Though, I want to make clear that it is permissible for some theoretical code to stop compiling when going from 1.4 to 1.5, for example. Check out https://github.com/nikomatsakis/rfcs/blob/projection-and-lif..., for an example of this.

This is because the compiler wasn't checking some cases strictly enough, and you could "hide" constraints from the compiler, causing them to fail to be enforced.

They checked against crates.io when raising this RFC, and found that there were 35 crates (packages) that would be able to keep compiling if this turned into a future-compat warning, to become an error in the release afterwards.

The example

    trait Test {
        fn test(&self) -> Option<Self>;
        //                ~~~~~~~~~~~~
        //            Incorrectly permitted before.
    }

Should be

    trait Test: Sized {
        fn test(&self) -> Option<Self>;
    }

Because `Option<T>` requires that `T` be `Sized`. Poking around on 1.3, I think that it would be an error to instantiate this trait with a [i8] or something that was not sized anyways, so you're not losing much by needing to declare the trait properly.