So far in my toy projects I've mostly seen this crop up from depending on a lot of 0.x crates still going through significant version churn and legitimate breaking changes, where A and B legitimately can't use the same version of C as-is due to API changes, because they haven't been keeping their dependencies up to date.

The fix is simple, when it happens: Patch A/B to use the latest major version of C, fixing the source code as necessary. You can [patch] locally until upstream accepts your Pull Request - which might include a https://deps.rs/repo/github/rust-lang/cargo badge in their README.md, to encourage them to continue to keep C up-to-date.

Clippy has an (off by default) lint to warn/deny using multiple versions of the same crate.

i would love to, too!

:)