feat(plugins): add plugin for xo config

Adds a plugin for [xo](https://github.com/xojs/xo#config). Essentially its an opinionated wrapper
around eslint with a bunch of bundled rules and plugins, and the config object is just the eslint
config object with some extra options. So to keep it consistent with the eslint plugin and avoid
duplication I've just used the eslint helper to parse out any additional eslint plugins that have
been added in the xo config.

packages/knip/fixtures/plugins/xo/.xo-config.js

@@ -0,0 +1,9 @@
+module.exports = {
+  space: true,
+  prettier: true,
+  plugins: ['unused-imports'],
+  parserOptions: {emitDecoratorMetadata: true},
+  rules: {
+    'func-names': ['error', 'always'],
+  },
+};

packages/knip/fixtures/plugins/xo/package.json

@@ -0,0 +1,10 @@
+{
+  "name": "@fixtures/xo",
+  "devDependencies": {
+    "xo": "^0.24.0"
+  },
+  "xo": {
+    "space": true,
+    "plugins": ["eslint-comments"]
+  }
+}

packages/knip/fixtures/plugins/xo/xo-config.cjs

@@ -0,0 +1,8 @@
+const mySharedConfig = require('glob');
+
+module.exports = {
+  ...mySharedConfig,
+  rules: {
+    'func-names': 'off',
+  },
+};

packages/knip/schema.json

@@ -507,6 +507,10 @@
           "title": "wrangler plugin configuration (https://github.com/webpro/knip/blob/main/src/plugins/wrangler/README.md)",
           "$ref": "#/definitions/plugin"
         },
+        "xo": {
+          "title": "xo plugin configuration (https://github.com/webpro/knip/blob/main/src/plugins/xo/README.md)",
+          "$ref": "#/definitions/plugin"
+        },
         "yorkie": {
           "title": "yorkie plugin configuration (https://github.com/webpro/knip/blob/main/src/plugins/yorkie/README.md)",
           "$ref": "#/definitions/plugin"

packages/knip/src/ConfigurationValidator.ts

@@ -77,6 +77,7 @@ export const pluginSchema = z.union([
 ]);
 
 const pluginsSchema = z.object({
+  xo: pluginSchema,
   astro: pluginSchema,
   angular: pluginSchema,
   ava: pluginSchema,

packages/knip/src/plugins/index.ts

@@ -58,3 +58,4 @@ export { default as webpack } from './webpack/index.js';
 export { default as wireit } from './wireit/index.js';
 export { default as wrangler } from './wrangler/index.js';
 export { default as yorkie } from './yorkie/index.js';
+export { default as xo } from './xo/index.js';

packages/knip/src/plugins/xo/index.ts

@@ -0,0 +1,34 @@
+import type { EnablerPatterns } from '#p/types/config.js';
+import type { IsPluginEnabled, Plugin, ResolveConfig } from '#p/types/plugins.js';
+import { hasDependency } from '#p/util/plugin.js';
+import { getDependenciesDeep } from '../eslint/helpers.js';
+import type { XOConfig } from './types.js';
+
+// link to xo docs: https://github.com/xojs/xo#config
+
+const title = 'xo';
+
+const enablers: EnablerPatterns = ['xo'];
+
+const isEnabled: IsPluginEnabled = ({ dependencies, config }) =>
+  hasDependency(dependencies, enablers) || 'xo' in config;
+
+const packageJsonPath = 'xo';
+const config: string[] = ['{.,}xo-config.{js,cjs,json,}', 'package.json'];
+
+const entry: string[] = ['{.,}xo-config.{js,cjs}'];
+
+const resolveConfig: ResolveConfig<XOConfig> = async (config, options) => {
+  const dependencies = await getDependenciesDeep(config, options);
+  return [...dependencies];
+};
+
+export default {
+  title,
+  enablers,
+  isEnabled,
+  packageJsonPath,
+  entry,
+  config,
+  resolveConfig,
+} satisfies Plugin;

packages/knip/src/plugins/xo/types.ts

@@ -0,0 +1,15 @@
+import type { ESLintConfig } from '../eslint/types.js';
+
+export type XOConfig = ESLintConfig & {
+  envs?: string[] | undefined;
+  globals?: string[] | undefined;
+  parser?: string | undefined;
+  plugins?: string[];
+  ignores?: string[] | undefined;
+  nodeVersion?: string | boolean | undefined;
+  prettier?: boolean | undefined;
+  printConfig?: string | undefined;
+  semicolon?: boolean | undefined;
+  space?: boolean | number | undefined;
+  webpack?: boolean | object | undefined;
+};

packages/knip/test/plugins/xo.test.ts

@@ -0,0 +1,27 @@
+import { test } from 'bun:test';
+import assert from 'node:assert/strict';
+import { main } from '../../src/index.js';
+import { resolve } from '../../src/util/path.js';
+import baseArguments from '../helpers/baseArguments.js';
+import baseCounters from '../helpers/baseCounters.js';
+
+const cwd = resolve('fixtures/plugins/xo');
+
+test('Find dependencies with the xo plugin', async () => {
+  const { issues, counters } = await main({
+    ...baseArguments,
+    cwd,
+  });
+
+  assert(issues.unlisted['.xo-config.js']['eslint-plugin-unused-imports']);
+  assert(issues.unlisted['xo-config.cjs']['glob']);
+  assert(issues.unlisted['package.json']['eslint-plugin-eslint-comments']);
+
+  assert.deepEqual(counters, {
+    ...baseCounters,
+    devDependencies: 1,
+    processed: 2,
+    unlisted: 3,
+    total: 2,
+  });
+});