> If a library exists for Rust 1.0, and then a library author releases a new version of the library with Rust 1.1 features, the Rust 1.0 library does not cease to exist. And Cargo supports semantic versioning...
That's true, but it ignores the way modern software unfortunately works. Programmers are lazy, and maintaining multiple development branches takes work, so they will try not to do that if they can get away with it. How many bug fixes will get backported to Rust 1.0.x, or to the 1.0-supporting version of that library? Probably not very many. Most developers will just bump the major version every 6 weeks, because that's easier than worrying about stability.