I haven't exactly thought it through, but there is an alternative scanning mechanism which could look through all tags, and read versions out of Cargo.toml, this is perhaps more expensive than mapping versions to tags, since it involves some amount of reading the index.
but has some benefit of perhaps not needing regexp and repository specific configuration? (It is obvious but i'll say it) If mapping version to tag is the thing, its good to also check that the Cargo.toml version is in sync with the version from which the tag is derived.