fix(plugin-eslint): use LegacyESLint if ESLINT_USE_FLAT_CONFIG=false in v9

Author: matejchalk

packages/plugin-eslint/src/lib/meta/versions/index.ts

@@ -5,6 +5,7 @@ import type { ConfigFormat } from './formats';
 import { loadRulesForLegacyConfig } from './legacy';
 
 export { detectConfigVersion } from './detect';
+export type { ConfigFormat } from './formats';
 
 export function selectRulesLoader(
   version: ConfigFormat,

packages/plugin-eslint/src/lib/meta/versions/legacy.ts

@@ -8,7 +8,7 @@ export async function loadRulesForLegacyConfig({
   eslintrc,
   patterns,
 }: ESLintTarget): Promise<RuleData[]> {
-  const eslint = setupESLint(eslintrc);
+  const eslint = await setupESLint(eslintrc);
 
   const configs = await toArray(patterns).reduce(
     async (acc, pattern) => [

packages/plugin-eslint/src/lib/runner/lint.ts

@@ -15,7 +15,8 @@ export async function lint({
   patterns,
 }: ESLintTarget): Promise<LinterOutput> {
   const results = await executeLint({ eslintrc, patterns });
-  const ruleOptionsPerFile = await loadRuleOptionsPerFile(eslintrc, results);
+  const eslint = await setupESLint(eslintrc);
+  const ruleOptionsPerFile = await loadRuleOptionsPerFile(eslint, results);
   return { results, ruleOptionsPerFile };
 }
 
@@ -45,11 +46,9 @@ async function executeLint({
 }
 
 function loadRuleOptionsPerFile(
-  eslintrc: ESLintTarget['eslintrc'],
+  eslint: ESLint,
   results: ESLint.LintResult[],
 ): Promise<RuleOptionsPerFile> {
-  const eslint = setupESLint(eslintrc);
-
   return results.reduce(async (acc, { filePath, messages }) => {
     const filesMap = await acc;
     const config = (await eslint.calculateConfigForFile(

packages/plugin-eslint/src/lib/setup.ts

@@ -1,9 +1,19 @@
 import { ESLint } from 'eslint';
 import type { ESLintTarget } from './config';
 
-export function setupESLint(eslintrc: ESLintTarget['eslintrc']) {
-  return new ESLint({
+export async function setupESLint(eslintrc: ESLintTarget['eslintrc']) {
+  const eslintConstructor = await loadESLint();
+  return new eslintConstructor({
     overrideConfigFile: eslintrc,
     errorOnUnmatchedPattern: false,
   });
 }
+
+async function loadESLint() {
+  const eslint = await import('eslint');
+  // loadESLint added to public API in v9, selects ESLint or LegacyESLint based on environment
+  if ('loadESLint' in eslint && typeof eslint.loadESLint === 'function') {
+    return (await eslint.loadESLint()) as typeof ESLint;
+  }
+  return ESLint;
+}