I think this is a legit complaint. I think I understand why the rust team wants folks to use nightlies for unstable features. But if you had to opt-in with special cargo/compiler flags then it would probably be sufficient.
(Such a flag exists as an environment var, but you're not supposed to use it)
I guess the idea is to dissuade people from using hacks to enable unstable features -- this will break things which depend on the library. If a library compiles on stable, it should never break due to a compiler upgrade.
(Tooling doesn't have this problem, it doesn't come into play when it's used on a dependency)
