Fixes #3028: make 'drush @self command' equivalent to 'drush command' (…

…#3030)

docs/usage.md

@@ -25,7 +25,7 @@ command line options just for Drush to work.
 If you wish to be able to select your Drupal site implicitly from the
 current working directory without using the --uri option, but you need your
 base_url to be set correctly, you may force it by setting the uri in
-a config.yml file located in the same directory as your settings.php file.
+a drush.yml file located in the same directory as your settings.php file.
 
 ```
 uri: "http://example.com"

src/Preflight/PreflightArgs.php

@@ -190,6 +190,10 @@ public function hasAlias()
      */
     public function setAlias($alias)
     {
+        // Treat `drush @self ...` as if an alias had not been used at all.
+        if ($alias == '@self') {
+            $alias = '';
+        }
         return $this->set(self::ALIAS, $alias);
     }
 

src/SiteAlias/SiteAliasManager.php

@@ -233,13 +233,15 @@ protected function buildSelf(PreflightArgsInterface $preflightArgs, Environment
         }
 
         // If the user provides the --root parameter then we don't want to use
-        // the site alias.
+        // the site-set alias.
         $selectedRoot = $preflightArgs->selectedSite();
         if (!$selectedRoot) {
             $aliasName = $environment->getSiteSetAliasName();
-            $alias = $this->getAlias($aliasName);
-            if ($alias) {
-                return $alias;
+            if (!empty($aliasName)) {
+                $alias = $this->getAlias($aliasName);
+                if ($alias) {
+                    return $alias;
+                }
             }
         }