should_panic

Write tests that expect a panic with should_panic.

should_panic

For some functions, panicking on invalid input is the expected behaviour. #[should_panic] counts the test as passed if it panics. With expected, a specific text is also required in the panic message:

fn bol(a: i32, b: i32) -> i32 {
    if b == 0 {
        panic!("sıfıra bölme");
    }
    a / b
}

#[test]
#[should_panic(expected = "sıfıra bölme")]
fn sifira_bolme_panik_verir() {
    bol(1, 0);
}

Without expected, a panic for the wrong reason would also pass the test.

Tests returning Result

For functions returning Result instead of panicking, checking the error case directly is clearer:

fn guvenli_bol(a: i32, b: i32) -> Result<i32, String> {
    if b == 0 { return Err("sıfıra bölme".into()); }
    Ok(a / b)
}

#[test]
fn sifir_hata_doner() {
    assert_eq!(guvenli_bol(1, 0), Err("sıfıra bölme".to_string()));
    assert!(guvenli_bol(4, 2).is_ok());
}

Exercise Panic test

Write the bol function above and a #[cfg(test)] mod tests containing a test marked #[should_panic(expected = "sıfıra bölme")]. In main, print the result of bol(10, 2).

Show example solution

fn bol(a: i32, b: i32) -> i32 {
    if b == 0 {
        panic!("sıfıra bölme");
    }
    a / b
}

fn main() {
    println!("{}", bol(10, 2));
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    #[should_panic(expected = "sıfıra bölme")]
    fn sifira_bolme_panik_verir() {
        bol(1, 0);
    }
}

Test yourself

What happens if a <code>#[should_panic]</code> test finishes without panicking?
- It warns
- The test fails
- The test passes
- It's skipped
Show answer

The test fails — If the expected panic didn't happen, the function isn't catching the invalid input.

Expected panic

#[test]
#[____(expected = "sıfıra bölme")]
fn sifira_bolme_panik_verir() {
    bol(1, 0);
}

Show answer

#[test]
#[should_panic(expected = "sıfıra bölme")]
fn sifira_bolme_panik_verir() {
    bol(1, 0);
}

Case and test result

should_panic + panic

Passes
should_panic + no panic

Fails
expected doesn't match

Fails (mismatch)
Normal test + panic

Fails (panic)

Next lesson: Summary - Testing

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