That you can have multiple versions of your crate is exactly what enables the issues caused by public dependencies. It's not about multiple versions of your cra

That you can have multiple versions of your crate is exactly what enables the issues caused by public dependencies. It's not about multiple versions of your crate, but rather, multiple versions of the failure crate. If failure 0.2 is released (and doesn't use the semver trick), then you'll have two versions of the Fail trait, and thus, two versions of the failure::Error trait object. Madness then ensues for anyone using failure::Error (or dyn Fail in general). failure is a victim of its own success. I think most everyone believes it's pretty close to what we want Rust's error handling story to look like. But it's a public dependdncy and everyone started using it. Now it's more difficult to evolve.