increasing-code-coverage

name	increasing-code-coverage
description	Writes tests to increase code coverage. Activates when the user asks to improve code coverage or to write tests.

Increasing Code Coverage

Write tests to increase the code coverage in a specific crate. The goal is to increase code coverage to be above 80%.

Rules

• Write unit tests in the same Rust file as the function being tested.
• Write integration tests in the tests folder inside the crate.
  • Create the folder if needed.
  • Common helper code goes in tests/common.rs.
• DO NOT CHANGE THE CRATE CODE WITHOUT USER CONSENT. Only write test code.
• Work on a single crate at the time.

Workflow

Useful commands:

• baseline/summary coverage: nix develop .#nightly code-coverage -p <crate-name>
• detailed/per file coverage: nix develop .#nightly code-coverage -p <crate-name> --text

1. Get the baseline code coverage for the crate being tested.
2. Note which files have the lowest coverage.
3. Run the detailed coverage command, focusing on the target files.
4. Starting from top level functions, write tests (better if integration tests) for the uncovered functions and paths.
5. Run the code coverage summary command again.
6. Repeat until the target coverage is reached OR if the coverage stopped increasing.
   • Warn the user if the coverage is not increasing despite adding more tests.

Writing tests

• Avoid writing unit tests for simple functions that are covered by integration tests.
• Follow existing testing patterns in the same crate.
• Follow the arrange-act-assert pattern for tests.
• If possible, use the insta::assert_snapshot! macro to test components that provide structured information through the Display trait, for example DataFusion objects such as LogicalPlan, ExecutionPlan etc.
• DO NOT test code generated by the #[derive(...)] macro, such as Debug, Clone, Serialize, etc.
• DO NOT test type aliases such as Ref or Result.
• DO NOT test creating structs for the sake of creating structs. Focus on testing functions.
• Always test error paths too. Use the insta::assert_debug_snapshot! macro if you need to test the error message/type.
• WARN users about flaky tests.
• Run cargo clippy -p <crate-name> --tests and fix any issues.