You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: ast.rs
    path of file: ./repos/erg/crates/erg_parser
    
    the code of the file until where you have to start completion: //! defines `Expr` (Expression, the minimum executing unit of Erg).
use std::borrow::Borrow;
use std::fmt;
use std::fmt::Write as _;

use erg_common::consts::ERG_MODE;
use erg
    
    fn loc(&self) -> Location {
        if let Some((lparen, rparen)) = &self.braces {
            Location::concat(lparen, rparen)
        } else if !self.attrs.is_empty() {
            let (first, _) = self.attrs.first().unwrap();
            let (_, last) = self.attrs.last().unwrap();
            Location::concat(first, last)
        } else {
            Location::Unknown
        }
    }