Enabling print statements in Cargo tests

January 23, 2015

When writing tests in Cargo, you sometimes need to debug failures and print statements are a great way to do that. When you run cargo test the output shows details about which tests pass or fail but filters out the stdout information.

The you can enable this by running tests with -- --nocapture like this:

$ cargo test -- --nocapture


Here is an example of what a test would look like:

#[test]
fn new_linked_list() {
    let mut list: super::LinkedList<String> = super::LinkedList::new();
    list.add("first".to_string());
    println!("Value added: {}", list.head.unwrap().payload);
}


When we run this with the no capture flag:

     Running target/algorithms-43e2eed4b681635b

running 3 tests
test datastructures::linkedlist::node_new ... ok
Value added: first
test datastructures::linkedlist::node_payload ... ok
test datastructures::linkedlist::tests::new_linked_list ... ok

test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured


Notice that the print statement is before the test passes (a false positive since I haven’t added an assertion), but it gets intertwined with our other tests which are running in parrallel. This can be annoying to see if you have a longer list of tests. We can remove that as well by only running the specific test that we want:

$ cargo test -- --nocapture datastructures::linkedlist::tests::new_linked_list


We then see:

     Running target/algorithms-43e2eed4b681635b

running 1 test
Value added: first
test datastructures::linkedlist::tests::new_linked_list ... ok

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