Slug Driver Support (#2456)

- Support slug drivers for core's sluggable models, easily extends to other models
- Add automated testing for affected single-model API routes
- Fix nickname selection UI
- Serialize slugs as `slug` attribute
- Make min search length a constant

src/User/UserRepository.php

@@ -40,6 +40,23 @@ class UserRepository
         return $this->scopeVisibleTo($query, $actor)->firstOrFail();
     }
 
+    /**
+     * Find a user by username, optionally making sure it is visible to a certain
+     * user, or throw an exception.
+     *
+     * @param int $id
+     * @param User $actor
+     * @return User
+     *
+     * @throws \Illuminate\Database\Eloquent\ModelNotFoundException
+     */
+    public function findOrFailByUsername($username, User $actor = null)
+    {
+        $query = User::where('username', $username);
+
+        return $this->scopeVisibleTo($query, $actor)->firstOrFail();
+    }
+
     /**
      * Find a user by an identification (username or email).
      *

src/User/UsernameSlugDriver.php

@@ -0,0 +1,36 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\User;
+
+use Flarum\Database\AbstractModel;
+use Flarum\Http\SlugDriverInterface;
+
+class UsernameSlugDriver implements SlugDriverInterface
+{
+    /**
+     * @var UserRepository
+     */
+    protected $users;
+
+    public function __construct(UserRepository $users)
+    {
+        $this->users = $users;
+    }
+
+    public function toSlug(AbstractModel $instance): string
+    {
+        return $instance->username;
+    }
+
+    public function fromSlug(string $slug, User $actor): AbstractModel
+    {
+        return $this->users->findOrFailByUsername($slug, $actor);
+    }
+}