perf: fix lazy loading of core rules

lib/config/flat-config-array.js

@@ -14,7 +14,6 @@ const { flatConfigSchema } = require("./flat-config-schema");
 const { RuleValidator } = require("./rule-validator");
 const { defaultConfig } = require("./default-config");
 const recommendedConfig = require("../../conf/eslint-recommended");
-const allConfig = require("../../conf/eslint-all");
 
 //-----------------------------------------------------------------------------
 // Helpers
@@ -79,7 +78,13 @@ class FlatConfigArray extends ConfigArray {
         }
 
         if (config === "eslint:all") {
-            return allConfig;
+
+            /*
+             * Load `eslint-all.js` here instead of at the top level to avoid loading all rule modules
+             * when it isn't necessary. `eslint-all.js` reads `meta` of rule objects to filter out deprecated ones,
+             * so requiring `eslint-all.js` module loads all rule modules as a consequence.
+             */
+            return require("../../conf/eslint-all");
         }
 
         return config;

package.json

@@ -122,6 +122,7 @@
     "node-polyfill-webpack-plugin": "^1.0.3",
     "npm-license": "^0.3.3",
     "nyc": "^15.0.1",
+    "pirates": "^4.0.5",
     "progress": "^2.0.3",
     "proxyquire": "^2.0.1",
     "puppeteer": "^9.1.1",

tests/_utils/test-lazy-loading-rules.js

@@ -0,0 +1,66 @@
+/**
+ * @fileoverview Tests lazy-loading of core rules
+ * @author Milos Djermanovic
+ */
+
+/*
+ * This module should be run as a child process, with `fork()`,
+ * because it is important to run this test with a separate, clean Node process
+ * in order to add hooks before any of the ESLint modules is loaded.
+ */
+
+"use strict";
+
+const path = require("path");
+const assert = require("assert");
+const { addHook } = require("pirates");
+
+const {
+    dir: rulesDirectoryPath,
+    name: rulesDirectoryIndexFilename
+} = path.parse(require.resolve("../../lib/rules"));
+
+// Show full stack trace. The default 10 is usually not enough to find the root cause of this problem.
+Error.stackTraceLimit = Infinity;
+
+const [cwd, pattern, usedRulesCommaSeparated] = process.argv.slice(2);
+
+assert.ok(cwd, "cwd argument isn't provided");
+assert.ok(pattern, "pattern argument isn't provided");
+assert.ok(usedRulesCommaSeparated, "used rules argument isn't provided");
+
+const usedRules = usedRulesCommaSeparated.split(",");
+
+// `require()` hook
+addHook(
+    (_code, filename) => {
+        throw new Error(`Unexpected attempt to load unused rule ${filename}`);
+    },
+    {
+
+        // returns `true` if the hook (the function passed in as the first argument) should be called for this filename
+        matcher(filename) {
+            const { dir, name } = path.parse(filename);
+
+            if (dir === rulesDirectoryPath && ![rulesDirectoryIndexFilename, ...usedRules].includes(name)) {
+                return true;
+            }
+
+            return false;
+        }
+
+    }
+);
+
+/*
+ * Everything related to loading any ESLint modules should be in this IIFE
+ */
+(async () => {
+    const { ESLint } = require("../..");
+    const eslint = new ESLint({ cwd });
+
+    await eslint.lintFiles([pattern]);
+})().catch(({ message, stack }) => {
+    process.send({ message, stack });
+    process.exit(1);
+});

tests/fixtures/lazy-loading-rules/.eslintrc.js

@@ -0,0 +1,6 @@
+module.exports = {
+    root: true,
+    rules: {
+        semi: 2
+    }
+};

tests/fixtures/lazy-loading-rules/foo.js

@@ -0,0 +1 @@
+/* content is not necessary */

tests/lib/eslint/eslint.js

@@ -27,6 +27,7 @@ const {
 const hash = require("../../../lib/cli-engine/hash");
 const { unIndent, createCustomTeardown } = require("../../_utils");
 const coreRules = require("../../../lib/rules");
+const childProcess = require("child_process");
 
 //------------------------------------------------------------------------------
 // Tests
@@ -6694,4 +6695,45 @@ describe("ESLint", () => {
             });
         });
     });
+
+    describe("loading rules", () => {
+        it("should not load unused core rules", done => {
+            let calledDone = false;
+
+            const cwd = getFixturePath("lazy-loading-rules");
+            const pattern = "foo.js";
+            const usedRules = ["semi"];
+
+            const forkedProcess = childProcess.fork(
+                path.join(__dirname, "../../_utils/test-lazy-loading-rules.js"),
+                [cwd, pattern, String(usedRules)]
+            );
+
+            // this is an error message
+            forkedProcess.on("message", ({ message, stack }) => {
+                if (calledDone) {
+                    return;
+                }
+                calledDone = true;
+
+                const error = new Error(message);
+
+                error.stack = stack;
+                done(error);
+            });
+
+            forkedProcess.on("exit", exitCode => {
+                if (calledDone) {
+                    return;
+                }
+                calledDone = true;
+
+                if (exitCode === 0) {
+                    done();
+                } else {
+                    done(new Error("Forked process exited with a non-zero exit code"));
+                }
+            });
+        });
+    });
 });