chore: refactor binary expression start position track

Author: gengjiawen

parser/lib.rs

@@ -145,15 +145,14 @@ impl<'a> Parser<'a> {
 
     fn parse_expression(&mut self, precedence: Precedence) -> Result<(Expression, Span), ParseError> {
         let start = self.current_token.span.start;
-        let mut left_start = self.current_token.span.start;
         let mut left = self.parse_prefix_expression()?;
         while self.peek_token.kind != TokenKind::SEMICOLON && precedence < get_token_precedence(&self.peek_token.kind) {
+            let left_start = self.current_token.span.start;
             match self.parse_infix_expression(
                 &left,
                 left_start
             ) {
                 Some(infix) => {
-                    left_start = self.current_token.span.end;
                     left = infix?
                 }
                 None => return Ok((left, Span {