chore: Adopt no-unused-vars-experimental for eslint to prevent incorrect linting errors

.eslintrc.js

@@ -33,6 +33,7 @@ module.exports = {
     // Disabled checks
     '@typescript-eslint/no-explicit-any': 0,
     '@typescript-eslint/no-use-before-define': 0,
+    '@typescript-eslint/no-unused-vars': 0,
 
     // Required checks
     'indent': ['error', 2],
@@ -46,5 +47,12 @@ module.exports = {
         'allowHigherOrderFunctions': true
       }
     ],
-  }
+    '@typescript-eslint/no-unused-vars-experimental': 2,
+  },
+  // Required by the @typescript-eslint/no-unused-vars-experimental rule.
+  // We use a separate tsconfig file for linting to reduce the time complexity of the operation.
+  // See github.com/typescript-eslint/typescript-eslint/issues/1320
+  parserOptions: {
+    project: './tsconfig.eslint.json',
+  },
 };

src/database.d.ts

@@ -1631,11 +1631,9 @@ export namespace admin.database {
    */
   interface ThenableReference extends admin.database.Reference, Promise<admin.database.Reference> { }
 
-  /* eslint-disable-next-line @typescript-eslint/no-unused-vars */
   function enableLogging(logger?: boolean | ((message: string) => any), persistent?: boolean): any;
 }
 
-/* eslint-disable @typescript-eslint/no-unused-vars */
 export namespace admin.database.ServerValue {
 
   /**

src/instance-id.d.ts

@@ -1,6 +1,5 @@
 import * as _admin from './index.d';
 
-/* eslint-disable-next-line @typescript-eslint/no-unused-vars */
 export namespace admin.instanceId {
   /**
    * Gets the {@link InstanceId `InstanceId`} service for the

src/machine-learning/machine-learning.ts

@@ -285,8 +285,7 @@ export class Model {
     return false;
   }
 
-  /* eslint-disable-next-line @typescript-eslint/no-unused-vars */
-  public waitForUnlocked(maxTimeSeconds?: number): Promise<void> {
+  public waitForUnlocked(_maxTimeSeconds?: number): Promise<void> {
     // Backend does not currently return locked models.
     // This will likely change in future.
     return Promise.resolve();

src/remote-config.d.ts

@@ -1,6 +1,5 @@
 import * as _admin from './index.d';
 
-/* eslint-disable-next-line @typescript-eslint/no-unused-vars */
 export namespace admin.remoteConfig {
 
   /**

test/integration/database.spec.ts

@@ -173,8 +173,6 @@ describe('admin.database', () => {
 // Check for type compilation. This method is not invoked by any tests. But it
 // will trigger a TS compilation failure if the RTDB typings were not loaded
 // correctly. (Marked as export to avoid compilation warning.)
-//
-// eslint-disable-next-line @typescript-eslint/no-unused-vars
 export function addValueEventListener(
   db: admin.database.Database,
   callback: (s: admin.database.DataSnapshot | null) => any): void {

test/resources/mocks.ts

@@ -221,7 +221,7 @@ export function generateSessionCookie(overrides?: object, expiresIn?: number): s
 
 export function firebaseServiceFactory(
   firebaseApp: FirebaseApp,
-  extendApp?: (props: object) => void, // eslint-disable-line @typescript-eslint/no-unused-vars
+  _extendApp?: (props: object) => void,
 ): FirebaseServiceInterface {
   const result = {
     app: firebaseApp,

tsconfig.eslint.json

@@ -0,0 +1,12 @@
+// Used for @typescript-eslint/no-unused-vars-experimental to glob the entire
+// source. An alternative is createDefaultProgram: true but it has exponential
+// complexity. See: github.com/typescript-eslint/typescript-eslint/issues/967
+{
+  "compilerOptions": {
+    "strict": true,
+  },
+  "include": [
+    "src/**/*.ts",
+    "test/**/*.ts",
+  ],
+}