Change the mount method to accept a function for defining engine routes

This change updates the mount API method to accept a function similar
to how the route API currently works so that engine routes can be defined
in the host application's router.js file.

Some Ember applications utilize in-repo ember-engines strictly for their
lazy loading and code organization features and not for their isolation features.

In this context it makes more sense to define engine routes in the host application's
router.js file.

packages/@ember/-internals/routing/lib/system/dsl.ts

@@ -31,7 +31,9 @@ export interface DSL {
   route(name: string, options: RouteOptions, callback: DSLCallback): void;
 
   mount(name: string): void;
+  mount(name: string, callback: DSLCallback): void;
   mount(name: string, options: MountOptions): void;
+  mount(name: string, options: MountOptions, callback: DSLCallback): void;
 }
 
 function isCallback(value?: RouteOptions | DSLCallback): value is DSLCallback {
@@ -173,9 +175,32 @@ export default class DSLImpl implements DSL {
     };
   }
 
-  mount(_name: string, options: MountOptions = {}): void {
-    let engineRouteMap = this.options.resolveRouteMap(_name);
+  mount(_name: string): void;
+  mount(_name: string, mountCallback: DSLCallback): void;
+  mount(_name: string, options: MountOptions): void;
+  mount(_name: string, options: MountOptions, mountCallback: DSLCallback): void;
+  mount(_name: string, _options?: MountOptions | DSLCallback, _mountCallback?: DSLCallback): void {
+    let engineRouteMap = null;
     let name = _name;
+    let options: MountOptions;
+    let mountCallback: Option<DSLCallback> = null;
+
+    if (isCallback(_options)) {
+      assert('Unexpected arguments', arguments.length === 2);
+      options = {};
+      mountCallback = _options;
+    } else if (isCallback(_mountCallback)) {
+      assert('Unexpected arguments', arguments.length === 3);
+      assert('Unexpected arguments', isOptions(_options));
+      options = _options;
+      mountCallback = _mountCallback;
+    } else {
+      options = _options || {};
+    }
+
+    if (!mountCallback) {
+      engineRouteMap = this.options.resolveRouteMap(name);
+    }
 
     if (options.as) {
       name = options.as;
@@ -198,7 +223,7 @@ export default class DSLImpl implements DSL {
 
     let callback;
     let dummyErrorRoute = `/_unused_dummy_error_path_route_${name}/:error`;
-    if (engineRouteMap) {
+    if (engineRouteMap || mountCallback) {
       let shouldResetEngineInfo = false;
       let oldEngineInfo = this.options.engineInfo;
       if (oldEngineInfo) {
@@ -212,7 +237,11 @@ export default class DSLImpl implements DSL {
       createRoute(childDSL, 'loading');
       createRoute(childDSL, 'error', { path: dummyErrorRoute });
 
-      engineRouteMap.class.call(childDSL);
+      if (mountCallback) {
+        mountCallback.call(childDSL);
+      } else if (engineRouteMap) {
+        engineRouteMap.class.call(childDSL);
+      }
 
       callback = childDSL.generate();
 

packages/@ember/-internals/routing/tests/system/dsl_test.js

@@ -223,6 +223,88 @@ moduleFor(
       );
     }
 
+    ['@test should allow engine routes to be defined instead of options when mounting an engine'](
+      assert
+    ) {
+      assert.expect(4);
+
+      Router = Router.map(function () {
+        this.route('bleep', function () {
+          this.route('bloop', function () {
+            this.mount('chat', function chatRoutes() {
+              this.route('foobar');
+            });
+          });
+        });
+      });
+
+      let engineInstance = buildOwner({
+        ownerOptions: { routable: true },
+      });
+
+      let router = Router.create();
+      setOwner(router, engineInstance);
+      router._initRouterJs();
+
+      assert.ok(
+        router._routerMicrolib.recognizer.names['bleep'],
+        'parent name was used as base of nested routes'
+      );
+      assert.ok(
+        router._routerMicrolib.recognizer.names['bleep.bloop'],
+        'parent name was used as base of nested routes'
+      );
+      assert.ok(
+        router._routerMicrolib.recognizer.names['bleep.bloop.chat'],
+        'parent name was used as base of mounted engine'
+      );
+      assert.ok(
+        router._routerMicrolib.recognizer.names['bleep.bloop.chat.foobar'],
+        'engine route was passed in from mount'
+      );
+    }
+
+    ['@test should allow engine routes to be defined in addition to options when mounting an engine'](
+      assert
+    ) {
+      assert.expect(4);
+
+      Router = Router.map(function () {
+        this.route('bleep', function () {
+          this.route('bloop', function () {
+            this.mount('chat', {}, function chatRoutes() {
+              this.route('foobar');
+            });
+          });
+        });
+      });
+
+      let engineInstance = buildOwner({
+        ownerOptions: { routable: true },
+      });
+
+      let router = Router.create();
+      setOwner(router, engineInstance);
+      router._initRouterJs();
+
+      assert.ok(
+        router._routerMicrolib.recognizer.names['bleep'],
+        'parent name was used as base of nested routes'
+      );
+      assert.ok(
+        router._routerMicrolib.recognizer.names['bleep.bloop'],
+        'parent name was used as base of nested routes'
+      );
+      assert.ok(
+        router._routerMicrolib.recognizer.names['bleep.bloop.chat'],
+        'parent name was used as base of mounted engine'
+      );
+      assert.ok(
+        router._routerMicrolib.recognizer.names['bleep.bloop.chat.foobar'],
+        'engine route was passed in from mount'
+      );
+    }
+
     ['@test should allow mounting of engines at a custom path'](assert) {
       assert.expect(1);