test(ivy): Add bazel flag to control building ViewEngine or Ivy (#23833)

PR Close #23833

packages/core/BUILD.bazel

@@ -9,7 +9,10 @@ ng_module(
             "*.ts",
             "src/**/*.ts",
         ],
-    ),
+        exclude = ["src/ivy_switch.ts"],
+    ) + [
+        ":ivy_switch",
+    ],
     module_name = "@angular/core",
     deps = [
         "//packages:types",
@@ -30,3 +33,20 @@ ng_package(
         "//packages/core/testing",
     ],
 )
+
+## Controls if Ivy is enabled. (Temporary target until we permanently switch over to Ivy)
+##
+## This file generates `src/ivy_switch.ts` file which reexports symbols for `ViewEngine` or `Ivy.`
+## - append `--define=ivy=false` (default) to `bazel` command to reexport `./ivy_switch_false`
+##   and use `ViewEngine`;
+## - append `--define=ivy=true` to `bazel` command to rexport `./ivy_switch_true` and use `Ivy`;
+##
+## NOTE: `--define=ivy=true` works with any `bazel` command or target across the repo.
+##
+## See: `//tools/bazel.rc` where `--define=ivy=false` is defined as default.
+## See: `./src/ivy_switch.ts` for more details.
+genrule(
+    name = "ivy_switch",
+    outs = ["src/ivy_switch.ts"],
+    cmd = "echo export '*' from \"'./ivy_switch_$(ivy)';\" > $@",
+)

packages/core/src/core_private_export.ts

@@ -15,6 +15,7 @@ export {ChangeDetectorStatus as ɵChangeDetectorStatus, isDefaultChangeDetection
 export {Console as ɵConsole} from './console';
 export {inject as ɵinject, setCurrentInjector as ɵsetCurrentInjector} from './di/injector';
 export {APP_ROOT as ɵAPP_ROOT} from './di/scope';
+export {ivyEnabled as ɵivyEnabled} from './ivy_switch';
 export {ComponentFactory as ɵComponentFactory} from './linker/component_factory';
 export {CodegenComponentFactoryResolver as ɵCodegenComponentFactoryResolver} from './linker/component_factory_resolver';
 export {ReflectionCapabilities as ɵReflectionCapabilities} from './reflection/reflection_capabilities';

packages/core/src/ivy_switch.ts

@@ -0,0 +1,34 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+/**
+ * This file is used to control if the default rendering pipeline should be `ViewEngine` or `Ivy`.
+ *
+ * Reexport from:
+ * - `./ivy_switch_false` => Use `ViewEngine`.
+ * - `./ivy_switch_true` => Use `Ivy`.
+ *
+ * This file is here for your IDE as well as for `google3`. The `bazel` build system
+ * specifically excludes this file and instead generates a new file which is controlled by
+ * command line:
+ *
+ * - `bazel build packages/core` => Use `ViewEngine`
+ * - `bazel build packages/core --define=ivy=true` => Use `Ivy`
+ *
+ * See: `bazel build packages/core:ivy_switch` for more details.
+ *
+ * ## How to use this
+ *
+ * Use this mechanism to have the same symbol be aliased to different implementation.
+ * 1) Create two implementations of a symbol (most likely a `function` or a `class`).
+ * 2) Export the two implementation under same name in `./ivy_switch_false` and `./ivy_switch_false`
+ *    respectively.
+ * 3) Import the symbol from `./ivy_switch`. The imported symbol will that point to either the
+ *    symbol in `./ivy_switch_false` and `./ivy_switch_false` depending on the compilation mode.
+ */
+export * from './ivy_switch_false';

packages/core/src/ivy_switch_false.ts

@@ -0,0 +1,9 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+export const ivyEnabled = false;

packages/core/src/ivy_switch_true.ts

@@ -0,0 +1,9 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+export const ivyEnabled = true;

tools/bazel.rc

@@ -53,3 +53,10 @@ test --experimental_ui
 ################################
 
 # Bazel flags for CircleCI are in /.circleci/bazel.rc
+
+################################
+# Temporary Settings for Ivy   #
+################################
+# to determine if the compiler used should be Ivy or ViewEngine one can use `--define=ivy=true` on 
+# any bazel target. This is a temporary flag until codebase is permanently switched to ViewEngine.
+build --define=ivy=false