RSpec's companion mocking library. In the wroclove.rb 2026 Rails.env-stubbing lightning talk, used with a 'with temporary scope' pattern and around/around_example hooks to let Rails.env.development?/test?/production? return different values within a block. Speaker noted it measurably slows down tests using the helper, likely because test-environment optimizations are bypassed when the Rails.env predicates are stubbed.