Why doesn't println! work in Rust unit tests?

Question

I\'ve implemented the following method and unit test:

use std::fs::File;
use std::path::Path;
use std::io::prelude::*;

fn read_file(path: &Path) {
    l         


        

Answer 1

While testing, standard output is not displayed. Don't use text messages for testing but assert!, assert_eq!, and fail! instead. Rust's unit test system can understand these but not text messages.

The test you have written will pass even if something goes wrong. Let's see why:

read_to_end's signature is fn read_to_end(&mut self) -> IoResult>

It returns an IoResult to indicate success or error. This is just a type def for a Result whose error value is an IoError. It's up to you to decide how an error should be handled. In this case, we want the task to fail, which is done by calling unwrap on the Result.

This will work:

let contents = File::open(&Path::new("message.txt"))
    .read_to_end()
    .unwrap();

unwrap should not be overused though.