I wondered why the Rust community seemed so averse to 1.0 releases, but Rust for Rustaceans makes the interesting point that there are a lot of cases in the Rust language where adding new features technically qualifies as a breaking change. There doesn't seem to be a lot of room for the minor version number - everything is either a revision or a major release, and people aren't ready to commit to that level of strictness until they're feature-complete.