@pierstoval cargo will "merge" semver compatible crate versions in your dependency tree, but if you explicitly depend on foo@1.0.0 and one of your dependencies explicitly depend on foo@2.0.0, then the only two options are "you can't do that" (can't compile) or "we'll let you do that and treat them as separate crates" (can compile but can't mix types from different versions). Under the former model, upgrading dependencies can mean an unpleasant trip to yak shaving central.