Rule authoring

Lands in v0.9.9. Codifies the four-step process every new rule kind, bundled ruleset, or rule-kind alias goes through so the coverage audits stay green and CI doesn’t surprise anyone.

This doc supplements (does not replace) docs/design/v0.9/coverage-and-dogfood.md, which captures the why. This doc captures the what to do.

Two-layer enforcement

Layer	Tool	Catches
1 — File presence	`alint check .` (the `.alint.yml` at the repo root, run via the `action-selftest.yml` workflow on every push)	Missing source files, missing scenario YAMLs, missing bundled-ruleset coverage. Fast feedback during local development.
2 — Semantic	`cargo test -p alint-e2e` (`coverage_audit_*.rs` integration tests)	Pass/fail symmetry, alias-aware kind coverage, bundled-ruleset symmetry, git-mode symmetry, registry consistency.

Both layers run in CI. A PR that lands a new rule without an e2e scenario fails layer 1 at lint time; a PR that lands only a passing scenario (no fires/silent counterpart) fails layer 2 at test time.

Adding a new rule kind

1. Implement under `crates/alint-rules/src/<kind>.rs`.
2. Register in `alint-rules/src/lib.rs::builtin_registry()`.
3. Add e2e scenarios:
     crates/alint-e2e/scenarios/check/<family>/<kind>_pass.yml
     crates/alint-e2e/scenarios/check/<family>/<kind>_fires.yml
4. If `wants_git_tracked()` / `wants_git_blame()` is opt-in or
   mode-dependent, add:
     <family>/<kind>_in_repo.yml         (given.git.init: true)
     <family>/<kind>_no_op_outside_git.yml  (no given.git block)
5. Run `cargo test -p alint-e2e -- --no-fail-fast`. Every
   `coverage_audit_*` should pass.

Family-directory conventions

The family directory is chosen by what the rule does:

content/ — content scanning rules (file_content_matches / no_trailing_whitespace / final_newline / line_endings / …)
existence/ — presence/absence (file_exists / file_absent / dir_exists / dir_absent)
cross_file/ — fan-out / relational (for_each_dir / for_each_file / pair / unique_by / every_matching_has / dir_only_contains / dir_contains)
encoding/ — Unicode + bytes (no_bom / no_bidi_controls / no_zero_width_chars / file_is_text / file_is_ascii)
metadata/ — file-property (file_min_size / file_max_size / file_min_lines / file_max_lines / file_hash)
naming/ — filename-shape (filename_case / filename_regex)
structure/ — layout (max_directory_depth / max_files_per_directory / no_empty_files)
structured/ — JSONPath / JSON-Schema / YAML / TOML
security/ — Trojan-source defense, denylists
git/ — git-aware rules (git_blame_age / git_commit_message / git_no_denied_paths / git_tracked_only behaviour)
unix_metadata/ — chmod / symlink shapes
when_iter/ / when_facts/ — when: expression coverage
interactions/ — multi-rule scenarios

If your rule doesn’t fit, add a new family. The audits walk scenarios/check/**/*.yml recursively, so directory layout is purely organisational.

Scenario shape

Every scenario YAML uses the same shape:

name: <human-readable description>
tags: [check, <kind>, <family>, passing|failing]

given:
  tree:
    <path>: <content>            # files
    <path>:                      # directories
      <child>: …
  config: |
    version: 1
    rules:
      - id: <test-rule-id>
        kind: <kind>
        paths: …
        level: …

when: [check]                    # or [fix]

expect:
  - violations: []               # pass
  - violations:                  # fires
      - {rule: <test-rule-id>, level: <level>, path: <where>}

For git-aware rules, add given.git: { init: true, add: […], commit: true }.

Native-test allowlist

Some rule kinds can’t have firing YAML scenarios because the testkit doesn’t yet materialise the required filesystem primitive (chmod, symlinks, backdated commits, custom commit messages). For these, add a Rust integration test under crates/alint-rules/tests/ or crates/alint-e2e/tests/ that covers the firing path directly, then add the kind to coverage_audit_pass_fail.rs::NATIVE_FIRES_ALLOWLIST with a pointer to the native test.

The allowlist is meant to shrink, not grow. As the testkit acquires mode: 0o755, symlink_to: <path>, custom commit messages, and GIT_AUTHOR_DATE overrides, allowlist entries move into native YAML coverage.

Adding a new bundled ruleset

1. Add `crates/alint-dsl/rulesets/v1/<...>.yml`. The first
   three lines must be:
     # alint://bundled/<name>@v<rev>
     #
     # <prose description>
   (enforced by the `bundled-ruleset-has-uri-header` rule in
    .alint.yml — caught by `alint check .`)
2. Add e2e scenarios:
     crates/alint-e2e/scenarios/check/bundled-<name>/
       <name>_well_formed_passes.yml      (every expect.violations: [])
       <name>_*_flagged.yml               (≥1 non-empty violations entry)
3. Run `cargo test -p alint-e2e --test coverage_audit_bundled_rulesets`.

`scope_filter:` for ecosystem rulesets (v0.9.6+)

Bundled rulesets that target one ecosystem (rust@v1, node@v1, etc.) should pair their tree-level when: facts.has_<ecosystem> gate with a per-rule scope_filter: { has_ancestor: <manifest> } on per-file content rules so the rule fires only on files inside that ecosystem’s package subtree. The two gates compose: when: is a cheap tree-level short-circuit (no facts → no rule iteration); scope_filter: narrows per-file scope when the rule does run, useful in polyglot monorepos where one language’s package sits next to another’s.

# In a ruleset YAML:
facts:
  - id: has_rust
    any_file_exists: [Cargo.toml, "**/Cargo.toml"]    # broadened: catch nested manifests

rules:
  - id: rust-sources-no-bidi
    when: facts.has_rust                              # tree gate
    kind: no_bidi_controls
    paths: "**/*.rs"                                  # path glob
    scope_filter:                                     # ancestor walk
      has_ancestor: Cargo.toml                        # canonical per-package manifest
    level: error

Constraints:

Per-file rules only. scope_filter: is supported on PerFileRule-trait rules (engine consults it in the file- major dispatch loop). Cross-file rules (pair, for_each_dir, file_exists, etc.) reject scope_filter: at build time with a pointer to the for_each_dir + when_iter: pattern. Rule-major rules like filename_case silently ignore scope_filter: today — gate them via the rule’s paths: glob or skip the filter.
Literal filenames, not globs. Each has_ancestor: entry is a filename like Cargo.toml or package.json; no **/ prefix, no path separators. The walk handles “anywhere up the tree” by traversing Path::parent() upward.
File’s own dir counts as ancestor. A pyproject.toml matched by paths: pyproject.toml and gated by scope_filter: { has_ancestor: pyproject.toml } always passes its own ancestor walk — don’t add the filter when the rule’s paths: is already a literal manifest filename.
has_ancestor accepts a single string or a list. has_ancestor: pom.xml and has_ancestor: [pom.xml, build.gradle, build.gradle.kts] are both valid; first-match- wins on the upward walk.

Design + semantics: docs/design/v0.9/scope-filter.md.

The audit treats nested rulesets like monorepo/cargo-workspace.yml as a single unit — its scenarios live alongside monorepo’s under bundled-monorepo/. The audit doesn’t require a separate family directory per nested ruleset; the URI match (extends: alint://bundled/monorepo/cargo-workspace@v1) is what counts.

Adding a rule-kind alias

Aliases register the same builder under multiple names (e.g. max_size ↔ file_max_size). They don’t need new scenarios; add the alias to coverage_audit.rs::aliased AND coverage_audit_pass_fail.rs::ALIASES so the audits treat both spellings as one canonical kind.

Bench-scale coverage (soft)

Bench coverage isn’t a correctness requirement; the coverage_audit_bench_listing.rs test always passes and just emits an eprintln! summary of rule kinds absent from any xtask/src/bench/scenarios/*.yml. Run with cargo test -p alint-e2e -- --nocapture to see the listing.

If your new rule’s dispatch shape is novel (e.g. a new cross-file aggregation that today’s S6 / S7 / S8 don’t exercise), consider extending one of those scenarios so xtask bench-compare gates regressions of its perf shape. This is opt-in — most rule additions don’t need it.

Failure modes you’ll hit

coverage_audit_pass_fail fails with “missing FIRING” — add a <kind>_fires.yml scenario whose expect.violations: lists a rule with that kind. Or, if the firing case can’t be expressed in YAML, add to NATIVE_FIRES_ALLOWLIST.
coverage_audit_pass_fail fails with “missing SILENT” — add a <kind>_pass.yml (or similar) with expect: - violations: [].
coverage_audit_bundled_rulesets fails — the new ruleset hasn’t been referenced from any scenario via extends:. Add at least a well-formed scenario.
coverage_audit.rs fails with “missing kinds” — the audit doesn’t see your new kind in any scenario at all. The earlier failures usually surface first; this is the catch-all backstop.
alint check . fires bundled-ruleset-has-uri-header — the ruleset’s first three lines don’t match the docs-export parser’s expected header shape. See the rule’s message: in .alint.yml for the exact pattern.

docs/design/v0.9/coverage-and-dogfood.md — design rationale
docs/design/ARCHITECTURE.md — engine + rule layer overview
crates/alint-e2e/tests/coverage_audit_*.rs — the audits themselves; each is a single integration test with a clear panic message