Practical Rust

Assertions

Checking a Function

Assertions verify expected results. Real test functions use #[test] with cargo test; this example checks inline with assert_eq!.

Program

Play the program to verify a double function and confirm all checks pass.

unit_tests.rs
Replay: real traced execution (multi-file project)
fn double(n: i32) -> i32 {
    n * 2
}

fn main() {
    assert_eq!(double(2), 4);
    assert_eq!(double(0), 0);
    println!("all checks passed");
}

  1. fn double(n: i32) -> i32

    pass 1 of 2
    1fn double(n: i32) -> i32 {2    2n * 23}

  2. assert_eq!(double(2), 4);

    5fn main() {6    assert_eq!(double(2), 4);7    assert_eq!(double(0), 0);8    println!("all checks passed");

  3. fn double(n: i32) -> i32

    pass 2 of 2
    1fn double(n: i32) -> i32 {2    0n * 23}

  4. assert_eq!(double(0), 0);

    6    assert_eq!(double(2), 4);7    assert_eq!(double(0), 0);8    println!("all checks passed");9}
    outputall checks passed
assert_eq! `assert_eq!(a, b)` panics if the values differ.
function under test `double` is the code being checked.
cargo test Real tests live in `#[test]` functions run by `cargo test`.