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: mod.rs
    path of file: ./repos/tokio/tokio/src/io
    
    the code of the file until where you have to start completion: //! Traits, helpers, and type definitions for asynchronous I/O functionality.
//!
//! This module is the asynchronous version of `std::io`. Primarily, it
//! defines two traits, [`AsyncRead`] and [`AsyncWrite`], which are asynchronous
//! versions of the 
    
//! async fn main() -> io::Result<()> {
//!     let f = File::open("foo.txt").await?;
//!     let mut reader = BufReader::new(f);
//!     let mut buffer = String::new();
//!
//!     // read a line into buffer
//!     reader.read_line(&mut buffer).await?;
//!
//!     println!("{}", buffer);
//!     Ok(())
//! }