♻️ Extract type assertions out of log to multiple pure function calls

build-system/test-configs/dep-check-config.js

@@ -476,6 +476,18 @@ exports.rules = [
     allowlist: ['src/amp.js->src/polyfills/index.js'],
   },
 
+  // Base assertions should never be used explicitly; only the user/dev wrappers
+  // or the Log class should have access to the base implementations.
+  {
+    filesMatching: '**/*.js',
+    mustNotDependOn: 'src/core/assert/base.js',
+    allowlist: [
+      'src/core/assert/dev.js->src/core/assert/base.js',
+      'src/core/assert/user.js->src/core/assert/base.js',
+      'src/log.js->src/core/assert/base.js',
+    ],
+  },
+
   // Rules for main src.
   {
     filesMatching: 'src/**/*.js',

build-system/test-configs/forbidden-terms.js

@@ -573,7 +573,8 @@ const forbiddenTermsGlobal = {
   '/\\*\\* @type \\{\\!Element\\} \\*/': {
     message: 'Use assertElement instead of casting to !Element.',
     allowlist: [
-      'src/log.js', // Has actual implementation of assertElement.
+      'src/core/assert/base.js', // Has actual implementation of assertElement.
+      'src/core/assert/dev.js', // Has actual implementation of assertElement.
       'src/polyfills/custom-elements.js',
       'ads/google/imaVideo.js', // Required until #22277 is fixed.
       '3p/twitter.js', // Runs in a 3p window context, so cannot import log.js.

src/core/assert/assert.extern.js

@@ -0,0 +1,25 @@
+/**
+ * Copyright 2021 The AMP HTML Authors. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS-IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * @fileoverview Type externs for assertions.
+ * @externs
+ */
+
+/**
+ * @typedef {function(?, string=, ...*):?|function(?, !Array<*>)}
+ */
+let AssertionFunction;