Minitest warning: DEPRECAED global use of must_match. Use `_(obj)`

I'm writing the following test with Minitest 5.12.0:

require 'minitest/autorun'
require 'nokogiri'
require 'net/http'

class NetClass; end

describe NetClass do
  attr_accessor :uri, :net
  before do
    @uri=URI('http://example.com/index.html')
    @net=Net::HTTP.get('example.com', '/index.html')
  end

  it 'gets uri' do
    net.must_match /Example Domain/i
  end
end

When running it I get the warning:

DEPRECATED: global use of must_match from net_test.rb:16 Use _(obj).must_match instead. This will fail in Minitest 6

To eliminate the warning I change the line:

net.must_match /Example Domain/i

to

_(net).must_match /Example Domain/i

I have not seen the _(obj) syntax before so my question is what is _() doing in this case.