Copy project snippet routes under - scope

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

changelogs/unreleased/dz-scope-project-snippet-routes.yml

+---
+title: Copy project snippet routes under - scope
+merge_request: 35022
+author:
+type: other

config/routes/project.rb

@@ -369,6 +369,19 @@ constraints(::Constraints::ProjectUrlConstrainer.new) do
         end
       end
 
+      # Serve snippet routes under /-/snippets.
+      # To ensure an old unscoped routing is used for the UI we need to
+      # add prefix 'as' to the scope routing and place it below original routing.
+      # Issue https://gitlab.com/gitlab-org/gitlab/-/issues/29572
+      scope '-', as: :scoped do
+        resources :snippets, concerns: :awardable, constraints: { id: /\d+/ } do # rubocop: disable Cop/PutProjectRoutesUnderScope
+          member do
+            get :raw
+            post :mark_as_spam
+          end
+        end
+      end
+
       namespace :prometheus do
         resources :alerts, constraints: { id: /\d+/ }, only: [:index, :create, :show, :update, :destroy] do # rubocop: disable Cop/PutProjectRoutesUnderScope
           post :notify, on: :collection

spec/routing/project_routing_spec.rb

@@ -407,6 +407,10 @@ describe 'project routing' do
     it 'to #destroy' do
       expect(delete('/gitlab/gitlabhq/snippets/1')).to route_to('projects/snippets#destroy', namespace_id: 'gitlab', project_id: 'gitlabhq', id: '1')
     end
+
+    it 'to #show from scope routing' do
+      expect(get('/gitlab/gitlabhq/-/snippets/1')).to route_to('projects/snippets#show', namespace_id: 'gitlab', project_id: 'gitlabhq', id: '1')
+    end
   end
 
   # test_project_hook POST    /:project_id/hooks/:id/test(.:format) hooks#test