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Rust II: data, choices and confidence · Lesson 11 / 12
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Tests that find defects

A useful test checks a rule, including where it changes. Let us find a one-character defect.

Learning paths14 min

Let's discover together

  1. 1

    #[test] marks functions run by cargo test. #[cfg(test)] includes the module only in test builds. use super::* lets tests access items from their parent module.

  2. 2

    assert! requires true; assert!(!value) requires false. assert_eq! compares values. A failure should describe an expectation from the rule, not copy the implementation calculation.

  3. 3

    With 3 available, requests for 2 and 3 are allowed; 4 is not. Zero is also invalid. These cases exercise below, exactly at, above and a forbidden value.

  4. 4

    Scenario 2 replaces <= with < and rejects a valid request for 3. Run tests after the change: boundary should fail. Restoring the rule and seeing it pass shows the test detects that specific defect.

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fn can_take(available: u16, request: u16) -> bool {
    request > 0 && request <= available
}
fn main() {
    println!("{} {} {}", can_take(3, 2), can_take(3, 3), can_take(3, 4));
}
#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn boundary() {
        assert!(can_take(3, 2));
        assert!(can_take(3, 3));
        assert!(!can_take(3, 4));
    }
    #[test]
    fn rejects_zero() { assert!(!can_take(3, 0)); }
}

Official links and next step

✦
YOUR MISSION

Write boundary tests and show they detect a faulty change.

Now it's your turn

Discovery workshop

Read, predict, investigate

Predict before revealing. This panel shows prepared results; it does not compile your code. Compiler errors are summarised; exact wording can vary with the compiler version.
Mini experiment

Learning simulation: compare cards and check your predictions. The browser is not compiling Rust or starting a server.

fn can_take(available: u16, request: u16) -> bool {
    request > 0 && request <= available
}
fn main() {
    println!("{} {} {}", can_take(3, 2), can_take(3, 3), can_take(3, 4));
}
#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn boundary() {
        assert!(can_take(3, 2));
        assert!(can_take(3, 3));
        assert!(!can_take(3, 4));
    }
    #[test]
    fn rejects_zero() { assert!(!can_take(3, 0)); }
}
Build your answers and check when you are ready.
A moment to understand

Do all passing tests prove there are no defects?

I need a hint

A finite set of examples does not automatically cover every behaviour.

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Take it with you

Run cargo test. Replace <= with <, confirm failure and restore it. Add cases (0, 1) and (20, 20), explaining the rule each protects.

RUST II · .rs

Your Rust II workbench

Read, predict and answer here. To actually compile, extract the activity ZIP into a new folder, open a terminal beside Cargo.toml and use the commands below. Rust and your operating system tools must be installed.

cargo check --offline
cargo test --offline
cargo run --offline
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The editor below saves a draft in this browser and lets you download or publish your .rs. To run it in the project, save the edited file as src/main.rs. The ZIP contains the starter version; prepared results do not change with your edits. Online grading does not verify local execution.

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