RFC to align the mount and route APIs

This RFC describes the motivation and implementation for
aligning the mount and route API signatures.

text/0722-align-mount-route-apis.md

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+---
+Stage: Accepted
+Start Date: 2021-02-12
+Release Date: Unreleased
+Release Versions:
+  ember-source: vX.Y.Z
+  ember-data: vX.Y.Z
+Relevant Team(s): Ember.js
+RFC PR: https://github.com/emberjs/rfcs/pull/722
+---
+
+<!---
+Directions for above:
+
+Stage: Leave as is
+Start Date: Fill in with today's date, YYYY-MM-DD
+Release Date: Leave as is
+Release Versions: Leave as is
+Relevant Team(s): Fill this in with the [team(s)](README.md#relevant-teams) to which this RFC applies
+RFC PR: Fill this in with the URL for the Proposal RFC PR
+-->
+
+# Align mount and route APIs
+
+## Summary
+
+Update the Ember `mount` API to accept a function argument which defines the Engine’s routes similar to how the `route` API works today.
+
+## Motivation
+
+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 the isolation actually causes some maintenance overhead and prevents build optimization.
+
+If isolation is not a requirement and all engines in an application are in-repo it makes more sense to define all of the routes at the application level in the router.js.
+
+## Detailed design
+
+The required change will be made to the `mount` method in [packages/@ember/-internals/routing/lib/system/dsl.ts ](https://github.com/emberjs/ember.js/blob/b35106e4d8471e396eb1ee5a5044b6bbe72fa069/packages/%40ember/-internals/routing/lib/system/dsl.ts#L176). A third argument will be added matching the method signature of the `route` method.
+
+```js
+mount(name, options, mountCallback) {...}
+```
+
+If the argument is present and it is a function the route-map will not be resolved and the callback will be used in place of the resolved route-map.
+
+```js
+let engineRouteMap = null;
+
+if (!mountCallback) {
+  engineRouteMap = this.options.resolveRouteMap(name);
+}
+
+if (mountCallback) {
+  mountCallback.call(childDSL);
+} else if (engineRouteMap) {
+  engineRouteMap.class.call(childDSL);
+}
+```
+
+## How we teach this
+
+The alignment of the `route` and `mount` APIs should be easy to teach and is likely already inline with peoples mental model of the `route` API.
+
+This change also unlocks a more idomatic way of using in-repo engines when the use-case does not require isolation.
+
+The Ember-Engines documentation will need to be updated to include instructions for defining engine routes via the mount API.
+
+## Drawbacks
+
+One drawback is that this changes an existing public API and is something that existing Ember-Engines users may need to learn.
+
+## Alternatives
+
+Create an addon which extends the resolver to resolve route-maps from a custom location.