+
+        let sql2 = String::from("0/*multi-line\n* \n/* comment \n /*comment/**/ */ /comment*/*/1");
+        let tokens2 = Tokenizer::new(&dialect, &sql2).tokenize().unwrap();
+        let expected2 = vec![
+            Token::Number("0".to_string(), false),
+            Token::Whitespace(Whitespace::MultiLineComment(
+                "multi-line\n* \n/* comment \n /*comment/**/ */ /comment*/".into(),
             )),
             Token::Number("1".to_string(), false),
         ];
+        compare(expected2, tokens2);
+
+        let sql3 = String::from("SELECT 1 /* a /* b */ c */");
+        let tokens3 = Tokenizer::new(&dialect, &sql3).tokenize().unwrap();
+        let expected3 = vec![
+            Token::make_keyword("SELECT"),
+            Token::Whitespace(Whitespace::Space),
+            Token::Number("1".to_string(), false),
+            Token::Whitespace(Whitespace::Space),
+            Token::Whitespace(Whitespace::MultiLineComment(" a /* b */ c ".to_string())),
+        ];
+        compare(expected3, tokens3);
+    }
+
+    #[test]
+    fn tokenize_nested_multiline_comment_empty() {
+        let sql = "select 'foo' /*/**/*/";
+
+        let dialect = GenericDialect {};
+        let tokens = Tokenizer::new(&dialect, sql).tokenize().unwrap();
+        let expected = vec![
+            Token::make_keyword("select"),
+            Token::Whitespace(Whitespace::Space),
+            Token::SingleQuotedString("foo".to_string()),
+            Token::Whitespace(Whitespace::Space),
+            Token::Whitespace(Whitespace::MultiLineComment("/**/".to_string())),
+        ];
+
         compare(expected, tokens);
     }