Stick this in a library file...

file_provider_copy.rb

class Chef
  class Provider
    class File
      class Copy << Chef::Provider::File
       
        def compare_content
          checksum(@current_resource.path) == checksum(@new_resource.content)
        end

        def set_content
          unless compare_content
            backup @new_resource.path if ::File.exists?(@new_resource.path)
            ::File.cp_r(@new_resource.content, @new_resource.path)
            Chef::Log.info("#{@new_resource.content} copied to #{@new_resource.path}")
            @new_resource.updated_by_last_action(true)
          end
        end
      end
    end
  end
end

recipe.rb

file "/tmp/foo" do
  content "/tmp/bar"
  provider Chef::Provider::File::Copy
end

A safer solution would be to add a new attribute instead of re-using content for this.