MDL-77182 core: add debug template info setting

A new developer setting that adds comments in the page HTML specifying
which template is used to render each part of the page. It can be set
form the site admin development tab and it is stored
as $CFG->debugtemplateinfo.

admin/settings/development.php

@@ -51,6 +51,7 @@
              100 => new lang_string('debugsqltrace100', 'admin'))));
     $temp->add(new admin_setting_configcheckbox('debugvalidators', new lang_string('debugvalidators', 'admin'), new lang_string('configdebugvalidators', 'admin'), 0));
     $temp->add(new admin_setting_configcheckbox('debugpageinfo', new lang_string('debugpageinfo', 'admin'), new lang_string('configdebugpageinfo', 'admin'), 0));
+    $temp->add(new admin_setting_configcheckbox('debugtemplateinfo', new lang_string('debugtemplateinfo', 'admin'), new lang_string('debugtemplateinfo_desc', 'admin'), 0));
     $ADMIN->add('development', $temp);
 
     // "Profiling" settingpage (conditionally if the 'xhprof' extension is available only).

lang/en/admin.php

@@ -483,6 +483,8 @@
 $string['debugsqltrace_desc'] = 'If enabled, a partial or full PHP stack trace is added into the SQL as a comment.';
 $string['debugstringids'] = 'Show origin of languages strings';
 $string['debugstringids_desc'] = 'If enabled, language string components and identifiers are displayed when ?strings=1 or &strings=1 is appended to the page URL.';
+$string['debugtemplateinfo'] = 'Show template information';
+$string['debugtemplateinfo_desc'] = 'If enabled, templates used for rendering are shown as comments in the page HTML. Use for temporary debugging only, as it produces HTML validation errors and could break some page scripts.';
 $string['debugvalidators'] = 'Show validator links';
 $string['defaultcity'] = 'Default city';
 $string['defaultcity_help'] = 'A city entered here will be the default city when creating new user accounts.';

lib/classes/output/mustache_filesystem_loader.php

@@ -65,4 +65,19 @@ protected function getFileName($name) {
     protected function shouldCheckPath() {
         return true;
     }
+
+    /**
+     * Load a Template by name.
+     *
+     * @param string $name the template name
+     * @return string Mustache Template source
+     */
+    public function load($name): string {
+        global $CFG;
+        if (!empty($CFG->debugtemplateinfo)) {
+            // We use many templates per page. We don't want to allocate more memory than necessary.
+            return "<!-- template(PHP): $name -->" . parent::load($name) . "<!-- /template(PHP): $name -->";
+        }
+        return parent::load($name);
+    }
 }

lib/classes/output/mustache_template_source_loader.php

@@ -88,14 +88,17 @@ public function load(
         string $themename,
         bool $includecomments = false
     ) : string {
+        global $CFG;
         // Get the template source from the callback.
         $source = ($this->gettemplatesource)($component, $name, $themename);
 
         // Remove comments from template.
         if (!$includecomments) {
             $source = $this->strip_template_comments($source);
         }
-
+        if (!empty($CFG->debugtemplateinfo)) {
+            return "<!-- template(JS): $name -->" . $source . "<!-- /template(JS): $name -->";
+        }
         return $source;
     }