Support new biome config extends definition (#1177)

Author: Thomascogez

packages/knip/biome.json

@@ -2,7 +2,12 @@
   "$schema": "https://biomejs.dev/schemas/1.8.3/schema.json",
   "extends": ["../../biome.json"],
   "files": {
-    "ignore": ["**/dist", "package.json", "vendor/bash-parser/index.js"]
+    "ignore": [
+      "**/dist",
+      "package.json",
+      "vendor/bash-parser/index.js",
+      "fixtures/plugins/biome-workspace/packages/stub"
+    ]
   },
   "formatter": {
     "ignore": ["ignore-exports-used-in-file-alias-exclude/more.ts"]

packages/knip/fixtures/plugins/biome-workspace/biome.jsonc

@@ -0,0 +1,4 @@
+{
+  "$schema": "https://biomejs.dev/schemas/2.1.1/schema.json",
+  "extends": ["@org/shared-configs/biome"]
+}

packages/knip/fixtures/plugins/biome-workspace/index.ts

@@ -0,0 +1,16 @@
+{
+  "name": "@plugins/biome-workspace",
+  "module": "index.ts",
+  "type": "module",
+  "private": true,
+  "workspaces": [
+    "packages/*"
+  ],
+  "scripts": {
+    "lint": "biome lint ."
+  },
+  "devDependencies": {
+    "@biomejs/biome": "*",
+    "@org/shared-configs": "*"
+  }
+}

packages/knip/fixtures/plugins/biome-workspace/node_modules/@biomejs/biome/package.json

@@ -0,0 +1,4 @@
+{
+  "$schema": "https://biomejs.dev/schemas/2.1.1/schema.json",
+  "extends": "//"
+}

packages/knip/fixtures/plugins/biome-workspace/node_modules/@org/shared-configs/biome.json

@@ -0,0 +1,3 @@
+{
+    "name": "stub-package"
+}

packages/knip/fixtures/plugins/biome-workspace/node_modules/@org/shared-configs/package.json

@@ -1,5 +1,7 @@
 import type { IsPluginEnabled, Plugin, PluginOptions, ResolveConfig } from '../../types/config.js';
+import { arrayify } from '../../util/array.js';
 import { type Input, toConfig } from '../../util/input.js';
+import { join } from '../../util/path.js';
 import { hasDependency } from '../../util/plugin.js';
 import type { BiomeConfig } from './types.js';
 
@@ -11,12 +13,20 @@ const isEnabled: IsPluginEnabled = ({ dependencies }) => hasDependency(dependenc
 
 const config: string[] = ['biome.json', 'biome.jsonc'];
 
+const isRootConfigReference = (specifier: string) => specifier === '//';
+
 const resolveExtends = (extendsArray: string[], options: PluginOptions): Input[] => {
-  return extendsArray.map(specifier => toConfig('biome', specifier, { containingFilePath: options.configFilePath }));
+  return extendsArray.map(specifier => {
+    if (isRootConfigReference(specifier)) {
+      return toConfig('biome', join(options.rootCwd, 'biome'), { containingFilePath: options.configFilePath });
+    }
+
+    return toConfig('biome', specifier, { containingFilePath: options.configFilePath });
+  });
 };
 
 const resolveConfig: ResolveConfig<BiomeConfig> = (config, options) => {
-  return [...resolveExtends(config.extends || [], options)];
+  return [...resolveExtends(arrayify(config.extends), options)];
 };
 
 export default {