Reading from stdin in rust

Error.txt

src/main.rs:8:36: 8:42 error: cannot borrow immutable local variable `buffer` as mutable
src/main.rs:8     try!(stdin.read_to_string(&mut buffer));
                                                 ^~~~~~
<std macros>:1:1: 6:48 note: in expansion of try!
src/main.rs:8:5: 8:45 note: expansion site
error: aborting due to previous error

stdin_read.rs

use std::env;
use std::io;
use std::io::Error;
use std::io::prelude::*;

fn read_from_stdin(buffer: &mut String) -> Result<(), Error> {
    let mut stdin = io::stdin();
    try!(stdin.read_to_string(&mut buffer));
    Ok(())
}

fn main() {
    let mut buffer = String::new();

    match read_from_stdin(&mut buffer) {
        Err(why) => println!("{:?}", why),
        Ok(_) => println!("{}", buffer),
    }
}