Allow filtering of unused rules, so only matching rules are checked.

docs/rules/no-unused-rules.md

@@ -18,6 +18,13 @@ foo = "1" // Good.  Default entry point.
 bar = "2" // Bad.  Not referenced.
 ```
 
+```peg.js
+// eslint @peggyjs/no-unused-rules: ["error", {filter: "^_"}]
+foo = "foo" // Good.
+bar = "bar" // Doesn't match the filter
+_boo = "boo" // Bad.  Not referenced, and matches the filter.
+```
+
 :+1: Examples of **correct** code for this rule:
 
 ```peg.js
@@ -27,6 +34,19 @@ foo = bar / baz
 bar = "2"
 ```
 
+```peg.js
+// eslint @peggyjs/no-unused-rules: ["error", {filter: "^_"}]
+foo = _boo
+bar = "bar" // Doesn't match the filter
+_boo = "boo"
+```
+
+### Options
+
+The first option can be an object with the key "filter".  The filter is treated
+as a regular expression; only rules that match this expression will cause
+errors if they are unused.
+
 ## 🔎 Implementation
 
 - [Rule source](../../src/rules/no-unused-rules.ts)

src/rules/no-unused-rules.ts

@@ -14,13 +14,26 @@ const rule: Rule.RuleModule = {
       unused: "Rule '{{ name }}' is never used.",
       unusedImport: "Library import '{{ name }}' is never used.",
     },
-    schema: [],
+    schema: [
+      {
+        type: "object",
+        properties: {
+          filter: {
+            type: "string",
+          },
+        },
+        additionalProperties: false,
+      },
+    ],
   },
   create(context: Rule.RuleContext): Rule.RuleListener {
     const imports = new Map<string, visitor.AST.Name>();
     const rules = new Map<string, visitor.AST.Rule>();
     const refs = new Set<string>();
     const importRefs = new Set<string>();
+    const filter = context.options[0]?.filter;
+    const match = filter ? new RegExp(filter) : undefined;
+
     return makeListener({
       import_binding(node: visitor.AST.ImportBinding): void {
         imports.set(node.binding.id.value, node.binding.id);
@@ -52,7 +65,11 @@ const rule: Rule.RuleModule = {
           rules.delete(name);
           imports.delete(name);
         }
+
         for (const [name, r] of rules) {
+          if (match && !match.test(name)) {
+            continue;
+          }
           context.report({
             node: n(r),
             messageId: "unused",

test/rules/no-unused-rules.test.js

@@ -40,6 +40,13 @@ foo = bar`,
 import {"foo" as bar} from 'bar'
 foo = bar`,
     },
+    {
+      options: [{ filter: "^_" }],
+      code: `
+foo = _bar
+baz = "boo"
+_bar = "bar"`,
+    },
   ],
   invalid: [
     {
@@ -76,5 +83,14 @@ foo = bar`,
         foo = "1"`,
       errors: [{ messageId: "unusedImport" }],
     },
+    {
+      options: [{ filter: "^_" }],
+      code: `
+foo = _bar
+baz = "boo"
+_bar = "bar"
+_boo = "boo"`,
+      errors: [{ messageId: "unused" }],
+    },
   ],
 });