use std::io;
use std::mem;

const BUF_SIZE: usize = 20;
pub enum String {
    Stack {
        data: [u8; BUF_SIZE],
        len: usize,
    },
    Heap(Box<[u8]>),
}

impl String {
    pub fn filled(r: &mut impl io::Read, len: usize) -> io::Result<Self> {
        if len > BUF_SIZE {
            let mut vec = Vec::with_capacity(len);
            unsafe { vec.set_len(len) };
            r.read_exact(&mut vec[..])?;
            Ok(String::Heap(vec.into_boxed_slice()))
        } else {
            let mut data = unsafe { mem::uninitialized() };
            r.read_exact(&mut data[..len])?;
            Ok(String::Stack { len, data })
        }
    }
}

fn main() {

}