Start with the idea
#[test] functions are compiled only for the test harness, so they cost nothing in a release build. Unit tests sit beside the code and can reach its private details; integration tests live in tests/ and see only the public API, which is a useful check on your own interface.
A test records an observable promise. #[test] marks a function that the test harness runs under cargo test; an assertion compares the actual value with a prediction. The sample puts tests beside the implementation in a #[cfg(test)] module, which is a good place for small unit tests of edge cases and private helpers. The module is only compiled when testing, so it does not become a normal production code path.
Start with a function that adds two numbers.
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