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: body.rs
    path of file: ./repos/reqwest/src/blocking
    
    the code of the file until where you have to start completion: use std::fmt;
use std::fs::File;
use std::future::Future;
#[cfg(feature = "multipart")]
use std::io::Cursor;
use std::io::{self, Read};
use std::mem;
use std::ptr;

use bytes::buf::UninitSlice;
use bytes::Bytes;
use futures_channel::mpsc;

use crate::async_impl;

/// The body of a `Request`.
///
/// In most cases, this is not needed directly, as the
/// [`RequestBuilder.body`][builder] method uses `Into<Body>`,
    
    pub fn buffer(&mut self) -> Result<&[u8], crate::Error> {
        match self.kind {
            Kind::Reader(ref mut reader, maybe_len) => {
                let mut bytes = if let Some(len) = maybe_len {
                    Vec::with_capacity(len as usize)
                } else {
                    Vec::new()
                };
                io::copy(reader, &mut bytes).map_err(crate::error::builder)?;
                self.kind = Kind::Bytes(bytes.into());
                self.buffer()
            }
            Kind::Bytes(ref bytes) => Ok(bytes.as_ref()),
        }
    }