{"id":2815,"date":"2022-06-12T07:54:01","date_gmt":"2022-06-12T12:54:01","guid":{"rendered":"https:\/\/www.becomebetterprogrammer.com\/?p=2815"},"modified":"2022-07-19T20:39:43","modified_gmt":"2022-07-20T01:39:43","slug":"rust-println-doesnt-work-in-unit-tests-solved","status":"publish","type":"post","link":"https:\/\/www.becomebetterprogrammer.com\/staging\/4563\/rust-println-doesnt-work-in-unit-tests-solved\/","title":{"rendered":"Rust | Println! Doesn’t Work in Unit Tests (Solved)"},"content":{"rendered":"\n

If you are new to Rust programming language, there is a chance you are experiencing a deep learning curve in understanding how the programming language works. On top of that, you start writing your first couple of unit tests and suddenly realize that the logs from using the print<\/a> or println!<\/a> macro are not displayed in the terminal. <\/p>\n\n\n\n

To fix println! not showing logs when running succesful unit tests, pass the --show-ouput<\/code> flag at the moment of executing the command to run tests cargo test<\/code>. Hence, the new command to run tests and display logs generated from using the println macro is cargo test -- --show-output<\/code>.<\/strong><\/p>\n\n\n\n

cargo test -- --show-output<\/code><\/p>\n\n\n\n

Let’s look at the following example. We have a simple display_text<\/code> function that only displays a log of a &str using println!, and returns a &\"Text displayed\"<\/code>.<\/p>\n\n\n\n

fn main() {\n    \n    display_text(\"Hello World!\");\n\n}\n\nfn display_text(text: &str) -> &str {\n    println!(\"Displaying Text {0}\", text);\n\n    &\"Text displayed\"\n}\n\n#[cfg(test)]\nmod test {\n    use super::*;\n\n    #[test]\n    fn test_display_text() {\n        println!(\"Running test on display_text function\");\n        assert_eq!(display_text(\"Hello!\"), \"Text displayed\")\n    }\n}<\/code><\/pre>\n\n\n\n
Notice there is a test to test the display_text<\/code> function that checks whether the returned &str<\/code> is “Text displayed”.<\/p>\n\n\n\n
Running the standard cargo test<\/code> command will give us the following result.<\/p>\n\n\n\n
$ cargo test\n    Finished test [unoptimized + debuginfo] target(s) in 0.23s\n     Running unittests (target\\debug\\deps\\rust_playground-afbcb92314fac06b.exe)\n\nrunning 1 test\ntest test::test_display_text ... ok\n\ntest result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.06s<\/code><\/pre>\n\n\n\n
Notice the logs that should have been generated from println!(\"Displaying Text {0}\", text);<\/code> and println!(\"Running test on display_text function\");<\/code> are not visible. Luckily, now that we know about the --show-output<\/code> flag, we can get the outputs in the terminal similar to the following:<\/p>\n\n\n\n
$ cargo test --  --show-output\n    Finished test [unoptimized + debuginfo] target(s) in 0.24s\n     Running unittests (target\\debug\\deps\\rust_playground-afbcb92314fac06b.exe)\n\nrunning 1 test\ntest test::test_display_text ... ok\n\nsuccesses:\n\n---- test::test_display_text stdout ----\nRunning test on display_text function\nDisplaying Text Hello!\n\n\nsuccesses:\n    test::test_display_text\n\ntest result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.06s<\/code><\/pre>\n\n\n\n
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Table of Contents<\/p>\n<\/div>