Shared via Rust Playground

playground.rs

use std::io::{Cursor, Read, self};

trait ReadExt {
    fn read_to_vec(&mut self, buf: &mut Vec<u8>) -> io::Result<usize>;
}

impl<T: Read> ReadExt for T {
    fn read_to_vec(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
        self.take(buf.capacity() as u64).read_to_end(buf)
    }
}

fn main() {
    let mut source = Cursor::<&[u8]>::new(b"stuff");
    let mut buf = Vec::<u8>::with_capacity(4);
    source.read_to_vec(&mut buf).unwrap();
    println!("{:?}", buf);
}