feat: Support Stylelint custom syntax object (#937)

packages/knip/fixtures/plugins/stylelint2/.stylelintrc.cjs

@@ -0,0 +1,4 @@
+/** @type {import('stylelint').Config} */
+module.exports = {
+  customSyntax: require('postcss-less'),
+};

packages/knip/fixtures/plugins/stylelint2/.stylelintrc.mjs

@@ -0,0 +1,18 @@
+import less from 'postcss-less';
+
+/** @type {import('stylelint').Config} */
+const config = {
+  customSyntax: less,
+  extends: ['stylelint-config-standard'],
+  rules: {
+    'alpha-value-notation': 'number',
+  },
+  overrides: [
+    {
+      files: ['**/*.html'],
+      extends: ['stylelint-config-html/html.js', 'stylelint-config-standard'],
+    },
+  ],
+};
+
+export default config;

packages/knip/fixtures/plugins/stylelint2/myExtendableConfig.js

@@ -0,0 +1,6 @@
+/** @type {import('stylelint').Config} */
+const myExtendableConfig = {
+  plugins: ['stylelint-order'],
+};
+
+module.exports = myExtendableConfig;

packages/knip/fixtures/plugins/stylelint2/package.json

@@ -0,0 +1,14 @@
+{
+  "name": "@fixtures/stylelint2",
+  "version": "*",
+  "devDependencies": {
+    "postcss-less": "*",
+    "postcss-styl": "*",
+    "stylelint": "*",
+    "stylelint-config-recommended": "*",
+    "stylelint-config-standard": "*"
+  },
+  "dependencies": {
+    "stylelint-config-html": "*"
+  }
+}

packages/knip/fixtures/plugins/stylelint2/stylelint.config.js

@@ -0,0 +1,23 @@
+const less = require('postcss-less');
+const stylus = require('postcss-styl');
+
+/** @type {import('stylelint').Config} */
+const config = {
+  customSyntax: less,
+  extends: [require.resolve('stylelint-config-recommended'), './myExtendableConfig'],
+  rules: {
+    'alpha-value-notation': 'number',
+  },
+  overrides: [
+    {
+      files: ['**/*.html'],
+      extends: ['stylelint-config-html/html', 'stylelint-config-standard'],
+    },
+    {
+      files: ['*.styl', '**/*.styl', '*.stylus', '**/*.stylus'],
+      customSyntax: stylus,
+    },
+  ],
+};
+
+module.exports = config;

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

@@ -17,7 +17,8 @@ const config = ['package.json', ...toCosmiconfig('stylelint')];
 const resolve = (config: StyleLintConfig | BaseStyleLintConfig): Input[] => {
   const extend = config.extends ?? [];
   const plugins = config.plugins ?? [];
-  const customSyntax = config.customSyntax ? [config.customSyntax] : [];
+  const customSyntax: string[] = typeof config.customSyntax === 'string' ? [config.customSyntax] : [];
+
   const overrideConfigs = 'overrides' in config ? config.overrides.flatMap(resolve) : [];
   return [...[extend, plugins, customSyntax].flat().map(toDeferResolve), ...overrideConfigs];
 };

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

@@ -1,5 +1,5 @@
 export type BaseStyleLintConfig = {
-  customSyntax?: string;
+  customSyntax?: unknown;
   extends?: string | string[];
   plugins?: string[];
 };

packages/knip/test/plugins/stylelint2.test.ts

@@ -0,0 +1,21 @@
+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/stylelint2');
+
+test('Find dependencies with the stylelint plugin (2)', async () => {
+  const { counters } = await main({
+    ...baseArguments,
+    cwd,
+  });
+
+  assert.deepEqual(counters, {
+    ...baseCounters,
+    processed: 4,
+    total: 4,
+  });
+});