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 (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.