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) {
    let mut file = File::open(path).unwrap();
    let mut contents = String::new();
    file.read_to_string(&mut contents).unwrap();
    println!("{}", contents);
}

#[test]
fn test_read_file() {
    let path = &Path::new("/etc/hosts");
    println!("{:?}", path);
    read_file(path);
}

I run the unit test this way:

rustc --test app.rs; ./app

I could also run this with

cargo test

I get a message back saying the test passed but the println! is never displayed on screen. Why not?

Answer 1

This happens because Rust test programs hide the stdout of successful tests in order for the test output to be tidy. You can disable this behavior by passing the --nocapture option to the test binary or to cargo test:

#[test]
fn test() {
    println!("Hidden output")
}

Invoking tests:

% rustc --test main.rs; ./main

running 1 test
test test ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured

% ./main --nocapture

running 1 test
Hidden output
test test ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured

% cargo test -- --nocapture

running 1 test
Hidden output
test test ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured

If tests fail, however, their stdout will be printed regardless if this option is present or not.