Why doesn't println! work in Rust unit tests?
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.
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
To include print outs with println!()
and keep colors for the test results, use the color
and nocapture
flags in cargo test
.
$ cargo test -- --color always --nocapture
(cargo version: 0.13.0 nightly)