Conditionally render create project button

This commit checks whether the current user is allowed to create a
project prior to rendering the button.
It also return a `404` error for the Project#new method when the current
user is not allowed to create a new project.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/333450

Changelog: fixed

app/controllers/projects_controller.rb

@@ -54,6 +54,8 @@ class ProjectsController < Projects::ApplicationController
 
   # rubocop: disable CodeReuse/ActiveRecord
   def new
+    return access_denied! unless current_user.can_create_project?
+
     @namespace = Namespace.find_by(id: params[:namespace_id]) if params[:namespace_id]
     return access_denied! if @namespace && !can?(current_user, :create_projects, @namespace)
 

app/helpers/nav/top_nav_helper.rb

@@ -267,7 +267,11 @@ module Nav
       builder.add_primary_menu_item(id: 'your', title: _('Your projects'), href: dashboard_projects_path)
       builder.add_primary_menu_item(id: 'starred', title: _('Starred projects'), href: starred_dashboard_projects_path)
       builder.add_primary_menu_item(id: 'explore', title: _('Explore projects'), href: explore_root_path)
+
+      if current_user.can_create_project?
         builder.add_secondary_menu_item(id: 'create', title: _('Create new project'), href: new_project_path)
+      end
+
       builder.build
     end
 

spec/controllers/projects_controller_spec.rb

@@ -42,6 +42,32 @@ RSpec.describe ProjectsController do
             expect(response).not_to render_template('new')
           end
         end
+
+        context 'when user is an external user' do
+          let_it_be(:user) { create(:user, external: true) }
+
+          it 'responds with status 404' do
+            group.add_owner(user)
+
+            get :new, params: { namespace_id: group.id }
+
+            expect(response).to have_gitlab_http_status(:not_found)
+            expect(response).not_to render_template('new')
+          end
+        end
+
+        context 'when user is a group guest' do
+          let_it_be(:user) { create(:user) }
+
+          it 'responds with status 404' do
+            group.add_guest(user)
+
+            get :new, params: { namespace_id: group.id }
+
+            expect(response).to have_gitlab_http_status(:not_found)
+            expect(response).not_to render_template('new')
+          end
+        end
       end
     end
   end