(PostCSS plugin): Add postcss to used dependencies when using PostC…

…SS with Tailwind CSS (#764)

Co-authored-by: Lars Kappert <[email protected]>

packages/knip/fixtures/plugins/postcss-tailwindcss/package.json

@@ -0,0 +1,6 @@
+{
+  "name": "@fixtures/postcss-tailwindcss",
+  "devDependencies": {
+    "postcss": "*"
+  }
+}

packages/knip/fixtures/plugins/postcss-tailwindcss/postcss.config.js

@@ -0,0 +1,5 @@
+module.exports = {
+  plugins: {
+    tailwindcss: {},
+  },
+};

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

@@ -18,13 +18,16 @@ const config = [
 ];
 
 const resolveConfig: ResolveConfig<PostCSSConfig> = config => {
-  return config.plugins
+  const plugins = config.plugins
     ? (Array.isArray(config.plugins) ? config.plugins : Object.keys(config.plugins)).flatMap(plugin => {
         if (typeof plugin === 'string') return plugin;
         if (Array.isArray(plugin) && typeof plugin[0] === 'string') return plugin[0];
         return [];
       })
     : [];
+
+  // Because postcss is not included in peerDependencies of tailwindcss
+  return plugins.includes('tailwindcss') ? [...plugins, 'postcss'] : plugins;
 };
 
 export default {

packages/knip/test/plugins/postcss-tailwindcss.test.ts

@@ -0,0 +1,24 @@
+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/postcss-tailwindcss');
+
+test('Find dependencies with the PostCSS plugin (with tailwindcss)', async () => {
+  const { issues, counters } = await main({
+    ...baseArguments,
+    cwd,
+  });
+
+  assert(issues.unlisted['postcss.config.js']['tailwindcss']);
+
+  assert.deepEqual(counters, {
+    ...baseCounters,
+    unlisted: 1,
+    processed: 1,
+    total: 1,
+  });
+});