A hash without braces became keyword arguments

A Rails app started adding a signed unsubscribe link to some customer emails. I booked a few test appointments on an account that gets those emails, and none of the emails arrived. Sidekiq was retrying the job with this:

ArgumentError: wrong number of arguments (given 0, expected 1)

The backtrace pointed at the helper that builds the unsubscribe token. The code passed ruby -c and every other path through the mailer, and only this kind of account ever reached it:

verifier.generate('o' => org_id, 'e' => email)

ActiveSupport::MessageVerifier#generate takes one positional argument, the value, plus keyword arguments like expires_in: and purpose:. In Ruby 3, when a method has keyword parameters, a hash at the end of the call without braces goes to the keywords. That’s true even with string keys. So the whole hash went to the keywords, and value got nothing. Braces make it one positional argument:

verifier.generate({ 'o' => org_id, 'e' => email })

A runner script showed the token now round-trips:

{"o"=>42, "e"=>"test@example.com"}

After the deploy to staging, the emails arrived.

Ruby 2.7 warned about this case, and Ruby 3 made it an error. Code like this passes a syntax check and fails only when that line runs, so a spec that builds the token, or a review rule for braceless hashes in calls to methods that take keywords, catches it earlier.