# pi-lens ast-grep Rule Reference Deep-dive companion to `SKILL.md`. Read this when you're debugging a rule that isn't matching (or over-matching) the way you expect: ReDoS-safe regex authoring, string-escape quirks in node text, `has`/`inside` `stopBy` defaults and when to override them, boolean-parameter matching across two grammars, and precision-over-recall heuristics for denylist-shaped rules. ## Hard-won gotchas (NAPI runner specifics — verified) ``` ⚠ `has`/`inside` default to DIRECT child/parent — add `stopBy: end` for a recursive search. This cuts BOTH ways, so think about where the target node actually lives: - Target is a grandchild+ → you MUST add `stopBy: end` or the `has` never matches. `switch-without-default` = `switch_statement` not has `switch_default`: the default lives under `switch_body`, so without `stopBy: end` it matches nothing and every switch (even ones WITH a default) is flagged. Same for `nested-ternary` catching a parenthesized `a ? (b ? c : d) : e`. - Target is the direct child → leave it at `neighbor` (default). Adding `stopBy: end` OVER-reports: `throw_statement` has `string` + `stopBy: end` flags `throw new Error("x")` (the string is nested), and `expression_statement` has `new_expression` + `stopBy: end` flags `fn(new Error())` as a discarded error. Keep these direct. napi's `has` never matches the node itself, so a self-referential `kind: X` has `kind: X` (with `stopBy: end`) correctly flags only genuinely-nested X. ✅ Prefer `regex` on the matched node's OWN text over `has` when you only need to inspect the node — avoids recursive-descendant false positives: kind: export_statement regex: '^export\s+(let|var)\b' # precise; no has-recursion FP (NAPI evaluates `regex` with JS RegExp on node.text() — keep it LINEAR so the detector can't itself ReDoS.) ⚠ String-literal regexes match SOURCE text, not the runtime string value. Inspect the exact node text before writing constraints: ast-grep run --kind string --lang ts sample.ts --json=compact Example: source `"\\|"` is node text `"\\\\|"` in JSON; to match a source-level escaped backslash (`\\`) followed by a non-backslash, the rule regex needs FOUR regex backslashes, preferably in a YAML block scalar: regex: >- ^["'`]\\\\[^\\A-Za-z0-9$] This is how `incomplete-string-escaping` catches both `"\\|"` and `'\\"'`. Avoid shell here-doc probes for this class — shell/JSON escaping can silently eat a backslash and make the rule look broken. ⚠ `-js` twins: remember there are TWO execution surfaces. - ast-grep CLI/LSP language-gates by `language:`. A `language: TypeScript` rule is not enough for standalone `.js` coverage, so shipped user-facing TS/JS rules that should fire under the ast-grep LSP usually need a `-js` twin with `language: JavaScript` plus its own fixture. - the in-process NAPI fallback (`ast-grep-napi.ts` — pi-lens source checkout only, not present in the installed package) already DEDUPES by grammar (#657): `ruleLanguageForFile` (`ast-grep-napi.ts:205-216`) maps each file extension to its actual grammar, and the matcher (`:450-455`) skips any `language:`-tagged rule whose tag doesn't match that grammar. A TS rule and its JS twin no longer double-fire on the same file in fallback mode. - **`-js` twins are needed only when the rule body itself is grammar-divergent** (different node kinds/fields between the TS and JS grammars). A grammar-agnostic body doesn't need a twin for fallback coverage — the dedup above already scopes it correctly; ship a twin only when standalone `.js` coverage through the ast-grep CLI/LSP baseline is required (see the first bullet). - **Grammar-divergent bodies** still need separate variants regardless: e.g. `no-flag-argument` uses `required_parameter` in TS and `assignment_pattern` in JS. ✅ Node-kind facts (tree-sitter-typescript grammar — NOT the TS compiler / Roslyn): - let / const → `lexical_declaration` (var is NOT here) - var → `variable_declaration` - a regex literal's pattern text → `regex_pattern` - x[i] index access → `subscript_expression` (NOT element_access_expression) - obj.prop access → `member_expression` (NOT property_access_expression) - !x / -x / typeof x → `unary_expression` - a ? b : c → `ternary_expression` ❌ Wrong-grammar kind names = silent dead rule. `element_access_expression`, `property_access_expression`, `binary_operator`, etc. are TS-compiler/Roslyn names, not tree-sitter's. napi REJECTS the whole rule ("invalid kind matcher") so it never runs. Verify a kind exists before shipping: node -e 'import("@ast-grep/napi").then(s=>{const r=s.ts.parse("x[i]").root(); const f=(n,k)=>{let c=n.kind()===k?1:0;for(const x of n.children())c+=f(x,k);return c}; console.log(f(r,"subscript_expression"))})' # >0 means the kind is real ✅ Test through the REAL runner from the pi-lens source checkout's repo root — it loads the actual shipped rules from rules/ast-grep-rules/rules. Assert on diagnostic `rule` ids: const res = await runner.run(ctx); // ctx.filePath = temp .ts, cwd = repo (pi-lens source checkout only — not present in the installed package) For pattern/kind/regex-only rules (CLI-identical semantics) `ast-grep scan` is fine. ✅ Before shipping any text/regex detector, FP-scan the codebase: ast-grep scan -r .yml clients tools Real safe variants bite (e.g. ReDoS: (ba+)+ is safe — a mandatory prefix makes the partition unique; flag only a single quantified atom inside the group). ``` ## Scoping to the nearest enclosing X (#1794 F1) `stopBy` controls how far the ancestor/descendant walk searches. It is not a stop CONDITION. An `any:` kind list inside `inside`/`has` decides which nodes the walk is ALLOWED to match — it does not tell the walk where to halt. `stopBy: end` alone searches every ancestor up to the file root, so a guard several scopes out can suppress a finding inside an unrelated closure. To scope a relation to the nearest enclosing function (or any other boundary), give `stopBy` its OWN rule — the same kind list used as the match target — so the walk stops at the first satisfying ancestor and can never escalate past it: ```yaml inside: stopBy: # the BOUNDARY: halts the walk here any: - kind: function_declaration - kind: method_definition - kind: arrow_function - kind: function_expression any: # the MATCH TARGET: same list, different job - kind: function_declaration - kind: method_definition - kind: arrow_function - kind: function_expression has: stopBy: end pattern: $M.has($K) ``` This is the fix for #1794 F1: an earlier `no-non-null-assertion` exclusion used `stopBy: end` with only the `any:` kind list as the match target, so a `.has()`/`.length` guard in an OUTER function suppressed a closure's `!` several scopes in. The same defect shape — `stopBy: end` mistaken for a boundary — shipped twice in the same 2026-08-20 window: once here, and once in `redundant-unsafe-function`'s `# Safety` valve (an unbounded backward comment scan, fixed in `00284bcc`). See `rules/ast-grep-rules/rules/no-non-null-assertion.yml` for the full working rule. ## Matching things a pattern can't express (#305) ``` ❌ A parameter default is NOT a `$X = false` pattern. `pattern: $FLAG = false` parses as an `assignment_expression` (statement context) and never matches a function parameter. Match the PARAM NODE + its child literal instead, capturing the name for reuse: # TS grammar - kind: required_parameter all: - has: { field: pattern, pattern: $FLAG } - has: { any: [ { kind: "true" }, { kind: "false" } ] } # JS grammar (assignment_pattern, with fields left/right) - kind: assignment_pattern all: - has: { field: left, pattern: $FLAG } - has: { field: right, any: [ { kind: "true" }, { kind: "false" } ] } ✅ Metavar consistency works ACROSS sibling clauses of an `all` — a metavar bound in one `has` must match the SAME text everywhere it reappears. Use it to CORRELATE nodes, which is what makes a structural rule precise: all: - has: { stopBy: end, kind: required_parameter, has: { field: pattern, pattern: $FLAG } } - has: { stopBy: end, any: [ { pattern: "if ($FLAG) $$$" }, { pattern: "if (!$FLAG) $$$" } ] } This fires ONLY when the function branches on the SAME param it declared boolean — a boolean default that's never branched on, or a branch on a different var, won't match. ❌ Two `has:` keys in one mapping silently OVERWRITE (YAML: last key wins). For multiple descendant constraints use `all:` with a LIST of `has` entries, never repeated `has:`. ✅ Prefer a high-precision structural guard over an unbounded denylist. Message-chain (Demeter) floods on fluent/promise/builder chains; rather than denylist every fluent method name, REQUIRE the chain's first calls to be accessors (`get*`/`is*`/`has*`) via `constraints` regex — promise/fluent/builder methods aren't accessor-named, so they're excluded by construction. Precision over recall. ```