RSpec any_instance deprecation: how to fix it?

Use allow_any_instance_of:

describe (".create") do
  it 'returns error when...' do
    allow_any_instance_of(User).to receive(:save).and_return(false)
    post :create, user: {name: "foo", surname: "bar"}, format: :json
    expect(response.status).to eq(422)
  end
end