I could have perhaps written "would be generally useful enough to justify the additional annotation burden" (alluded to later in the paragraph). Also, in what year were you discovering these use cases? The discussions in question would have had to have happened in 2014 or earlier; for people who began using Rust post-1.0, it may very well be that these use cases were encountered because the language was stable by then, and that people might not have bothered enough with an unstable language to bother pushing the language to its limits like this. Though I guess we can take solace in the idea that the language that comes after Rust someday might benefit from these case studies. :)