Build, break, and test a study planner

15-minute session: read for 3 minutes, trace for 3, experiment for 7, and complete the check for 2. Spend longer whenever you need.

A useful test compares a result with a requirement. The planner must never report negative remaining work for its stated nonnegative input domain. Run the starter, then deliberately remove the protection for completed counts above total. At least one check should fail. Restore it. Add two named study tasks, a summary, and boundary cases. Save the source and evidence on Agila Forge. Explain every line, including anything suggested by AI.

fn main is the entry point. let introduces a binding; mut permits reassignment. println! is a macro. A reference such as &plan borrows rather than transfers ownership.

fn remaining(total: u32, done: u32) -> u32 { total.saturating_sub(done) }
fn main() {
    assert_eq!(remaining(4,1),3);
    assert_eq!(remaining(4,4),0);
    assert_eq!(remaining(4,5),0);
    assert_eq!(remaining(0,0),0);
    println!("Passed");
}

Expected result: Passed

Practice and evidence

Keep three notes: your prediction, the actual result, and why they match or differ. The completion check grades understanding; the practice task builds skill.

Language reference