`implied_by` not working with const-stability?

[steffahn]

@rustbot label T-compiler, C-enhancement

Opening an issue for the problem described in #99212 (comment)

Context: The only use-case of implied_by that’s currently on nightly was introduced in #102556, but as far as I can tell, that doesn’t actually do anything. Given that the general implied_by feature has a behavior test, I would assume that the problem is that it isn’t working with (and isn’t tested with) rustc_const_unstable/rustc_const_stable instead of unstable/stable.

cc @davidtwco