(PostCSS plugin): Add postcss to used dependencies when using PostCSS with Tailwind CSS

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/WorkspaceWorker.ts

@@ -289,6 +289,10 @@ export class WorkspaceWorker {
 
         for (const configFilePath of configFilePaths) {
           const opts = { ...options, configFileDir: dirname(configFilePath), configFileName: basename(configFilePath) };
+
+          // biome-ignore lint/suspicious/noExplicitAny: raw incoming user data
+          let config: any;
+
           if (hasResolveEntryPaths || shouldRunConfigResolver) {
             const isManifest = basename(configFilePath) === 'package.json';
             const fd = isManifest ? undefined : this.cache.getFileDescriptor(configFilePath);
@@ -299,7 +303,7 @@ export class WorkspaceWorker {
               if (fd.meta.data.resolveConfig)
                 for (const id of fd.meta.data.resolveConfig) addDependency(id, configFilePath);
             } else {
-              const config = await loadConfigForPlugin(configFilePath, plugin, opts, pluginName);
+              config = await loadConfigForPlugin(configFilePath, plugin, opts, pluginName);
               const data: CacheItem = {};
               if (config) {
                 if (hasResolveEntryPaths) {
@@ -316,16 +320,16 @@ export class WorkspaceWorker {
               }
             }
           }
+
+          if (hasResolve) {
+            const dependencies = (await plugin.resolve?.(config, options)) ?? [];
+            for (const id of dependencies) addDependency(id, join(cwd, 'package.json'));
+          }
         }
 
         const finalEntryPaths = getFinalEntryPaths(plugin, options, configEntryPaths);
         for (const id of finalEntryPaths) addDependency(id);
 
-        if (hasResolve) {
-          const dependencies = (await plugin.resolve?.(options)) ?? [];
-          for (const id of dependencies) addDependency(id, join(cwd, 'package.json'));
-        }
-
         debugLogArray([name, plugin.title], 'dependencies', pluginDependencies);
       }
     }

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

@@ -13,7 +13,7 @@ const entry = ['astro.config.{js,cjs,mjs,ts}', 'src/content/config.ts'];
 
 const production = ['src/pages/**/*.{astro,mdx,js,ts}', 'src/content/**/*.mdx'];
 
-const resolve: Resolve = options => {
+const resolve: Resolve = (_, options) => {
   const { isProduction, manifest } = options;
   const dependencies = [];
 

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

@@ -1,4 +1,4 @@
-import type { IsPluginEnabled, Plugin, ResolveConfig } from '#p/types/plugins.js';
+import type { IsPluginEnabled, Plugin, Resolve, ResolveConfig } from '#p/types/plugins.js';
 import { hasDependency, toLilconfig } from '#p/util/plugin.js';
 import type { PostCSSConfig } from './types.js';
 
@@ -27,10 +27,19 @@ const resolveConfig: ResolveConfig<PostCSSConfig> = config => {
     : [];
 };
 
+const resolve: Resolve<PostCSSConfig> = async (options, config) => {
+  for (const plugin of await resolveConfig(options, config)) {
+    // Because postcss is not included in peerDependencies of tailwindcss
+    if (plugin === 'tailwindcss') return ['postcss'];
+  }
+  return [];
+};
+
 export default {
   title,
   enablers,
   isEnabled,
   config,
   resolveConfig,
+  resolve,
 } satisfies Plugin;

packages/knip/src/types/plugins.ts

@@ -33,7 +33,8 @@ export type ResolveEntryPaths<T = any> = (config: T, options: PluginOptions) =>
 // biome-ignore lint/suspicious/noExplicitAny: TODO
 export type ResolveConfig<T = any> = (config: T, options: PluginOptions) => Promise<string[]> | string[];
 
-export type Resolve = (options: PluginOptions) => Promise<string[]> | string[];
+// biome-ignore lint/suspicious/noExplicitAny: TODO
+export type Resolve<T = any> = (config: T, options: PluginOptions) => Promise<string[]> | string[];
 
 export interface Plugin {
   title: string;

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,
+  });
+});