Rust's built-in lints are configurable with compiler directives, so you'll be able to add

    #![deny(bare_trait_objects)]

To the top of your crate and cause a compiler warning if someone uses the old syntax on your project.

EDIT: just realized I mis-parsed "you" in parent. Oh well.

This kind of change is the kind an edition can make, though I’m not 100% sure if the plan for 2018 is to warn or error on the older syntax.

I feel like a warning for one edition and error on the next would make it straightforward as to how much time is left to update the code.

That’s generally true, the question is, do you warn in 2015 or 2018?

That being said, this particular fix is 100% automatable, and rustfix can already do it, so the time to update is “a few seconds”.

IIRC, it will take two editions before removing something: at the first edition the compiler will lint against it (which is already quite a big change: you get tons of new warnings you need to fix) and one edition later, it's an error.

Edit: I didn't see the answer from steveklabnik below. Apparently, it's not clearly decided yet.

Yes, the next edition will allow breaking changes, while still allowing you to use crates written in the older edition.

some forms of breaking changes. This is of that kind, though we cannot make arbitrary ones.