what's a 'test balloon'?

A viral commit

Ironically it's original use was in political* parlance.

From wiki it's "information sent out to the media in order to observe the reaction of an audience. It is used by companies sending out press releases to judge customer reaction, and by politicians who deliberately leak information on a policy change."

Yup I have no doubt that there's a Rust 'evangelist' group somewhere aiming for inorganic growth of the language.