Automatic Versioning in Package Managers

Reading http://crates.io/manifest.html it mentions a section on following semver:

> Before you reach 1.0, anything goes.
> After 1.0, only make breaking changes when you increment the major version. In Rust, breaking changes include adding fields to structs or variants to enums. Don’t break the build.
> After 1.0, don’t add any new public API (no new pub anything) in tiny versions. Always increment the minor version if you add any new pub structs, traits, fields, types, functions, methods or anything else.

Wait, why can't we just automatically version code based on this? It's hard for other languages, but Rust compiler is probably able to generate semver for us. It would certainly make less code break if semver was definitely followed.

The one downside is less flexibility for the authors. But when doing semver, you don't really have that flexibility anyway-- versioning is spec-ed out for you.