Prevent private project name and namespace from leaking in the new MR view

Fixes #15591.

Signed-off-by: Rémy Coutable <remy@rymai.me>

app/services/merge_requests/build_service.rb

@@ -7,6 +7,9 @@ module MergeRequests
       merge_request.can_be_created = false
       merge_request.compare_commits = []
       merge_request.source_project = project unless merge_request.source_project
+
+      merge_request.target_project = nil unless can?(current_user, :read_project, merge_request.target_project)
+
       merge_request.target_project ||= (project.forked_from_project || project)
       merge_request.target_branch ||= merge_request.target_project.default_branch

spec/features/merge_requests/create_new_mr_spec.rb

@@ -30,4 +30,14 @@ feature 'Create New Merge Request', feature: true, js: true do
     expect(page).to have_content 'git checkout -b orphaned-branch origin/orphaned-branch'
   end
+
+  context 'when target project cannot be viewed by the current user' do
+    it 'does not leak the private project name & namespace' do
+      private_project = create(:project, :private)
+
+      visit new_namespace_project_merge_request_path(project.namespace, project, merge_request: { target_project_id: private_project.id })
+
+      expect(page).not_to have_content private_project.to_reference
+    end
+  end
 end