I’d like to have a function that either writes to stdout or to a Vec. This is for an interactive CLI I would like to write tests for.

This works:

// To stdout
write!(std::io::stdout(), "{}", "fooxn".to_string()).unwrap();

// To a Vec<u8>
let mut vec_buf = Vec::new();
write!(vec_buf, "{}", "foox".to_string()).unwrap();
print!("Via string: {:?}", std::str::from_utf8(&vec_buf).unwrap());

But when I try something like this:

fn write_stuff(w: &mut Box<dyn Write>) {
    write!(w, "foo");
}

I can run it with stdout:

    let mut buf = Box::new(io::stdout()) as Box<dyn Write>;
    write_stuff(&mut buf);

I can run it with a Vec<u8>:

    let mut buf = Box::new(Vec::new()) as Box<dyn Write>;
    write_stuff(&mut buf);

But then, how can I assert the contents after the call?

print!("Via string: {:?}", std::str::from_utf8(&buf).unwrap());

Results in:

error[E0308]: mismatched types
  --> src/pipeline.rs:96:56
   |
96 |         print!("Via string: {:?}", std::str::from_utf8(&buf).unwrap());
   |                                    ------------------- ^^^^ expected `&[u8]`, found `&Box<dyn Write>`
   |                                    |
   |                                    arguments to this function are incorrect
   |
   = note: expected reference `&[u8]`
              found reference `&Box<dyn std::io::Write>`