users_controller.rb

class UsersController < ApplicationController 

  def create
    @user = User.new params[:user]
    
    return render 'new' unless @user.save 
    
    flash[:notice] = "The user was created succesfully" 
    redirect_to :index
  end

end

users_controller_service.rb

class UsersController < ApplicationController 

  def create 
    CreateUser.new(User).call(
      params[:user], 
      on_success: ->(user) do 
        flash[:notice] ="The user was created successfully" 
        redirect_to :index
      end, 
      on_failure: ->(user) do 
        render 'new'
      end
  end
end

class CreateUser
   def initialize(users_repository)
     @users_repository = users_repository
   end
   
   def call(attributes: {}, on_success: nil, on_failure: nil)
      user = @users_repository.new attributes
      
      return on_faiulre[user] unless user.save
      
      on_success[user]
   end
end

I like how you inject User into the CreateUser model, rather than using it directly in CreateUser#call. In the codebase I have at work, I might default the argument in the CreateUser constructor so that client code doesn't need to worry about the collaborators.

However, I can definitely see why you wouldn't want to do that, so that CreateUser isn't tightly coupled to User.