use core::cell::RefCell;

struct Interface {
    buf: RefCell<[u8; 8]>,
}

impl Interface {
    pub fn new() -> Self {
        Self { buf: RefCell::new([0; 8]) }
    }
}

#[derive(Debug)]
struct Foo<'a>(&'a [u8]);

impl Interface {
    fn receive<F>(&self, mut f: F) where F: FnMut(Option<Foo<'_>>) {
        f(Some(Foo(&*self.buf.borrow())));
    }

    fn transmit<'a>(&self, foo: Foo<'a>) {
        (&mut self.buf.borrow_mut()[..foo.0.len()]).copy_from_slice(&foo.0);
    }
}

fn main() {
    let interface = Interface::new();

    interface.receive(|foo| println!("{:?}", foo));
    interface.transmit(Foo(&[1; 4][..]));
    interface.receive(|foo| println!("{:?}", foo));
}