ADR-0018: 70% line coverage is a required check on every crate and wrapper

Status: Accepted
Date: 2026-09-20
Derives from: the user's project brief (70% test coverage); design.md § What Rust does not solve (the suites as reviewer)
Constrains: architecture.md § Verification strategy, CLAUDE.md § Testing and coverage, .github/workflows/ci.yml
Implemented by: Wave 0 plan

Context

The conformance suites (ADR-0009) prove parity with the upstream tools but do not cover the native format, the new reporters, the agent commands or the wrappers. A single-maintainer project needs a floor that a contributor's pull request cannot lower.

Decision

Rust: cargo llvm-cov --workspace --fail-under-lines 70 is a required CI check. Coverage is measured per workspace and reported per crate; a crate below 70% fails the check even when the workspace is above it. Conformance fixtures count toward coverage when they run under cargo test, and they do (gate 1 layer 1 and gate 2 are #[test]s over committed fixtures).
TypeScript (wrappers/npm, adapters/vitest, eslint-plugin-rulebearing): vitest with coverage.thresholds.lines = 70.
C# (Rulebearing.TestAdapter, Rulebearing.Analyzer, the fallback extractor if invoked): coverlet with a 70% line threshold in the test project.
Python (wrappers/pip, pytest-rulebearing): pytest --cov --cov-fail-under=70.
The threshold is a floor, not a target: the extractors and rb-rules are expected to sit well above it because the fixtures drive them.
Generated code (stdlib snapshots, schema output) is excluded by path in the coverage configuration and the exclusions are listed in CLAUDE.md.

Consequences

A pull request that adds a reporter without a fixture fails CI.
The wave 0 plan installs cargo-llvm-cov in CI and sets the gate before any feature code lands.

Alternatives considered

A higher threshold. Not chosen by the brief; the conformance suites already push the core crates higher.
Branch coverage. Not required; line coverage is what the brief asks for and what all four toolchains report consistently.