Guidelines for Writing Rust Tests

Guidelines for writing new tests for Rust code.

Guidelines

1. Test against the public API of the code under test.
2. Test private APIs if and only if the private component is highly complex and difficult to test through the public API.
3. Use insta whenever you are testing output that is difficult to predict or compare.
4. Where appropriate, use proptest to add property-based tests for key invariants.
5. Testing code should be written with the same care reserved to production code. Avoid unnecessary duplication, introduce helpers to reduce boilerplate and ensure readability. The intent of a test should be obvious or, if not possible, clearly documented.
6. Do not reference exact line numbers in comments, as they may change over time.

Code organization

1. Put tests for public (pub) items under the crate's tests directory. Two layouts are in use and both are fine — match whichever the crate already has:
   • tests/integration/ — one test crate with its own main.rs and a module per area (trie_rs, geo, query_eval, …). Prefer this for a new crate: it compiles as a single unit instead of one binary per file.
   • Cargo's default layout — one integration binary per tests/*.rs file (varint, fork_gc, rlookup, …).
2. If the test must rely on private APIs, co-locate it with the code it tests, using a #[cfg(test)] module. Integration tests cannot reach pub(crate) or private items, so this is the only option for them — but prefer exercising the behavior through the public API where you can, per guideline 2 above.

Dealing with extern C symbols

Check out CONTRIBUTING.md for instructions on how to deal with undefined C symbols in Rust tests.