Rust does use semver and takes it very seriously. RFC 1105 for example documents exactly what is considered a breaking change to stabilized library APIs. The Rust team has an explicit allowance for breaking changes related to type soundness or memory changes without performing a semver bump. This is the first such change, there is one more coming in the future.

Both changes are very small; they do not impact most users and the users who are impacted can easily fix them unless they are exploiting the existing "bug" to do something unsound.

https://github.com/rust-lang/rfcs/blob/8e2d3a3341da533f846f6...