feat: emit warning when .eslintignore file is detected

lib/eslint/eslint.js

@@ -613,6 +613,13 @@ class ESLint {
             });
         }
 
+        // Check for the .eslintignore file, and warn if it's present.
+        if (existsSync(path.resolve(processedOptions.cwd, ".eslintignore"))) {
+            process.emitWarning(
+                "The \".eslintignore\" file is no longer supported. For an upgrade, switch to using the \"ignores\" property in a config object. See https://eslint.org/docs/latest/use/configure/migration-guide#ignoring-files for details.",
+                "ESLintIgnoreWarning"
+            );
+        }
     }
 
     /**

tests/bin/eslint.js

@@ -123,7 +123,7 @@ describe("bin/eslint.js", () => {
             const exitCodePromise = assertExitCode(child, 0);
             const stdoutPromise = getOutput(child).then(output => {
                 assert.strictEqual(output.stdout.trim(), expectedOutput);
-                assert.strictEqual(output.stderr, "");
+                assert.match(output.stderr, /ESLintIgnoreWarning: The ".eslintignore" file is no longer supported./u);
             });
 
             child.stdin.write("var foo = bar;;\n");

tests/lib/eslint/eslint.js

@@ -4603,6 +4603,20 @@ describe("ESLint", () => {
 
             await assert.rejects(() => eslint.isPathIgnored(null), /'filePath' must be a non-empty string/u);
         });
+
+        it("should warn if .eslintignore file is present", async () => {
+            const cwd = getFixturePath("ignored-paths");
+            const processStub = sinon.stub(process, "emitWarning");
+
+            // eslint-disable-next-line no-new -- for testing purpose only
+            new ESLint({ cwd });
+
+            assert.strictEqual(processStub.callCount, 1, "calls `process.emitWarning()` once");
+            assert.strictEqual(processStub.getCall(0).args[0], "The \".eslintignore\" file is no longer supported. For an upgrade, switch to using the \"ignores\" property in a config object. See https://eslint.org/docs/latest/use/configure/migration-guide#ignoring-files for details.");
+            assert.strictEqual(processStub.getCall(0).args[1], "ESLintIgnoreWarning");
+
+            processStub.restore();
+        });
     });
 
     describe("loadFormatter()", () => {