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

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?

Solution 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 (but, in this case after -- – see below):

#[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.

Solution 2:

TL;DR

$ cargo test -- --nocapture

With the following code:

#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum PieceShape {
    King, Queen, Rook, Bishop, Knight, Pawn
}

fn main() {
    println!("Hello, world!");
}

#[test]
fn demo_debug_format() {
    let q = PieceShape::Queen;
    let p = PieceShape::Pawn;
    let k = PieceShape::King;
    println!("q={:?} p={:?} k={:?}", q, p, k);
}

Then run the following:

 $ cargo test -- --nocapture

And you should see

Running target/debug/chess-5d475d8baa0176e4

running 1 test
q=Queen p=Pawn k=King
test demo_debug_format ... ok

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