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?
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.