twin4rust

A source file under src/ should have a test file mirroring it under tests/. twin4rust fails the build when one doesn't.

twin4rust report

Files without a matching mirrored test file:
- node: src/raft_node.rs -> tests/raft_node_tests.rs
- node: src/state/raft_state.rs -> tests/state/raft_state_tests.rs

summary: packages_with_gaps=1 missing_files=2

What it is not

This is not a coverage tool. It does not measure lines, it does not measure branches, and it has no opinion on whether your tests are any good — it never opens the test file it is looking for.

It answers one structural question: does the mirrored test file exist?

That question is worth asking on its own, because line coverage cannot answer it. A source file can be at 90% coverage purely as collateral from an end-to-end test three layers up, with nothing anywhere that names it as its subject. Coverage reports that as covered. twin4rust reports it as untested, which is the more useful answer when the file later breaks and no failing test points at it.

Install

cargo install cargo-twin4rust

Use

cargo twin4rust --manifest-path Cargo.toml --package my-crate

Exit code is 0 when every expected mirror exists and 1 when any is missing, so it drops into CI as-is. Gaps are sorted by package, then source path, so the output is stable across runs and diffable.

The mirror rule

src/<path>/<name>.rs   ->   tests/<path>/<name>_tests.rs

The rule is src/-rooted, and that is currently a hard requirement rather than a default. A file outside src/ yields no mirror path, and a file with no mirror path is not reported — see Known limitations.

What it skips

The gate is deliberately conservative — it would rather stay quiet than force a test file for something with no behaviour to test.

• Entry points: src/lib.rs, src/main.rs, build.rs, and any file directly under src/bin/. A module inside a src/bin/<name>/ binary stays in scope
• every mod.rs, unconditionally — including one that carries behaviour
• Definition-only files: every top-level item is a struct, enum, type, trait, const or static declaration, or an ignorable use, extern crate or bodiless mod — with at least one declaring kind present. A top-level macro invocation keeps the file in scope, since its expansion is never seen
• Method-less trait impls, such as impl Marker for T {} or a blanket impl<T> Alias for T where ..., which introduce nothing to assert
• Single-type files whose only impl is a trivial new — one method, no generics, returns Self, body is a single struct literal, no branching, no loops, no helper calls. Pure data holders do not earn a test file.
• Humble adapters — a single type whose methods all either hold what they were given or forward it: return nothing, one statement, and that statement is a call. A method returning a value stays in scope however short its body, and one branch takes the whole file back out of the exemption
• The src/ tree of any package whose name ends in -validation
• Items carrying #[cfg(test)], stripped before any of the above is evaluated

Anything else stays in scope. A new with a branch in it, a second method, or a trait impl carrying at least one method all put the file back on the list. A file with no top-level items at all is reported rather than skipped — an empty source file is more likely an accident than a decision.

Known limitations

Files outside src/ are analyzed and then silently dropped. Source roots come from each production target's own path, so a [[bin]] at path = "tools/x.rs" contributes tools/ as a root and the file is walked, read and classified — but no mirror path can be derived for it, and a file with no mirror path is treated as having no expectation. It does not appear in the report and does not affect the exit code.

Relatedly, a package with no target rooted under src/ never walks src/ at all, because the fallback that adds it fires only when no root was collected.

Until this is resolved, twin4rust is only trustworthy on packages whose production sources live under src/. Both behaviours are recorded in docs/OPEN_POINTS.md, along with the ordering and mod.rs quirks behind them.

Choosing what to point it at

Only pass packages whose test tree takes files as its subject.

A repository running a tiered test ladder has trees with different subjects: one covers files, another covers a single-node cluster, another covers a multi-node cluster. node/tests/raft/replication_tests.rs and validation/tests/cluster/replication_tests.rs can both exist and both be correct — one covers the file, the other covers a cluster replicating. Neither substitutes for the other.

Point this tool at a cluster-level harness and every one of its sources is reported as a gap, because by design none of them has a mirror. The -validation suffix rule covers the common naming convention; anything else is your call.

There is deliberately no flag letting one package's tests satisfy another's expectation. Such a flag existed briefly and was removed: aimed at a harness tree it made the number improve while coverage did not, because a cluster test was silencing a missing file-level test. If a source file has no test whose subject is that file, that is the finding — wherever else it happens to be exercised.

Documentation

Related

• cargo-crap4rust — CRAP scores across Rust crates
• slotgate — bounded-parallelism job runner giving each slot a disjoint port range

License

MIT. See LICENSE.