[BUGFIX release] Deprecate htmlSafe via prototype

In Ember 3.24 various string methods added to the `String.prototype` were
deprecated for removal in Ember 4.0. `htmlSafe` (the version available
via string prototype) was supposed to be included in those deprecations,
however dues to its implementation being different it was missed. This
ommission can be understood as a bug.

This patch deprecates `String.prototype.htmlSafe` targeting Ember 4.0.
This will allow the removal of *all* string prototype extensions in 4.0
as intended by the original deprecation.

packages/@ember/-internals/glimmer/tests/utils/string-test.js

@@ -11,6 +11,15 @@ moduleFor(
       this.assert.ok(safeString instanceof SafeString, 'should be a SafeString');
     }
 
+    ['@test [deprecated] htmlSafe via string prototype should return an instance of SafeString']() {
+      let safeString;
+      expectDeprecation(() => {
+        safeString = 'you need to be more <b>bold</b>'.htmlSafe();
+      }, /String prototype extensions are deprecated./);
+
+      this.assert.ok(safeString instanceof SafeString, 'should be a SafeString');
+    }
+
     ['@test htmlSafe should return an empty string for null']() {
       let safeString = htmlSafe(null);
 
@@ -24,6 +33,15 @@ moduleFor(
       this.assert.equal(safeString instanceof SafeString, true, 'should be a SafeString');
       this.assert.equal(safeString.toString(), '', 'should return an empty string');
     }
+
+    ['@test [deprecated] htmlSafe via string prototype should return an instance of SafeString for an empty string']() {
+      let safeString;
+      expectDeprecation(() => {
+        safeString = ''.htmlSafe();
+      }, /String prototype extensions are deprecated./);
+
+      this.assert.ok(safeString instanceof SafeString, 'should be a SafeString');
+    }
   }
 );
 

packages/ember/index.js

@@ -493,6 +493,15 @@ Ember._captureRenderTree = captureRenderTree;
 
 if (ENV.EXTEND_PROTOTYPES.String) {
   String.prototype.htmlSafe = function () {
+    deprecate(`String prototype extensions are deprecated. Please import htmlSafe from '@ember/template' instead.`, true, {
+      id: 'ember-string.prototype-extensions',
+      for: 'ember-source',
+      since: {
+        enabled: '3.27.1',
+      },
+      until: '4.0.0',
+      url: 'https://deprecations.emberjs.com/v3.x/#toc_ember-string-htmlsafe-ishtmlsafe',
+    });
     return htmlSafe(this);
   };
 }