Comment by demurgos

I assume that they mean that you can use Rust as a language without its standard library. This matters here since the Kernel does not use Rust's standard library as far as I know (only the core module).

I'm not aware of semver breakage in the language.

Another important aspect is that Semver is a social contract, not a mechanical guarantee. The Semver spec dedicates a lot of place to clarify that it's about documented APIs and behaviors, not all visible behavior. Rust has a page where it documents its guarantees for libraries [0].

[0] https://doc.rust-lang.org/cargo/reference/semver.html