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: parse.rs
    path of file: ./repos/phaser/phaser/vendor/syn/tests/common
    
    the code of the file until where you have to start completion: extern crate rustc_ast;
extern crate rustc_expand;
extern crate rustc_parse as parse;
extern crate rustc_session;
extern crate rustc_span;

use rustc_ast::ast;
use rustc_ast::ptr::P;
use rustc_session::parse::ParseSess;
use rustc_span::source_map::FilePathMapping;
use rustc_span::FileName;
use std::panic;

pub fn librustc_expr(input: &str) -> Option<P<ast::Expr>> {
    match panic::catch_unwind(|| {
        let sess = ParseSess::new(FilePathMapping::empty());
        let e = parse::new_parser_from_source_str(
            &ses
    
pub fn librustc_expr(input: &str) -> Option<P<ast::Expr>> {
    match panic::catch_unwind(|| {
        let sess = ParseSess::new(FilePathMapping::empty());
        let e = parse::new_parser_from_source_str(
            &sess,
            FileName::Custom("test_precedence".to_string()),
            input.to_string(),
        )
        .parse_expr();
        match e {
            Ok(expr) => Some(expr),
            Err(mut diagnostic) => {
                diagnostic.emit();
                None
            }
        }
    }) {
        Ok(Some(e)) => Some(e),
        Ok(None) => None,
        Err(_) => {
            errorf!("librustc panicked\n");
            None
        }
    }
}