testlog

Yes, One macro

A micro-crate that provides test_log! – a macro that prints to stderr only when tests are running, and only for the crate where it's used.

Why?

You need to debug test failures. println! gets captured by the test harness. eprintln! clutters production logs. Adding print statements of any kind pollute the logs of the consumers of your library. What you want is something in your library that only prints for your tests, and ideally is only shown during testing on failure.

This does exactly what you want: debug output that only appears during testing of your crate, and since output from a test is captured on success, you only see the log when the test fails. Perfect!.

Usage

Add to Cargo.toml:

[dependencies]
testlog = "0.1"

Use in your code:

use testlog::test_log;

fn complex_function(data: &[i32]) -> Vec<i32> {
    test_log!("Processing {} items", data.len());
    
    let result: Vec<i32> = data.iter()
        .map(|&x| {
            test_log!("Processing item: {}", x);
            x * 2
        })
        .collect();
        
    test_log!("Result: {:?}", result);
    result
}

#[test]
fn test_complex_function() {
    let input = vec![1, 2, 3];
    let output = complex_function(&input);
    assert_eq!(output, vec![2, 4, 6]);
    // Debug output appears here when running `cargo test`
}

How it works

The test_log! macro checks cfg!(test) at compile time:

• In test builds: Expands to eprintln!(...)
• In production builds: Expands to nothing (zero runtime cost)
• Crate-local: Only activates when the current crate is in test mode

That's it