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: message_body.rs
    path of file: ./repos/actix-web/actix-http/src/body
    
    the code of the file until where you have to start completion: //! [`MessageBody`] trait and foreign implementations.

use std::{
    convert::Infallible,
    error::Error as StdError,
    mem,
    pin::Pin,
    tas
    
    async fn test_vec() {
        assert_eq!(vec![0; 0].size(), BodySize::Sized(0));
        assert_eq!(Vec::from("test").size(), BodySize::Sized(4));

        let pl = Vec::from("test");
        pin!(pl);
        assert_poll_next!(pl, Bytes::from("test"));
    }