Add can_merge option to autocomplete response

It's used only if merge_request_iid param is provided

app/controllers/autocomplete_controller.rb

@@ -16,7 +16,7 @@ class AutocompleteController < ApplicationController
       .new(params: params, current_user: current_user, project: project, group: group)
       .execute
 
-    render json: UserSerializer.new.represent(users)
+    render json: UserSerializer.new(params).represent(users, project: project)
   end
 
   def user

app/serializers/user_serializer.rb

@@ -2,4 +2,21 @@
 
 class UserSerializer < BaseSerializer
   entity UserEntity
+
+  def represent(resource, opts = {}, entity = nil)
+    if params[:merge_request_iid]
+      merge_request = opts[:project].merge_requests.find_by_iid!(params[:merge_request_iid])
+      preload_max_member_access(merge_request.project, Array(resource))
+
+      super(resource, opts.merge(merge_request: merge_request), MergeRequestAssigneeEntity)
+    else
+      super
+    end
+  end
+
+  private
+
+  def preload_max_member_access(project, users)
+    project.team.max_member_access_for_user_ids(users.map(&:id))
+  end
 end

spec/controllers/autocomplete_controller_spec.rb

@@ -222,6 +222,20 @@ describe AutocompleteController do
         expect(response_user_ids).to contain_exactly(non_member.id)
       end
     end
+
+    context 'merge_request_iid parameter included' do
+      before do
+        sign_in(user)
+      end
+
+      it 'includes can_merge option to users' do
+        merge_request = create(:merge_request, source_project: project)
+
+        get(:users, params: { merge_request_iid: merge_request.iid, project_id: project.id })
+
+        expect(json_response.first).to have_key('can_merge')
+      end
+    end
   end
 
   context 'GET projects' do

spec/serializers/user_serializer_spec.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe UserSerializer do
+  let(:user1) { create(:user) }
+  let(:user2) { create(:user) }
+
+  context 'serializer with merge request context' do
+    let(:merge_request) { create(:merge_request) }
+    let(:project) { merge_request.project }
+    let(:serializer) { described_class.new(merge_request_iid: merge_request.iid) }
+
+    before do
+      allow(project).to(
+        receive_message_chain(:merge_requests, :find_by_iid!)
+          .with(merge_request.iid).and_return(merge_request)
+      )
+
+      project.add_maintainer(user1)
+    end
+
+    it 'returns a user with can_merge option' do
+      serialized_user1, serialized_user2 = serializer.represent([user1, user2], project: project).as_json
+
+      expect(serialized_user1).to include("id" => user1.id, "can_merge" => true)
+      expect(serialized_user2).to include("id" => user2.id, "can_merge" => false)
+    end
+  end
+end