Disclaimer: I'm not a Rust author, but I actively follow the development process. Rust does use semver. But as the concept of "breaking change" is ill-defined

Disclaimer: I'm not a Rust author, but I actively follow the development process. Rust does use semver. But as the concept of "breaking change" is ill-defined – as I've understood – in the definition, the Rust project is using the following heuristic: * If it's a safety/soundness-releated bugfix, it's considered a minor (1.x.0-changing) change even if it makes some existing crates stop compiling. Because the crate was relying on broken/unsound functionality, that's acceptable. * Impact of all such breaking changes are measured and assessed against the ecosystem using the Crater tool. * Even if crates stop building, the changes should be such that it's trivial to fix them. (In the best case, one-liner type annotations etc.) Btw. Rust sometimes deprecates APIs that have been supplanted by better alternatives. Deprecations are warnings, and they are allowed to go away only on a x.0.0 version change. Additionally, careful consideration for stability and breaking changes is done when designing new language features.